radiational, adj.
/ray'dee ay"sheuhn/, n.
1. Physics.
a. the process in which energy is emitted as particles or waves.
b. the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and absorbed by another body.
c. the energy transferred by these processes.
2. the act or process of radiating.
3. something that is radiated.
4. radial arrangement of parts.
[1545-55; < L radiation- (s. of radiatio) a glittering, shining. See RADIATE, -ION]

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Process by which energy is emitted from a source and propagated through the surrounding medium, or the energy involved in this process.

Radiation consists of a flow of atomic or subatomic particles or of waves. Familiar examples are light (a form of electromagnetic radiation) and sound (a form of acoustic radiation). Both electromagnetic and acoustic radiation can be described as waves with a range of frequencies and intensities. Electromagnetic radiation is also often treated as discrete packets of energy, called photons. All matter is constantly bombarded by radiation from cosmic and terrestrial sources, and radioactive elements emit several types of radiation (see radioactivity). See also Cherenkov ratiation, Hawking radiation, infrared radiation, synchrotron radiation, thermal radiation, ultraviolet radiation.
(as used in expressions)
braking radiation
enhanced radiation warhead

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      flow of atomic and subatomic particles and of waves, such as those that characterize heat rays, light rays, and X rays. All matter is constantly bombarded with radiation of both types from cosmic and terrestrial sources. This article delineates the properties and behaviour of radiation and the matter with which it interacts and describes how energy is transferred from radiation to its surroundings. Considerable attention is devoted to the consequences of such an energy transfer to living matter, including the normal effects on many life processes (e.g., photosynthesis in plants and vision in animals) and the abnormal or injurious effects that result from the exposure of organisms to unusual types of radiation or to increased amounts of the radiations commonly encountered in nature. The applications of various forms of radiation in medicine and technological fields are touched upon as well.

General background

Types of radiation
      Radiation may be thought of as energy in motion either at speeds equal to the speed of light in free space—approximately 3 × 1010 centimetres (186,000 miles) per second—or at speeds less than that of light but appreciably greater than thermal velocities (e.g., the velocities of molecules forming a sample of air). The first type constitutes the spectrum of electromagnetic radiation that includes radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays, as well as the neutrino (see below). These are all characterized by zero mass when (theoretically) at rest. The second type includes such particles as electrons, protons, and neutrons. In a state of rest, these particles have mass and are the constituents of atoms and atomic nuclei. When such forms of particulate matter travel at high velocities, they are regarded as radiation. In short, the two broad classes of radiation are unambiguously differentiated by their speed of propagation and corresponding presence or absence of rest mass. In the discussion that follows, those of the first category are referred to as “electromagnetic rays” (plus the neutrino) and those of the second as “matter rays.”

      At one time, electromagnetic rays were thought to be inherently wavelike in character—namely, that they spread out in space and are able to exhibit interference when they come together from two or more sources. (Such behaviour is typified by water waves in the way they propagate and periodically reinforce and cancel one another.) Matter rays, on the other hand, were considered to be inherently particle-like in character—i.e., localized in space and incapable of interference. During the early 1900s, however, major experiments and attendant theories revealed that all forms of radiation, under appropriate conditions, can exhibit both particle-like and wavelike behaviour. This is referred to as the wave–particle duality (wave-particle duality) and provides in large part the foundation for the modern quantum theory of matter and radiation. The wave behaviour of radiation is apparent in its propagation through space, while the particle behaviour is revealed by the nature of interactions with matter. Because of this, care must be exercised to use the terms waves and particles only when appropriate.

Electromagnetic rays and neutrinos

Visible light and the other components of the electromagnetic spectrum
      According to the theory of relativity, the velocity of light is a fixed quantity independent of the velocity of the emitter, the absorber, or a presumably independent observer, all three of which do affect the velocities of common wavelike disturbances such as sound. In an extended definition, the term light embraces the totality of electromagnetic radiation. It includes the following: the long electromagnetic waves predicted by the Scottish physicist James Clerk Maxwell in 1864 and discovered by the German physicist Heinrich Hertz in 1887 (now called radio waves); infrared and ultraviolet rays; the X rays discovered in 1895 by Wilhelm Conrad Röntgen of Germany; the gamma rays that accompany many radioactive-decay processes; and some even more energetic (with higher energy) X rays and gamma rays produced as the normal accompaniment of the operations of ultrahigh-energy machines (i.e., particle accelerators such as the Van de Graaff generator, the cyclotron and its variants, and the linear accelerator).

      The behaviour of light seems to have interested ancient philosophers but without stimulating them to experiment, though all of them were impressed by vision. The first meaningful optical experiments on light were performed by the English physicist and mathematician Isaac Newton (Newton, Sir Isaac) (beginning in 1666), who showed (1) that white light diffracted by a prism into its various colours (colour) can be reconstituted into white light by a prism oppositely arranged and (2) that light of a particular colour selected from the diffracted spectrum of a prism cannot be further diffracted into beams of other colour by an additional prism. Newton hypothesized that light is corpuscular in its nature, each colour represented by a different particle speed, an erroneous assumption. Furthermore, in order to account for the refraction of light, the corpuscular theory required, contrary to the wave theory of the Dutch scientist Christiaan Huygens (developed at about the same time), that light corpuscles travel with greater velocity in the denser medium. Support for the wave theory came in the electromagnetic theory of Maxwell (1864) and the subsequent discoveries of Hertz and of Röntgen of both the very long and the very short waves Maxwell had included in his theory. The German physicist Max Planck (Planck, Max) proposed a quantum theory of radiation to counter some of the difficulties associated with the wave theory of light, and in 1905 Einstein proposed that light is composed of quanta (later called photons). Thus, experiment and theory had led around from particles (of Newton) that behave like waves (Huygens) to waves (Maxwell) that behave like particles (Einstein), the apparent velocity of which is unaffected by the velocity of the source or the velocity of the receiver. Furthermore it was found, in 1922, that the shorter-wavelength electromagnetic radiations (e.g., X rays) have momentum such as may be expected of particles, part of which can be transferred to electrons with which they collide (i.e., the Compton effect).

Neutrinos (neutrino) and antineutrinos
      Neutrinos and their antiparticles are forms of radiation similar to electromagnetic rays in that they travel at the speed of light and have little or no rest mass and zero charge. They too are produced by ultrahigh-energy particle accelerators and certain types of radioactive decay.

Matter rays (de Broglie wave)
      Unlike X rays and gamma rays, some high-energy radiations travel at less than the speed of light. Some of these were identified initially by their particulate nature and only later were shown to travel with wavelike character. One example of this kind of radiation is the electron, first established as a negatively charged particle in 1897 by the English physicist Joseph John Thomson (Thomson, Sir J.J.) and later as the component of beta rays emitted by radioactive elements. The electron was shown by the American physicist Robert Millikan (Millikan, Robert Andrews) in 1910 to have a fixed charge and by George Paget Thomson, an English physicist, and the American physicists Clinton J. Davisson and Lester H. Germer (1927) to have wavelike as well as particulate character. Electrons classified as radiation have velocities that range from as low as 108 centimetres per second to almost the speed of light. The negative electron, still commonly called an electron, is identified more precisely as a negatron. In 1932 the American physicist Carl Anderson (Anderson, Carl David) demonstrated the existence of a positive electron, generally called a positron and identified as one of the antiparticles of matter. The collision of a positron and an electron results in the intermediate production of a short-lived atomlike system called positronium, which decays in about 10-7 second into two gamma rays. Other entities commonly classified as matter when traveling with high velocity include the positively charged nucleus of the hydrogen atom, or proton; the nucleus of deuterium (i.e., heavy hydrogen, the nucleus of which has double the mass of normal hydrogen's nucleus), or deuteron, also positively charged; and the nucleus of the helium atom, or alpha particle, which has a double positive charge. The more-massive positive nuclei of other atoms show similar wavelike properties when sufficiently accelerated in an electric field. All charged matter rays have a charge exactly equal to that of the negative or positive electron or to some integral multiple of that charge.

      The neutron also is a matter ray. It is emitted in certain radioactive-decay processes and in fission, the process in which a nucleus splits into two smaller nuclei. The neutron decays in free space with a 12- to 13-minute half-life—i.e., one-half of any given number of neutrons decay within 12–13 minutes, each into a proton and a negatron plus an antineutrino (see above). The mass of the neutron approximates that of the hydrogen atom, about 1,850 times the mass of the electron.

      Another class of the so-called elementary particles is the meson, which comes both positively and negatively charged (i.e., with the same charge as that of an electron), as well as electrically neutral. The masses of mesons are always greater than those of electrons, and most have a mass less than that of the proton; a few have slightly greater mass. Although all mesons are classified as matter rays when traveling at high velocities, they are so few that their chemical effects are not presently studied. Because they are part of the constant bombardment from free space to which all matter is constantly exposed, however, they may have considerable effects, such as contributing to the processes of aging and evolution.

The structure and properties of matter
      Matter in bulk comprises particles that, compared to radiation, may be said to be at rest, but the motion of the molecules that compose matter, which is attributable to its temperature, is equivalent to travel at the rate of hundreds of metres per second. Although matter is commonly considered to exist in three forms, solid, liquid, and gas, a review of the effects of radiation on matter must also include mention of the interactions of radiation with glasses (glass), attenuated (low-pressure) gases, plasmas, and matter in states of extraordinarily high density. A glass appears to be solid but is actually a liquid of extraordinarily high viscosity, or a mixture of such a liquid and embedded microcrystalline material, which unlike a true solid remains essentially disorganized at temperatures much below its normal freezing point. Low-pressure gases are represented by the situation that exists in free space, in which the nearest neighbour molecules, atoms, or ions may be literally centimetres apart. Plasmas (plasma), by contrast, are regions of high density and temperature in which all atoms are dissociated into their positive nuclei and electrons.

 The capability of analyzing and understanding matter depends on the details that can be observed and to an important extent on the instruments that are used. Bulk, or macroscopic, matter is detectable directly by the senses supplemented by the more common scientific instruments, such as microscopes, telescopes, and balances. It can be characterized by measurement of its mass and, more commonly, its weight, by magnetic effects, and by a variety of more sophisticated techniques, but most commonly by optical phenomena—by the visible or invisible light (i.e., photons) that it absorbs, reflects, or emits or by which its observable character is modified. Energy absorption, which always involves some kind of excitation, and the opposed process of energy emission depend on the existence of ground-state and higher energy levels of molecules and atoms. A simplified system of energy states (energy state), or levels, is shown schematically in Figure 1—>. Such a system is exactly fixed for each atomic and molecular system by the laws of quantum mechanics; the “allowed,” or “permitted,” transitions between levels, which may involve energy gain or loss, are also established by those same laws of nature. Excitation to energy levels above those of the energetically stable molecules or atoms may result in dissociation or ionization: molecules can dissociate into product molecules and free radicals, and, if the energy absorption is great enough, atoms as well as molecules can yield ions and electrons (i.e., ionization occurs). Atomic nuclei themselves may exist in various states in which they absorb and emit gamma rays under certain conditions, and, if the nuclei are raised to, or by some process left in, energy states that are sufficiently high, they may themselves emit positrons, negatrons, alpha particles, or neutrons (and neutrinos) or dissociate into the nuclei of two or more lighter atoms. The resulting atoms may be similarly short-lived and unstable, or they may be extremely long-lived and quite stable.

The effects of radiation
      The interaction of radiation with matter can be considered the most important process in the universe. When the universe began to cool down at an early stage in its evolution, stars, like the Sun, and planets appeared, and elements such as hydrogen (H), oxygen (O), nitrogen (N), and carbon (C) combined into simple molecules such as water (H2O), ammonia (NH3), and methane (CH4). The larger hydrocarbons, alcohols, aldehydes, acids, and amino acids were ultimately built as a result of the action (1) of far-ultraviolet light (wavelength less than 185 nanometres) before oxygen appeared in the atmosphere, (2) of penetrating alpha, beta, and gamma radiations, and (3) of electric discharges from lightning storms when the temperature dropped and water began to condense. These simple compounds interacted and eventually developed into living matter. To what degree—if at all—the radiations from radioactive decay contributed to the synthesis of living matter is not known, but the occurrence of high-energy-irradiation effects on matter at very early times in the history of this world is recorded in certain micas as microscopic, concentric rings, called pleochroic (pleochroism) halos, produced as the result of the decay of tiny specks of radioactive material that emitted penetrating products, such as alpha particles. At the termini of their paths, particles of this kind produced chemical changes, which can be seen microscopically as dark rings. From the diameters of the rings and the known penetrating powers of alpha particles from various radioactive elements, the nature of the specks of radioactive matter can be established. In some cases, alpha particles could not have been responsible for the effects observed; in other cases, the elementary specks that occupied the centres of the halos were not those of any presently known elements.

      It can be readily surmised that some of the elements that participated in the evolution of the world were not originally present but were produced as the result of external high-energy bombardment, that some disappeared as the result of such processes, and that many compounds required for the living processes of organisms evolved as a consequence of the high-energy irradiation to which all matter is subjected. Hence, radiation is believed to have played a major role in the evolution of the universe and is ultimately responsible not only for the existence of life but also for the variety of its forms.

Fundamental processes involved in the interaction of radiation with matter

The passage of electromagnetic (electromagnetic field) rays
The field concept
      A discussion of this subject requires preliminary definition of a few of the more common terms. Around every particle, whether it be at rest or in motion, whether it be charged or uncharged, there are potential fields of various kinds. As one example, a gravitational field exists around the Earth and indeed around every particle of mass that moves with it. At every point in space, the field has direction in respect to the particle. The strength of the gravitational field around a specific particle of mass, m, at any distance, r, is given by the product of g, the universal gravitational constant, and m divided by the square of r, or gm/r2. The field extends indefinitely in space, moves with the particle when it moves, and is propagated to any observer with the velocity of light. Newton showed that the mass of a homogeneous spherical object can be assumed to be concentrated at its centre and that all distances can be measured from it. Similarly, electric fields exist around electric charges and move with them. Magnetic fields (magnetic field) exist around electric charges in motion and change in intensity with all changes in the accompanying electric field, with the magnetic field at any point being perpendicular to the electric field in free space. Any regular oscillation is time-dependent, as is any change in field strength with time.

      Time-dependent electric and magnetic fields occur jointly; together they propagate as what are called electromagnetic waves (electromagnetic radiation). In an assumed ideal free space (without intrusion from other fields or forces of any kind, devoid of matter, and, thus, in effect without any intrusions, demarcations, or boundaries), such waves propagate with the speed of light in the so-called transverse electromagnetic mode—one in which the directions of the electric field, the magnetic field, and the propagation of the wave are mutually perpendicular. They constitute a right-handed coordinate system; i.e., with the thumb and first two fingers of the right hand perpendicular to each other, the thumb points in the direction of the electric field, the forefinger in that of the magnetic field, and the middle finger in that of propagation. A boundary may be put on the space by appropriate physical means (bound space), or the medium may be something other than a vacuum (material medium). In either case, other forces and other fields come into the picture, and propagation of the wave is no longer exclusively in the transverse electromagnetic mode. Either the electric field or the magnetic field (a matter of arbitrary choice) may be considered to have a component parallel to the direction of propagation itself. It is this parallel component that is responsible for attenuation of energy of the waves as they propagate.

frequency range
 Electromagnetic waves span an enormous range of frequencies (number of oscillations per second), only a small part of which fall in the visible region. Indeed, it is doubtful that lower or upper limits of frequency exist, except in regard to the applicability of present-day instrumentation. Figure 2—> indicates the usual terminology employed for electromagnetic waves of different frequency or wavelength. Customarily, scientists designate electromagnetic waves by fields, waves, and particles in increasing order of the frequency ranges to which they belong. Traditional demarcations into fields, waves, and particles (e.g., gamma-ray photons) are shown in the figure. The distinctions are largely of classical (i.e., nonquantum) origin; in quantum theory there is no need for such distinctions. They are preserved, however, for common usage. The term field is used in a situation in which the wavelength of the electromagnetic waves is larger than the physical size of the experimental setup. For wave designation, the wavelength is comparable to or smaller than the physical extent of the setup, and at the same time the energy of the photon is low. The particle description is useful when wavelength is small and photon energy is high.

Properties of light
      The ordinary properties of light, such as straight-line propagation, reflection and refraction (bending) at a boundary or interface between two mediums, and image formation by mirrors or lenses, can be understood by simply knowing how light propagates, without inquiring into its nature. This area of study essentially is geometrical optics. On the other hand, the extraordinary properties of light do require answers to questions regarding its nature (physical optics). Thus, interference, diffraction, and polarization relate to the wave aspect, while photoelectric effect, Compton scattering, and pair production relate to the particle aspect of light. As noted above, light has dual character. It is the duality in the nature of light, as well as that of matter, that led to quantum theory.

Wave aspects of light
      In general, radiation interacts with matter; it does not simply act on nor is it merely acted upon. Understanding of what radiation does to matter requires also an appreciation of what matter does to radiation.

      When a ray of light is incident upon a plane surface separating two mediums (e.g., air and glass), it is partly reflected (thrown back into the original medium) and partly refracted (transmitted into the other medium). The laws of reflection and refraction state that all the rays (incident, reflected, and refracted) and the normal (a perpendicular line) to the surface lie in the same plane, called the plane of incidence. Angles of incidence and reflection are equal; for any two mediums the sines of the angles of incidence and refraction have a constant ratio, called the mutual refractive index. All these relations can be derived from the electromagnetic theory of Maxwell (Maxwell's equations), which constitutes the most important wave theory of light. The electromagnetic theory, however, is not necessary to demonstrate these laws.

      In double refraction, light enters a crystal the optical properties of which differ along two or more of the crystal axes. What is observed depends on the angle of the beam with respect to the entrant face. Double refraction was first observed in 1669 by Erasmus Bartholin (Bartholin, Erasmus) in experiments with Iceland spar crystal (calcite) and elucidated in 1690 by Huygens.

      If a beam of light is made to enter an Iceland spar crystal at right angles to a face, it persists in the crystal as a single beam perpendicular to the face and emerges as a single beam through an opposite parallel face. If the exit face is at an angle not perpendicular to the beam, however, the emergent beam is split into two beams at different angles, called the ordinary and extraordinary rays, and they are usually of different intensities. Clearly, any beam that enters an Iceland spar crystal perpendicular to its face and emerges perpendicular to another face is of changed character—although superficially it may not appear to be changed. Dependent on the relative intensities and the phase relationship of its electric components (i.e., their phase shift), the beam is described as either elliptically or circularly polarized (polarization). There are other ways of producing partially polarized, plane-polarized, and elliptically (as well as circularly) polarized light, but these examples illustrate the phenomena adequately.

      Polarization of an electromagnetic wave can be shown mathematically to relate to the space-time relationship of the electromagnetic-field vector (conventionally taken as the electric vector, a quantity representing the magnitude and direction of the electric field) as the wave travels. If the field vector maintains a fixed direction, the wave is said to be plane-polarized, the plane of polarization being the one that contains the propagation direction and the electric vector. In the case of elliptic polarization, the field vector generates an ellipse in a plane perpendicular to the propagation direction as the wave proceeds. Circular polarization is a special case of elliptic polarization in which the so-described ellipse degenerates into a circle.

      An easy way to produce circularly polarized light is by passage of the light perpendicularly through a thin crystal, as, for example, mica. The mica sample is so selected that the path difference for the ordinary and the extraordinary rays is one-quarter the wavelength of the single-wavelength, or monochromatic, light used. Such a crystal is called a quarter-wave plate, and the reality of the circular polarization is shown by the fact that, when the quarter-wave plate is suitably suspended and irradiated, a small torque—that is, twisting force—can be shown to be exerted on it. Thus, the action of the crystal on the light wave is to polarize it; the related action of the light on the crystal is to produce a torque about its axis.

      The ratio of the intensity of the reflected light to that of the incident light is called the reflection coefficient (albedo). This quantitative measure of reflection depends on the angles of incidence and refraction, or the refractive index, and also on the nature of polarization.

      It can be shown that the reflection coefficient at any angle of incidence is greater for polarization perpendicular to the plane of incidence than for polarization in the plane of incidence. As a result, if unpolarized light is incident at a plane surface separating two media, reflected light will be partially polarized perpendicular to the plane of incidence, and refracted light will be partially polarized in the plane of incidence. An exceptional case is the Brewster angle (Brewster's law), which is such that the sum of the angles of incidence and refraction is 90°. When that happens, the reflection coefficient for polarization in the plane of incidence equals zero. Thus, at the Brewster angle, the reflected light is wholly polarized perpendicular to the plane of incidence. At an air-glass interface, the Brewster angle is approximately 56°, for which the reflection coefficient for perpendicular polarization is 14 percent. Another extremely important angle for refraction is the critical angle of incidence when light passes from a denser medium to a rarer one. It is that angle for which the angle of refraction is 90° (in this case the angle of refraction is greater than the angle of incidence). For angles of incidence greater than the critical angle there is no refracted ray; the light is totally reflected internally. For a glass-to-air interface the critical angle has a value 41°48′.

 The variation of the refractive index with frequency is called dispersion. It is this property of a prism that effects the colour separation, or dispersion, of white light. An equation that connects the refractive index with frequency is called a dispersion relation. For visible light the index of refraction increases slightly with frequency, a phenomenon termed normal dispersion. The degree of refraction depends on the refractive index. The increased bending of violet light over red by a glass prism is therefore the result of normal dispersion. If experiments are done, however, with light having a frequency close to the natural electron frequency, some strange effects appear. When the radiation frequency is slightly greater, for example, the index of refraction becomes less than unity (<1) and decreases with increasing frequency; the latter phenomenon is called anomalous dispersion. A refractive index less than unity refers correctly to the fact that the speed of light in the medium at that frequency is greater than the speed of light in vacuum. The velocity referred to, however, is the phase velocity or the velocity with which the sine-wave (wave velocity) peaks are propagated. The propagation velocity of an actual signal or the group velocity is always less than the speed of light in vacuum. Therefore, relativity theory is not violated. An example is shown in Figure 3—>, in which a light source is initially pointed in the direction A. The source rotates in such a way that the velocity of the light image moves from D to E with a velocity v approximating c. Thus, the phase velocity with which the image moves from A to B is greater than c, but the relativity principle is not violated because the velocity of transmission of matter or energy does not exceed the velocity of light.

Electromagnetic waves and atomic structure

Quantum concepts
    quantum mechanics includes such concepts as “allowed states”—i.e., stationary states of energy content exactly stipulated by its laws. The energy states shown in Figure 1—> are of that kind. A transition between such states depends not only on the availability (e.g., as radiation) of the precise amount of energy required but also on the quantum-mechanical probability of such a transition. That probability, the oscillator strength, involves so-called selection rules (selection rule) that, in general terms, state the degree to which a transition between two states (which are described in quantum-mechanical terms) is allowed. As an illustration of allowed transition in Figure 1—>, the only electronic transitions permitted are those in which the change in vibrational quantum number accompanying a change in electronic excitation is plus or minus one or zero, except that a 0 ↔ 0 (zero-to-zero) change is not permitted. All electronic states include vibrational and rotational levels, so that the probability of a specific electronic transition includes the probabilities of transition between all the vibrational and rotational states that can conceivably be involved. Figure 1—> is, of course, a simplified picture of a compendium of energy states available to a molecule (polyatomic structure)—and the selection rules are accordingly more involved in such a case. The selection rules are worked out by scientists in a process of discovery; the attempt is to state them systematically so that the applicable rules in an experimentally unstudied case may be stated on the basis of general principle.

absorption and emission
      In transit through matter, the intensity of light decreases exponentially with distance; in effect, the fractional loss is the same for equal distances of penetration. The energy loss from the light appears as energy added to the medium, or what is known as absorption. A medium can be weakly absorbing at one region of the electromagnetic spectrum and strongly absorbing at another. If a medium is weakly absorbing, its dispersion and absorption can be measured directly from the intensity of refracted or transmitted light. If it is strongly absorbing, on the other hand, the light does not survive even a few wavelengths of penetration. The refracted or transmitted light is then so weak that measurements are at best difficult. The absorption and dispersion in such cases, nevertheless, may still be determined by studying the reflected light only. This procedure is possible because the intensity of the reflected light has a refractive index that separates mathematically into contributions from dispersion and absorption. In the far ultraviolet it is the only practical means of studying absorption, a study that has revealed valuable information about electronic energy levels and collective energy losses (see below Molecular activation (radiation)) in condensed material.

      Experimental studies of the chemical effects of radiation on matter can be greatly forwarded by the use of beams of high intensity and very short duration. Such studies are made possible by employment of the laser, a light source developed by the American physicists Arthur L. Schawlow (Schawlow, Arthur L.) and Charles H. Townes (Townes, Charles Hard) (1958) from the application of one of the Einstein (Einstein, Albert) equations. Einstein suggested (on the basis of a principle of detailed balancing, or microscopic reversibility (microscopic reversibility, principle of)) that, just as the amount of light absorbed by a molecular system in a light field must depend on the intensity of the light, the amount of light emitted from excited states of the same system must also exhibit such dependency. In this fundamentally important idea of microscopic reversibility can be seen one of the most dramatic illustrations of the physical effects of radiation.

      Under any circumstance, the absorption probability in the ground state is given by the number of molecules (or atoms), Ni, in that state multiplied both by the probability, Bij, for transition from state i to state j and by the light intensity, I(ν), at frequency symbolized by the Greek letter nu, ν; i.e., Ni Bij I(ν). Light emission from an excited (excitation) state to the ground state depends on the number of molecules (or atoms) in the upper state, Nj, multiplied by the probability of spontaneous emission, Aji, to the ground state plus the additional induced emission (stimulated emission) term, Nj Bji I(ν), in which Bji is a term that Einstein showed to be equal to Bij and that relates the probability of such induced emission, so that in the general case in any steady-state situation (in which light absorption and emission are occurring at equal rates):

      There is a well-developed theoretical relationship (not here presented) of a quantum-mechanical nature between Aji and Bij. Ordinarily, the light intensity, I(ν), is so low that the second term on the right can be neglected. At sufficiently high light intensities, however, that term can become important. In fact, if the light intensity is high, as in a laser, the induced-emission probability can easily exceed that of spontaneous emission.

      Spontaneous emission of light is random in direction and phase. Induced emission has the same direction of polarization and propagation as that of the incident light. If by some means a greater population is created in the upper level than in the lower one, then, under the stimulus of an incident light of appropriate frequency, the light intensity actually increases with path length provided that there is enough stimulated emission to compensate for absorption and scattering. Such stimulated emission is the basis of laser light. Practical lasers such as the ruby or the helium-neon lasers work, however, on a three-level principle.

Particle aspects of light
      The energy required to remove an orbital electron from an atom (or molecule) is called its binding energy in a given state. When light of photon energy greater than the minimum binding energy is incident upon an atom or solid, part or all of its energy may be transformed through the photoelectric effect, the Compton effect, or pair production—in increasing order of importance with increase of photon energy. In the Compton effect, the photon is scattered from an electron, resulting in a longer wavelength, thus imparting the residual energy to the electron. In the other two cases the photon is completely absorbed or destroyed. In the pair-production (pair production) phenomenon, an electron–positron pair is created from the photon as it passes close to an atomic nucleus. A minimum energy (1,020,000 electron volts [eV]) is required for this process because the energy of the electron–positron pair at rest—the total mass, 2m, times the velocity of light squared (2mc2)—must be provided. If the photon energy (hν) is greater than the rest mass, the difference (hν - 2mc2), called the residual energy, is distributed between the kinetic energies of the pair with only a small fraction going to the nuclear recoil.

      The photoelectric effect is caused by the absorption of electromagnetic radiation and consists of electron ejection from a solid (or liquid) surface, usually of a metal, though nonmetals have also been studied. In the case of a gas, the term photoionization (photo-ionization) is more common, though there is basically little difference between these processes. In spite of experimental difficulties connected with surface-adsorbed gas and energy loss of ejected electrons in penetrating a layer of the solid into vacuum, early experimenters established two important features about the photoelectric effect. These are: (1) although the photoelectric current (i.e., the number of photoelectrons) is proportional to the incident-light intensity, the energy of the individual photoelectrons is independent of light intensity; and (2) the maximum energy of the ejected electron is roughly proportional to the frequency of light. These observations cannot be explained in terms of wave theory. Einstein argued that the light is absorbed in quanta of energy equal to Planck's constant (h) times light frequency, hν, by electrons, one at a time. A minimum energy symbolized by the Greek letter psi, ψ, called the photoelectric work function of the surface, must be supplied before the electron can be ejected. When a quantum of energy is greater than the work function, photoelectric emission is possible with the maximum energy symbolized by the Greek letter epsilon, ε, of the photoelectron (εmax) being stated by Einstein's photoelectric equation as equaling the difference between the photon energy and the work function; i.e., εmax = hν - ψ. Einstein's interpretation gave strong support for the quantum theory of radiation. Early experiments determined Planck's constant, h, independently through the above equation and also established the fact that an immeasurably small time delay is involved between absorption of a quantum of light and the ejection of an electron. The latter is clearly indicative of particle-like interaction.

      Accurate and reliable values of the work function and ejection energy are now available for most solids; the chief obstacles to the development of such data were the difficulty of preparing clean surfaces and the energy loss of electrons in penetration into vacuum. The photoelectric threshold frequency, symbolized by the Greek letter nu with subscript zero, ν0, is that frequency at which the effect is barely possible; it is given by the ratio of the work function symbolized by the Greek letter psi, ψ, to Planck's constant (ν0 = ψ/h). The photoelectric yield, defined as the ratio of the number of photoelectrons to that of incident photons, serves as a measure of the efficiency of the process. Photoelectric yield starts from a zero value at threshold, reaches a maximum value (about 1/1,000) at about twice the threshold frequency, and falls again when frequency is further increased. Some unusual alloys exhibit yields up to 100 times greater than normal (i.e., about 0.1). Normally the yield depends also on polarization and angle of incidence of the radiation. Parallel polarization (polarization in the plane of incidence) gives higher yield than does perpendicular polarization, in some instances by almost 10 times.

cross section and Compton scattering (Compton effect)
      A useful concept in describing the absorption of radiation in matter is called cross section; it is a measure of the probability that photons interact with matter by a particular process. When the energy of each individual photon (hν) is much smaller than the rest energy of the electron (its mass times the velocity of light squared [mc2]), the scattering of photons is described by a cross section derived by J.J. Thomson. This cross section is called the Thomson cross section, symbolized by the Greek letter sigma with subscript zero, σ0, and is equal to a numerical factor times the square of the term, electric charge squared divided by electron rest energy, or σ0 = (8π/3) (e2/mc2)2. When the photon energy is equal to or greater than the electron's rest energy of (hν ⋜ mc2), inelastic (i.e., energy loss) scatterings begin to appear. One such is Compton scattering, in which an X ray or gamma ray (electromagnetic radiation from an atomic nucleus) experiences an increase in wavelength (reduction in energy) after being scattered through an angle. Arthur Holly Compton (Compton, Arthur Holly), an American physicist, correctly interpreted the effect by using the laws of classical relativistic mechanics. He showed that the increase in wavelength symbolized by the Greek letters delta and lambda, Δλ, is independent of the energy of the photon and is given by an expression in which the product of two terms appears. The first is a universal constant symbolized by the Greek letter lambda with subscript zero, λ0, generally called the Compton wavelength, and itself equal to Planck's constant, h, divided by the mass of the electron at rest and the velocity of light; i.e., λ0 = h/mc = 2.4 × 10-10 centimetre. The second is a term dependent on the angle symbolized by the Greek letter theta, θ, through which the photon is scattered; it is one minus the cosine of that angle, or 1 - cos θ. The increase in wavelength observed at that angle is simply Δλ = λ0(1 - cos θ). In discussing the Compton effect, the electron is treated as free—that is, not bound to a nucleus—because, in the study of that effect for most materials of low atomic number, the incident photon has energy much greater than the binding energy. For bound electrons, the corrections to the Compton relation are small but complicated. When photons are scattered, the concept of differential cross sections may be used; differential cross section is a measure of the probability that a photon will be scattered within a given small angle.

      The differential cross section for the Compton process was derived by the Swedish physicist Oskar Klein and the Japanese physicist Yoshio Nishina. The Klein–Nishina formula shows almost symmetrical scattering for low-energy photons about 90° to the beam direction. As the photon energy increases, the scattering becomes predominantly peaked in the forward direction, and, for photons with energies that are greater than five times the rest energy of the electron, almost the entire scattering is confined within an angle of 30°. When averaged over the angle, the Klein–Nishina cross section shows variation with the incident photon energy. At low energy this cross section increases uniformly and approaches the classical Thomson value as energy is decreased; at high energy the cross section is inversely proportional to the energy. The energy distribution of Compton electrons (recoil or scattered electrons) and outgoing photons may also be derived from the Klein–Nishina theory. The result shows a wide distribution; for atoms of low atomic number and incident photon energies in the region of importance (i.e., 1,000,000 to 100,000,000 eV), the probability of scattering per unit energy interval is fairly constant—except that, for the case of nearly total conversion of the photon energy into electron kinetic energy, a plot of energy versus angle shows a sharp, narrow peak. Thus, as a crude approximation, the average energy of a Compton electron is about half the incident photon energy.

      Compton scattering plays a key role in the interaction of matter with intermediate-energy gamma rays and high-energy X rays. For these radiations, it is almost the exclusive mechanism by which energy is transferred from the radiation and added to the matter. An example may be cited of the penetration of gamma rays (gamma ray) from the radioactive substance cobalt-60 into a sample of water or aqueous solution. The electron density is approximately 3 × 1023 per millilitre. Taking the Compton cross section as approximately 3 × 10-25 square centimetre per electron, calculation yields a mean free path for Compton scattering of about 10 centimetres—that is to say, a photon will move about 10 centimetres between successive encounters with electrons. The dominant radiation effect produced by a gamma ray therefore is attributable to the recoil electron and the vast number of progeny (such as secondary and tertiary electrons) that are produced. These higher generation electrons are produced through electron-impact ionization (an electron is removed from an atom by the collision of another electron), a process that continues until barred either by energetic limitation or by low cross section. For cobalt-60 gamma rays the average Compton energy in a material of low atomic number, such as water, is approximately 600,000 eV.

      Pair production is a process in which a gamma ray of sufficient energy is converted into an electron and a positron. A fundamental law of mechanics, given by Newton, is that in any process total linear (as well as angular) momentum remains unchanged. In the pair-production process a third body is required for momentum conservation. When that body is a heavy nucleus, it takes very little recoil energy, and therefore the threshold is just twice the rest energy of the electron; i.e., twice its mass, m, times the square of the velocity of light, c2, or 2mc2. Pair production also can occur in the field of an atomic electron, to which considerable recoil energy is thereby imparted. The threshold for such a process is four times the rest energy of an electron, or 4mc2. The total pair-production cross section is the sum of the two components, nuclear and electronic. These cross sections depend on the energy of the gamma ray and are usually calculated in an electron theory proposed by the British physicist P.A.M. Dirac through a method of approximation that is a simplification of a method (a “first approximation”) devised by the German physicist Max Born (Born, Max) (i.e., a “first Born approximation”). The process is envisaged by Dirac as the transition of an electron from a negative to a positive energy state. Corrections are required for these cross sections at high energy, at high atomic number, and for atomic screening (the intrusion of the field of the electrons in an atom); these are normally made via numerical procedures. The fraction of residual energy, symbolized by the Greek letter alpha, unexpended in conversion of energy to mass, that appears in any one particle (e.g., the electron) is thus given by the kinetic energy of that electron Ee minus its rest energy mc2 divided by the energy of the gamma ray hν (i.e., the product of Planck's constant and the frequency of the gamma ray) minus twice the rest energy of the electron 2mc2, or α = (Ee -mc2)/(hν - 2mc2). Because the same equation applies to each of the two electrons that are formed, it must be symmetric about the condition that each of the particles has half the residual energy, symbolized by the Greek letter alpha, α (in excess of that conveyed to the “third body”); i.e., that α = 0.5. Below an energy of about 10,000,000 eV for the gamma ray, the probability for pair production (i.e., the pair-production cross section) is almost independent of the atomic number of the material, and, up to about 100,000,000 eV of energy, it is also almost independent of the quantity α. Even at extremely high energies the probability that a certain fraction of the total available energy will appear in one particle is almost independent of the fraction as long as energy is comparably distributed between the two particles (excepting in cases in which almost all energy is dumped into one particle alone). Typical pair-production cross sections at 100 MeV (million electron volts) are approximately 10-24 to 10-22 square centimetre, increasing with atomic number. At high energies, approximately equal to or greater than 100 MeV, pair production is the dominant mechanism of radiation interaction with matter.

      Clearly, as the photon energy increases, the dominant interaction mechanism shifts from photoelectric effect to Compton scattering to pair production. Rarely do photoelectric effect and pair production compete at a given energy. Compton scattering, however, at relatively low energy competes with the photoelectric effect and at high energy competes with pair production. Thus, in lead, interaction below 0.1 MeV is almost exclusively photoelectric; between 0.1 MeV and 2.5 MeV both photoelectric and Compton processes occur; and between 2.5 MeV and 100 MeV Compton scattering and pair production share the interaction. In the pair process the photon is annihilated, and an electron–positron pair is created. On the other hand, an electron or positron with energy approximately equal to or greater than 100 MeV loses its energy almost exclusively by production of high-energy bremsstrahlung (X rays produced by decelerating electric charges) as the result of interaction with the field of a nucleus. The cross section for bremsstrahlung production is nearly independent of energy at high energies, whereas at low energies the dominant energy-loss mechanism is by the creation of ionizations and excitations. A succession of bremsstrahlung and pair-production processes generates a cascade or shower in the absorber substance. This phenomenon can be triggered by an electron, a positron, or a photon, the triggering mechanism being unimportant as long as the starting energy is high. A photon generates a pair through pair production, and the charged particles generate photons through bremsstrahlung, and so on repeatedly as long as the energy is kept sufficiently high. With penetration into the substance, the shower increases in size at first, reaches a maximum, and then gradually decreases. Loss of particles by degradation to lower energies (in which the yield of bremsstrahlung is low), ionization loss, and production and absorption of low-energy photons eventually reduce the size of the cascade. The mathematical theory of cascades has been developed in great detail.

X rays (X-ray) and gamma rays
      When light of sufficiently high frequency (or energy equal to hν), independent of its source, is absorbed in a molecular system, the excited molecular state so produced, or some excited state resultant from it, may either interact with other molecules or decompose to produce intermediate or ultimate products; i.e., chemical reactions ensue. Study of such processes is encompassed in the subject of photochemistry (see also below Molecular activation (radiation)).

 Electromagnetic waves of energy greater than those usually described as ultraviolet light (see Figure 2—>) are included in the classes of X rays or gamma rays. X-ray and gamma-ray photons may be distinguished by definition on the basis of source. They are indistinguishable on the basis of effects when their energy is absorbed in matter.

      The total effect of X-ray or gamma-ray irradiation of matter, in the almost immediate time interval, is the production of high-energy electrons of energy related to that of the incident ray. Such electrons behave like beta rays (electrons emitted from atomic nuclei) or electrons from a machine source of the same energy. They lose energy by excitation and ionization of atoms and molecules of the systems they traverse. The amount of energy such an electron gives to an atom or molecule tends to exceed that deposited in photochemical processes, and the variety of initial physical (and consequent chemical) effects is more numerous and diverse. The situation is further complicated by the fact that the secondary electrons produced in ionization processes in which the input of energy is high may themselves initiate other ionization and excitation processes that can yield further chemistry, the totality of which is embraced in the title of radiation chemistry (see below Molecular activation (radiation) and Ionization and chemical change (radiation)).

The passage of matter rays (de Broglie wave)
Heavy charged particles
      Charged particles, such as atomic or molecular ions or molecular fragments, that travel in a material medium deposit energy along their paths, or tracks. If the medium is sufficiently thick, the velocity of the charged particle is reduced to near zero so that its energy is all but totally absorbed and is totally utilized in producing physical, chemical, and, in viable (living) matter, biologic changes. If the sample is sufficiently thin, the particle may ultimately emerge, but with reduced energy.

Linear energy transfer and track structure
      The stopping power of a medium toward a charged particle refers to the energy loss of the particle per unit path length in the medium. It is specified by the differential -dE/dx, in which -dE represents the energy loss and dx represents the increment of path length. What is of interest to the radiation scientist is the spatial distribution of energy deposition in the particle track. In approximate terms, it is customary to refer to linear energy transfer (LET), the energy actually deposited per unit distance along the track (i.e., -dE/dx). For not-so-fast particles, stopping power and LET are numerically equal; this situation covers all heavy particles studied so far in chemistry and biology but not electrons. In a refined study and redefinition of LET or restricted linear collision stopping power, a quantity symbolized by the letter L with subscript Greek letter delta, LΔ, is defined as equal to the fractional energy lost (-dE) per unit distance traversed along the track (dl), or LΔ = -(dE/dl)Δ, in which the subscript delta (Δ) indicates that only collisions with energy transfer less than an amount Δ are included. The quantity LΔ may be expressed in any convenient unit of energy per unit length. For Δ equal to 100 eV, even the most energetic secondary electrons (i.e., electrons ejected by the penetrating particle) produce on average only about three subsequent ionizations. The latter, however, are closely spaced because of the low energy of the electron, and hence the corresponding energy density is high. It is higher yet for lower-energy secondary electrons. In contrast, for Δ much in excess of 100 eV, more subsequent ionizations are produced, but their spacing is increased significantly and the corresponding density of energy deposition is low. Since only the region of high energy density is of concern for many applications, the quantity L100 is often used to characterize LET.

      The bulk of energy deposition resulting from the passage of a fast-moving, charged particle is concentrated in the “infratrack,” a very narrow region extending typically on the order of 10 interatomic distances perpendicular to the particle trajectory. The extent of the infratrack is dependent on the velocity of the particle, and it is defined as the distance over which the electric field of the particle is sufficiently strong and varies rapidly enough to produce electronic excitation. Inside the infratrack, electrons of the medium are attracted toward the trajectory of a positively charged particle. Many cross the trajectory, depositing energy on both sides. Consequently, the infratrack is characterized by an exceedingly high density of energy deposition and plays a vital role in determining the effects of ionizing radiation on the medium. (The magnitude of energy deposition in the infratrack is further increased by the preponderance of collective [plasma] excitations in that region.) The concept of the infratrack was developed by the American physicists Werner Brandt and Rufus H. Ritchie and independently by Myron Luntz. The region outside the infratrack is beyond the direct influence of the penetrating particle. Energy deposition in this outer region, or “ultratrack,” is due primarily to electronic excitation and ionization by secondary electrons having sufficient energy to escape from the infratrack. In contrast to the infratrack, the ultratrack does not have well-defined physical bounds. Its spatial extent may reasonably be equated with the maximum range of secondary electrons transverse to the particle trajectory.

      For practical purposes, LET is associated with the main track, which may be thought of as including the infratrack and a portion of the ultratrack out to which energy density is still relatively high—i.e., the region over which excitation is caused by secondary electrons of initial energy less than some value Δ, say 100 eV. Energy deposited in “blobs” or “short tracks” to the side of the main track, as described in the Mozumder–Magee theory of track effects (named for Asokendu Mozumder, an Indian-born physicist, and John L. Magee, an American chemist) is purposefully excluded. LET, so defined, characterizes energy deposition within a limited volume—i.e., energy locally deposited about the particle trajectory.

Stopping power
      By use of classical mechanics, Bohr (Bohr, Niels) developed an equation of stopping power, -dE/dx, given as the product of a kinematic factor and a stopping number.

 The kinematic factor includes such terms as the electronic charge and mass, the number of atoms per cubic centimetre of the medium, and the velocity of the incident charged particle. The stopping number includes the atomic number and the natural logarithm of a term that includes the velocity of the incident particle as well as its charge, a typical transition energy in the system (see Figure 1—>; a crude estimate is adequate because the quantity appears within the logarithm), and Planck's constant, h. Bohr's stopping-power formula does not require knowledge of the details of atomic binding. In terms of the stopping number, B, the full expression for stopping power is given by -dE/dx = (4πZ12e4N/mv2)B, where Z1 is the atomic number of the penetrating particle and N is the atomic density of the medium (in atoms/volume).

      For a heavy incident charged particle in the nonrelativistic range (e.g., an alpha particle, a helium nucleus with two positive charges), the stopping number B, according to the German-born American physicist Hans Bethe (Bethe, Hans), is given by quantum mechanics as equal to the atomic number (Z) of the absorbing medium times the natural logarithm (ln) of two times the electronic mass times the velocity squared of the particle, divided by a mean excitation potential (I) of the atom; i.e., B = Z ln (2mv2/I).

      Bethe's stopping number for a heavy particle may be modified by including corrections for particle speed in the relativistic range (β2 + ln [1 - β2]), in which the Greek letter beta, β, represents the velocity of the particle divided by the velocity of light, and polarization screening (i.e., reduction of interaction force by intervening charges, represented by the symbol δ/2), as well as an atomic-shell correction (represented by the ratio of a constant C to the atomic number of the medium); i.e., B = Z (ln 2mv2/I - β2 - ln[1 - β2] - C/Z - δ/2).

      The most important nontrivial quantity in the equation for stopping number is the mean excitation potential, I. Experimental values of this parameter, or quantity, are known for most atoms, but no single theory gives it over the whole range of atomic numbers because the calculation would require knowledge of the ground states and all excited states. Statistical models of the atom, however, come close to providing a theory. Calculations by the American physicist Felix Bloch (Bloch, Felix) in 1933 showed that the mean excitation potential in electron volts is about 14 times the atomic number of the element through which the charged particle is passing (I = 14Z). A later calculation gives the ratio of the potential to atomic number as equal to a constant (a) plus another constant (b) times the atomic number raised to the -2/3 power in which a = 9.2 and b = 4.5—i.e., I/Z = a + bZ-2/3. This formula is widely applicable. Other exact quantum-mechanical calculations for hydrogen give its mean excitation potential as equal to 15 eV.

      Even though the basic stopping-power theory has been developed for atoms, it is readily applied to molecules by virtue of Bragg's rule (named for the British physicist William H. Bragg), which states that the stopping number of a molecule is the sum of the stopping numbers of all the atoms composing the molecule. For most molecules Bragg's rule applies impressively within a few percent, though hydrogen (H2) and nitrous oxide (NO) are notable exceptions. The rule implies: (1) similarity of atomic binding in different molecules having one common atom or more, and (2) that the vacuum ultraviolet transitions, in which most electronic transitions are concentrated under such irradiation, involve energy losses much higher than the strengths of most chemical bonds.

      The charge on a heavy positive ion fluctuates during penetration of a medium. In the beginning it captures an electron, which it quickly loses. As it slows down, however, the cross section of electron loss decreases relative to that for capture. Basically, the impinging ion undergoes charge-exchange cycles involving a single capture followed by a single loss. Ultimately, an electron is permanently bound when it becomes energetically impossible for the ion to lose it. A second charge-exchange cycle then occurs. This phenomenon continues repeatedly until the velocity of the heavy ion approximates the orbital velocity of the electron in Bohr's theory of the atom, when the ion spends part of its time as singly charged and another part as a neutral atom. The kinematic factor in the expression for stopping power is proportional to the square of the nuclear charge of the penetrating particle, and it is modified to account for electron capture as the particle slows down. On slowing down further, the electronic energy-loss mechanism becomes ineffective, and energy loss by elastic scattering dominates. The mathematical expressions presented here apply strictly in the high-velocity, electronic excitation domain.

      The total path length traversed by a charged particle before it is stopped is called its range. Range is considered to be taken as the sum of the distance traversed over the crooked path (track), whereas the net projection measured along the initial direction of motion is known as the penetration. The difference between range and penetration distances results from scattering encountered by the particle along its path. For heavy charged particles with high initial velocities (those that are appreciable fractions of the speed of light), large-angle scatterings are rare. The corresponding trajectories are straight, and the difference between range and penetration distance is, for most purposes, negligible.

      Particle ranges may be obtained by (numerical) integration of a suitable stopping-power formula. Experimentally, range is more easily measured than is stopping power. For heavy particles a critical incident energy in low-atomic-number mediums is 1,000,000 eV divided by the mass of the particle in atomic mass units (amu)—i.e., 1 MeV/amu. For incident energies higher than this critical value, range is usually well-known, and computation agrees with experiment within about 5 percent. In the case of aluminum, which is the best studied material, the accuracy is within about 0.5 percent. For incident energies less than the critical value, however, range calculations are usually uncertain, and agreement with experiment is poor. The range–energy relation is often given adequately as a power law, that range (R) is proportional to energy (E ) raised to some power (n); that is, REn. Protons in the energy interval of a few hundred MeV conform to this kind of relation quite well with the exponent n equal to 1.75. Similar situations exist for other heavy particles. Measurements of range and stopping power are of great importance in particle identification and measurement of their energies. Many experimental data and computations are available for ranges of heavy particles as well as of electrons. The theory by which Bethe derived a stopping number is generally accepted as providing the framework for understanding the variation of range with energy, though in practice the mean excitation potential, I, must be obtained in many cases by experimental curve fitting.

      Both stopping power and range should be understood as mean (or average) values over an ensemble of atoms or molecules, because energy loss is a statistical phenomenon. Fluctuations are to be expected. In general, these fluctuations are called straggling, and there are several kinds. Most important among them is the range straggling, which suggests that, for statistical reasons, particles in the same medium have varying path lengths between the same initial and final energies. Bohr showed that for long path lengths the range distribution is approximately Gaussian (a type of relationship between number of occurrences and some other variable). For short path lengths, such as those encountered in penetration of thin films, the emergent particles show a kind of energy straggling called Landau type (for the Soviet physicist Lev Landau). This energy straggling means that the distribution of energy losses is asymmetric when a plot is drawn, with a long tail on the high-energy-loss side. The intermediate case is given by a distribution according to Sergey Ivanovich Vavilov, a Soviet physicist, that must be evaluated numerically. There is evidence in support of all three distributions in their respective regions of validity.

      The ionization density (number of ions per unit of path length) produced by a fast charged particle along its track increases as the particle slows down. It eventually reaches a maximum called the Bragg peak close to the end of its trajectory. After that, the ionization density dwindles quickly to insignificance. In fact, the ionization density follows closely the LET. With slowing, the LET at first continues to increase because of the strong velocity denominator in the kinematic factor of the stopping-power formula. At low speeds, however, LET goes through a maximum because of: (1) progressive lowering of charge by electron capture, and (2) the effect of the logarithmic term in the stopping-power formula. In general, the maximum occurs at a few times the Bohr orbital velocity. A curve of ionization density (also called specific ionization or number of ion pairs—negative electron and associated positive ion—formed per unit path length) versus distance in a given medium is called a Bragg curve. The Bragg curve includes straggling within a beam of particles; thus, it differs somewhat from the specific ionization curve for an individual particle in that it has a long tail of low ionization density beyond the mean range. The mean range of radium-C′ alpha particles in air at normal temperature and pressure (NTP), for example, is 7.1 centimetres; the Bragg peak occurs at about 6.3 centimetres from the source with a specific ionization of about 60,000 ion pairs per centimetre.

Electrons (electron)
      In the first Born approximation, inelastic cross section depends only on velocity and the magnitude of the charge on the incident particle. Hence, an electron and a positron at the same velocity should have identical stopping powers, which should be the same as that of a proton at that velocity. In practice, there is some difference in the case of an electron because of the indistinguishability of the incident and atomic electrons. In describing an ionization caused by an incident electron, the more energetic of the two emergent electrons is called, by convention, the primary. Thus, maximum energy loss (ignoring atomic binding) is half the incident energy. Incorporating this effect, the stopping number of an electron is given by a complicated expression that involves a different arrangement of the parameters found in the stopping number of heavy charged particles; i.e.,

      This stopping-power formula has a wide range of validity, from approximately a few hundred electron volts to a few million electron volts in materials of low atomic number. For low velocities, the Born approximation gradually breaks down, and highly excited states begin to be inaccessible to transitions by virtue of small maximum energy transfer. Yet, with some corrections the electron-stopping-power formula may be extended down to about 50 eV. Below that value any stopping-power formula is of doubtful validity, even though it is certain that most of the energy is still being lost to electronic states down to a few eV of energy.

      On the high-velocity side, relativistic effects increase electron-stopping power from about 1,000,000 eV upward. Except for the term δ attributable to polarization screening, the relativistic stopping power tends to infinity as the electron velocity approaches the speed of light (v/c = β → 1). One-half of the stopping power, called the restricted stopping power, is numerically equal to the linear energy transfer and changes smoothly to a constant value, called the Fermi plateau (Fermi level), as the ratio β approaches unity. The other half, called the unrestricted stopping power, increases without limit, but its effect at extreme relativistic velocities (those very near the speed of light) becomes small compared with energy loss by nuclear encounters.

      At extremely high velocities an electron loses a substantial part of its energy by radiative nuclear encounter. Lost energy is carried by energetic X rays (i.e., bremsstrahlung). The ratio of energy loss by nuclear radiative encounter to collisional energy loss (excitation and ionization) is given approximately by the incident electron energy (E) in units of 1,000,000 eV times atomic number (Z) divided by 800; i.e., EZ/800. For a large class of mediums (atomic number equal to or greater than 8; i.e., that for oxygen), the electron stopping is dominated by bremsstrahlung radiation for energies greater than 100 MeV.

      When the speed of a charged particle in a transparent medium (air, water, plastics) is so high that it is greater than the group velocity of light in that medium, then a part of the energy is emitted as Cherenkov radiation, first observed in 1934 by Pavel A. Cherenkov, a Soviet physicist. Such radiation rarely accounts for more than a few percent of the total energy loss. Even so, it is invaluable for purposes of monitoring and spectroscopy. Cherenkov radiation is spread over the entire visible region and into the near ultraviolet and near infrared. The direction of its propagation is confined within a cone, the axis of which is the direction of electron motion.

Energy-transfer mechanism
 At the low-velocity end of its path, an electron continues to excite electronic levels of atoms or molecules until its kinetic energy falls below the lowest (electronically) excited state (see Figure 1—>). After that it loses energy mainly by exciting vibrations in a molecule. Such a mechanism proceeds through the intermediary of temporary negative ion states, for direct momentum-transfer collisions are very inefficient. In a condensed medium (liquid, solid, or glass) very low-energy (less than 1 eV) electrons continue to lose energy by a process called phonon emission and by interaction with other low-frequency intermolecular motions of the medium.

      An electron and a singly charged heavy particle with the same velocity have about equal stopping powers. Because of the small mass of the electron, however, the relative retardation (decrease in velocity per unit path length) is much more for it. This larger retardation for an electron means that, if an electron and a heavy particle start with the same velocity, the electron will have a much smaller range. Electron tracks show much more straggling and scattering compared with that of a heavy particle. The first effect results from the fact that the electron can lose a large fraction of its energy in a single encounter; the second is the result of small mass. A power law may be used to connect range and energy of electrons in a given medium—i.e., the range is proportional to energy raised to a power n; as in the case of a heavy particle, the index n is slightly less than two at high energies. At low energies the relationship is such that the exponent is one or less. Many formulas and tables are available for stopping powers and for ranges of electrons as well as of heavy particles over a wide range of energies.

Neutrons (neutron)
      A neutron is an uncharged particle with the same spin as an electron and with mass slightly greater than a proton mass. In free space it decays into a proton, an electron, and an antineutrino and has a half-life of about 12–13 minutes, which is so large compared with lifetimes of interactions with nuclei that the particle disappears predominantly by such interactions.

      Neutron beams may be produced in a variety of ways. A modern method is to extract a high-intensity beam from a nuclear reactor. A simpler but expensive device is one that employs a mixture of radium and beryllium. The reaction of the alpha (α) particles emitted by the radium with beryllium nuclei produces a copious output of neutrons. The neutron is a major nuclear constituent and is responsible for nuclear binding. A free neutron interacts with nuclei in a variety of ways, depending on its velocity and the nature of the target. Ordinary interactions include scattering (elastic and inelastic), absorption, and capture by nuclei to produce new elements. Unlike the electron, a neutron loses energy significantly through elastic collisions, because its mass is comparable to masses of atoms of low atomic number. (According to the laws of mechanics, in elastic collision, on the average, an object loses half its energy to another object of equal mass.)

      The average fraction of energy transferred from a neutron per collision, symbolized by (Δ E/E)av, is twice the atomic mass number (A) of the struck atom divided by the square of the mass number plus one; i.e.,

      Thus, only 18, 25, 42, 90, and 114 collisions are required to thermalize (reduce the energy of motion to that of the surrounding atoms) a fast neutron in hydrogen, deuterium, helium, beryllium, and carbon, respectively.

      Pure absorption does not result in a new element, even though it is sometimes accompanied by emission of gamma rays. In certain cases of capture, radioactivity follows, often with production of beta (β) particles. In another class of interaction, a heavy charged particle is ejected (such as an α-particle or proton); the resultant nucleus is often but not always radioactive. As an example, the reaction of neutrons on boron to produce alpha particles provides the basis for alpha-particle welding. The principle of such welding, invented by the Soviet chemist V.I. Goldansky, is to deposit a thin layer of a boron (or lithium) compound in the interface between diverse materials, which is thereafter irradiated with neutrons. The high-energy α-particles produced from the nuclear reaction weld the materials together.

      Extraordinary interactions of the neutron are represented by diffraction, nuclear fission, and nuclear fusion. Diffraction, exhibited by low-energy neutrons (approximately equal to or less than 0.05 eV), demonstrates their wave nature and is consistent with de Broglie's hypothesis of the wave character of matter. Neutron diffraction complements X-ray technique in locating the positions of atoms in molecules and crystals, especially atoms of low atomic number such as hydrogen. Fission (nuclear fission) is the breakup of a heavy nucleus (either spontaneously or under the impact, for example, of a neutron) into two smaller ones with liberation of energy and neutrons. Spontaneous-fission rates and cross sections of fission induced by agencies other than the neutron are so small that in most applications only neutron-induced fission is important. Also, the neutron-induced-fission cross section depends on the particular isotope (species of an element with the same atomic number and similar chemical behaviour but different atomic mass) involved and the neutron energy. The fission process itself generates fast neutrons, which, when suitably slowed down by elastic scattering (a process called moderation), are again ready to induce more fission. The ratio of neutrons produced to neutrons absorbed is called the reproduction factor. When that factor exceeds unity, a chain reaction may be started, which is the basis of nuclear-power reactors and other fission devices. The chain is terminated by a combination of adventitious absorption, leakage, and other reactions that do not regenerate a neutron. At the power level at which a reactor operates, the loss rate always balances the generation rate through fission. The Hungarian-born American physicist Eugene P. Wigner (Wigner, Eugene Paul), in the course of consideration of the possible effects of fast neutrons, suggested in 1942 that the process of energy transfer by collision from neutron to atom might result in important physical and chemical changes. The phenomenon, known as the Wigner effect and sometimes as a “knock on” process, was actually discovered in 1943 by the American chemists Milton Burton and T.J. Neubert and found to have profound influences on graphite and other materials.

Secondary effects of radiation

Purely physical effects
      With respect to radiation effects the terms primary and secondary are used in a relative sense; the usage depends on the situation under study. Thus, ionization and excitation may be considered as primary with respect to some physical and chemical effects. For other chemical effects, production of free radicals (molecular fragments) may be considered as primary even though that process requires a much longer time for its accomplishment. Still longer times are involved in biologic processes, in which the end product of an earlier chemical reaction may be considered as primary.

      Generally, an atomic solid (a material consisting of only one atomic species) exhibits little or no permanent chemical change upon irradiation. Important among the atomic solids are such materials as metals and graphite. Production of molecular carbon (C2) or bigger clusters upon irradiation of carbon and graphite may, in a certain marginal sense, be considered a chemical change. Ionization of a condensed atomic medium followed by recombination regenerates the same atom, but its locale may be affected. For a molecular medium the situation is quite different. Excited electronic states are often dissociative for a molecule and yield chemically reactive radicals. Positive ions, similarly produced, can experience a variety of reactions even before neutralization occurs. Such an ion may fragment all by itself, or it may react with a neutral molecule in what is called an ion–molecule reaction. In either case new chemical species are created. These transformed ions and radicals, as well as the electrons, parent ions, and excited states, are capable of reacting with themselves and with molecules of the medium, as well as with a solute (a dissolved substance) that may be present in homogeneous distribution. The end products of the reactions can be, on the one hand, new stable compounds or, on the other, regenerated molecules of the original species, as in the case of water irradiation.

      A variety of purely physical effects have been observed in different substances under irradiation. They may be broadly classified as: (1) structural change in the crystal, sometimes accompanied by change in the structural dimensions, (2) change in static mechanical properties, such as elasticity and hardness, (3) change in dynamic mechanical properties, such as internal friction and strain, and (4) changes in transport properties, such as heat conductivity and electrical resistivity. Such changes are considered below in Tertiary effects of radiation on materials (radiation).

Molecular activation
 A molecule is considered activated when it absorbs energy by interaction with radiation. In this energy-rich state it may undergo a variety of unusual chemical reactions that are normally not available to it in thermal equilibrium. Of special importance is electronic activation—i.e., production of an electronically excited state of the molecule (see Figure 1—>). This state can be reached (1) by direct excitation by photon absorption, (2) by impact of charged particles, either directly or indirectly through charge neutralization, or by excitation transfer from excited positive ions, and (3) by charge transfer in collision with (relatively) slow incident positive ions. Among the variety of ensuing processes is light emission, or luminescence.

      The language of luminescence is clouded by history. Originally, fast luminescence was called fluorescence and slow (i.e., delayed or protracted) luminescence was called phosphorescence. Present scientific practice is to define the terms on the basis of so-called quantum-mechanical selection rules: fluorescence is an allowed transition (e.g., singlet–singlet) and occurs in a typical time of about 10-9 second; phosphorescence is a forbidden transition (e.g., triplet–singlet) and may require 10-6 second or longer.

      In the gas phase (gaseous state), an excited molecule either luminesces, undergoes a process called internal conversion, or undergoes dissociation. Luminescence is the rule for anthracene, whereas for water it is dissociation into hydrogen (H) and hydroxide (OH). As a rule, luminescence processes occur by default—that is to say, only if dissociation is energetically impossible or involves a complicated energy-transfer process or if internal conversion to a nonluminescing state is inefficient.

 Fluorescence usually takes place from the lowest electronically excited state (see Figure 1—>); if higher states are excited they either dissociate or energetically cascade to the lowest excited state by one of several possible internal transition mechanisms before emission occurs. (A notable exception to this rule is afforded by azulene.)

      A similar situation exists for triplet excited molecules. The rate of emission, however, is even slower, for in this case it is forbidden by selection rules. If the triplet excitation energy is insufficient for molecular-bond breakage (dissociation), the molecule may remain in a metastable state (one of apparent, not real, stability) for a long time until it either phosphoresces, undergoes internal conversion, or combines with other triplets. Such a combination produces a highly excited state, which has enough energy for dissociation. Some of the latter excited states are formed as singlets capable of light emission. This discussion relates to the more common, general features. There are also special cases, not discussed, that do not follow the general pattern.

ionization phenomena
 Ionization (see Figure 1—>) is that extreme form of excitation in which an electron is ejected, leaving behind a positive molecular ion. The minimum energy required for this process is called the ionization potential (ionization energy) (IP). The actual energetics are described by the Franck–Condon principle, which simply recognizes that, during the extremely short time of an electronic transition, the nuclear configuration of a molecule experiences no significant change. As a consequence of this principle, in an optical process the ion is almost invariably formed in some kind of excited state by input of energy greater than the IP. Also, because of Franck–Condon restrictions, excitation of an inner electron may result in initial production of nonionized, superexcited molecules (suggested by R.L. Platzman, an American physicist) with energy exceeding the ionization potential. A superexcited molecule is short-lived and usually converts rapidly (in a time as short as 10-14 second) either to neutral products or to an ion plus a free electron with marked excess energy. The ion itself may fragment to give other species with excess kinetic or internal vibrational and rotational energy.

Excitation states
      All the various kinds of excitation that occur in the gas phase may also take place in the condensed states of matter (liquid, glass, or solid), but their relative contributions may be affected. In addition, special activated states are produced for which there is no analogue in the gaseous state. They owe their existence to the collective behaviour of atoms and molecules in close proximity. The more important of them are the exciton state, the polaron state, the charge-transfer (or charge-separated) state, and the plasmon state.

      The exciton state is a cooperative state of molecules in which the excitation energy belongs simultaneously to all.

      In a polaron state an electron belongs to the association of molecules, but its motion is relatively slow so that it carries with it its own polarization field, which is described as “a cloud of virtual phonons.” A solvated electron (an electron associated with a particular molecule or group of molecules) is an example of this.

      The charge-transfer state is an excited state. In a certain sense, electronic excitation involves motion of an electron from a lower orbit to a higher one. Quantum mechanics notes that the electron does not revolve around an atomic nucleus in a precise classical orbit but rather that it occupies an orbital in which it is to be found with maximum probability in the location of the classical orbit. When a molecule in a condensed system is excited, the resulting electronic orbital may overlay one or more adjacent molecules, and, in that sense, the electron belongs to the group because its excitation level does not correspond to the electronic properties of a single, isolated molecule.

      The plasmon state is a highly delocalized state formed collectively through Coulombian (electrostatic) interaction of weakly bound electrons. Energy losses, approximating 10–20 eV in most materials, resulting from formation of plasmon states are seen in the impact of electrons of a few tens of kilovolts energy on thin films. Both metals and nonmetals, including plastics, show plasma energy losses. The lost energy may reappear in the form of ultraviolet or visible radiation (Ferrell radiation, 1960); no chemical effect is known to have occurred from such losses.

Energy transfer

Fluorescence and phosphorescence
      In general, a small, simple molecule luminesces in the ultraviolet, and a more complex one emits near the blue-violet end of the visible spectrum. Dye molecules, on the other hand, may emit throughout the visible region, including the red end. The ground electronic state of most molecules is a singlet state. Usually, therefore, the optically allowed emission, or fluorescence, is from the lowest excited singlet state to the ground state. The lowest triplet state of the molecule lies somewhat below the excited singlet. Light emission from this triplet state is forbidden by the quantum-mechanical selection rules, but it does occur by default when other processes are even less probable. Such emission is called phosphorescence. It is relatively weak, slow, and shifted toward longer wavelength. Triplet states may be produced from higher singlets by processes called internal conversion and intersystem crossing. The states may also be produced in excitation from the ground state by impact of relatively slow charged particles, such as electrons.

      Much of the effect of optical radiation in a condensed system is not on the molecule in which the energy is initially absorbed but on a more remote molecule to which the energy is transferred in a variety of possible processes. They include excitation transfer either directly between adjacent molecules, by a direct quantum-mechanical interaction of an excited molecule with a remote one at a distance of 40 angstroms (4 × 10-7 centimetre) or less, or by the so-called trivial process of fluorescence emission from one molecule and reabsorption by one at any distance. These processes are studied mostly in regard to fluorescence and phosphorescence phenomena.

      With high-energy radiation (such as that of electrons, X rays, and gamma rays), an additional mechanism involving ions is also available. In the case of a solute M in a solvent S, for example, a simplified description of some possible effects of radiation is represented by the following expressions, in which the symbol  is read, “is acted upon by high-energy radiation to give” and e represents an ejected electron:

      Any actual process is considerably more complicated and involves a larger number of species.

      One of the most important effects of radiation on matter is seen in photographic action. Apart from its various uses in art, commerce, and industry, photography is an invaluable scientific tool. It is used extensively in spectroscopy, in photometry, and in X-ray examinations. Also, photographic emulsion techniques have been widely used in the detection and characterization of high-energy charged particles. It is important to note that all speculation regarding the primary phenomena involves the notion that, in an energy absorption process, either direct or sensitized, a chloride (or other halide) ion in a silver halide lattice loses (photo-ionization) an electron. That electron is thereafter captured by a silver ion located at such a point in the lattice that under suitable conditions of exposure and development a silver grain grows to a size representative of the duration and intensity of the light exposure.

ionization and chemical change
      Earlier in this section, the ionization phenomenon was briefly discussed as a special case of molecular activation. The ionization process, however, does have certain characteristic features. Most notably, the probabilities (or cross sections) for ionization by light (photoionization) and for ionization by charged-particle impact are different in magnitude and in lowest—radiation—energy of occurrence (i.e., threshold behaviour) for the same atom or molecule. The photoionization cross section shows abrupt onset (i.e., a step behaviour) to a high value at threshold, falling thereafter only gradually with increase of photon energy. Electron-impact ionization in simple atoms (such as hydrogen and helium) begins at the ionization potential, increases in direct proportion to the energy near the threshold, and shows a peak at an incident energy of about 100–200 eV. With molecules the behaviour is similar except that the peak is broad and much less pronounced. When the incident energy is high and the ejected electron has kinetic energy (energy of motion) largely in excess of its binding energy, the cross section for the process approaches a limit called the classical Rutherford value, after the British physicist Ernest Rutherford.

      In general, the initial processes resulting from the action of high-energy radiation on matter involve the intermediate production and participation of positive ions (both stable and unstable), electrons, negative ions, excited species, and free radicals and atoms, which in turn may enter into the processes of classical reaction kinetics.

      Ordinary low-energy (or optical) processes usually involve only excited species and free radicals and atoms—all formed by processes that do not involve outright transfer of electric charge (i.e., electrons) between different atoms and molecules.

      The important feature that characterizes the chemistry both of optical processes (photochemistry) and of high-energy radiation (radiation chemistry) is that they are conveniently employed and their kinetics studied at room temperature and lower.

      There are two “laws” of photochemistry. The first, the Grotthuss–Draper law (named for the chemists Christian J.D.T. von Grotthuss and John W. Draper), is simply: for light to produce an effect upon matter it must be absorbed. The second, or Stark–Einstein law (photochemical equivalence law) (for the physicists Johannes Stark and Albert Einstein), in its most modern form is: one resultant primary physical or chemical act occurs per photon absorbed. The quantum yield of a particular species of product is the number of moles of that product divided by the number of einsteins of light (units of 6.02 × 1023 photons)—or the number of molecules of product per photon—absorbed. In the ideal case the quantum yield, frequently denoted by the Greek letters gamma, γ, or phi, Φ, is unity. In real cases, Φ may approach zero on the one hand—particularly if a back reaction is involved—or it may be of the order of 1,000,000, in which case the primary product may start a chain reaction, as in a clean, dry mixture of hydrogen (H) and chlorine (Cl). In the following chemical equations each symbol for an element stands for one atom, and the number of atoms bonded into a molecule is given as a subscript following the symbol, while the number of molecules precedes the formula; the arrow indicates the course of the reaction:

      in which reactions 2—> and 3—> reoccur repeatedly in a chain reaction. The symbol →hν may be read “when a photon of light frequency, symbolized by the Greek letter nu, ν (which is always stipulated), is absorbed, gives.” Because h is Planck's constant of action (approximately 6.6 × 10-27 erg second) and ν is expressed in reciprocal seconds (i.e., second-1), the product hν indicates the energy absorbed per photon. Some reactions may give two primary products; e.g.,

      In that case, there are different quantum yields for each of the primary reactions, and the ratio of those yields varies with the frequency, ν, of the light absorbed.

Radiation chemistry
      When a target is bombarded by a positive ion such as the hydrogen ion H+ or the deuterium ion D+ from a particle accelerator or the alpha particle 4He2+ from nuclear decay, or indeed any high-energy heavy positive ion, the initial effects differ significantly from those of a high-energy electron. This situation results from the fact that, for the same kinetic energy, 1/2mv2, a particle of greater mass, m, travels with smaller velocity, v. The smaller the velocity of a particle of a particular charge in the domain of high (but not ultrarelativistic) velocities, the greater its probability of interaction with the medium traversed—that is to say, the greater the linear energy transfer. Thus, positive ions produce their initial effects close together in the ionization track in a condensed medium such as water (perhaps one or two angstroms, 1 or 2 × 10-8 centimetre, apart), whereas equally energetic electrons traveling through the same medium deposit energy in small collections called spurs, which may be 1,000 angstroms (10-5 centimetre) or so apart. The appearance of the excitation and ionization track has been likened to a rope (in the case of positive-ion bombardment), on the one hand, as compared with isolated beads on a string (in the case of electron bombardment), on the other. The dense track, as well as the isolated spurs, contains ions, excited molecules, and electrons; however, the distributions in the two essentially different types of track are so different that the ensuing chemical reactions (i.e., the track effects) may be quite dissimilar. As an example, alpha-particle irradiation of pure water produces substantial yields of hydrogen and hydrogen peroxide (H2O2), whereas irradiation with beta particles, X rays, or gammas is essentially without effect. One of the reaction sequences suggested in overall considerations of the radiation chemistry of water is

      in which reaction (1—>) summarizes the early chemical consequences both of ionization and of excitation. It has been suggested that reactions (2—>) and (3—>) occur with high probability in dense tracks (e.g., of alpha particles) but that, in isolated spurs (as in fast-particle tracks), such reactions may occur only with low probability. In such a case, according to the American chemist A. Oliver Allen, the hydrogen atoms and OH radicals enter with somewhat greater probability into back-reaction chains with any H2 + H2O2 already produced and existent in the body of the liquid:

      The H atom produced in reaction (5—>) thereupon enters into reaction (4—>), so that whatever small amounts of H2 and H2O2 are actually produced in reactions (2—>) and (3—>) are consumed in reactions (5—>) and (4—>), respectively, and remain essentially undetectable no matter how long the reaction is run.

Radiation chemical reactions
      In more detailed discussions of the mechanism of radiation chemical reactions, the roles of both excitation and ionization are considered. Information regarding the former is available from the extensive data of photochemistry; frequently, the initial excitation process leads to no significant chemical effect. By contrast, ionization may result in a large variety of chemical changes involving the positive ion, the outgoing electron, and the excited states resultant from charge neutralization, as well as (parent) positive-ion fragmentation and ion-molecule reactions. Some such consequences are summarized for a few cases.

      Different channels of fragmentation from the same parent ion (e.g., the propane ion C3H8+), such as

      compete unless barred by energetic considerations. Because ionization potentials of various possible fragments may differ greatly, charge localization may occur on only one of them. On the other hand, because the initial ionization rarely leads to the ground state of the positive ion, the energy is usually adequate for bond breakage.

      Ion-molecule reactions such as that between a water ion and a molecule,

      are more important in the condensed phase, and fragmentation is more important in the gas phase. The parent ion in liquid water almost invariably undergoes ion–molecule reaction as indicated above. Many ion–molecule reactions have high cross sections. The same ion may undergo fragmentation or ion–molecule reaction, depending on circumstances. Thus, methane (CH4), acted upon by high-energy gamma radiation, producing an electron, symbolized by

      may be followed by fragmentation,

      as well as an ion–molecule reaction,

      The electron ejected in an initial ionization process may further ionize and excite other molecules in its path, thus causing other chemical transformations. In addition, it may produce chemical changes of its own by dissociative attachment, as in carbon tetrachloride (CCl4) and nitrous oxide (N2O),

      and by formation of negative ions of either permanent or virtual (i.e., very short-lived) nature. Many of the negative ions produced in a dissociation process are chemically reactive (H-, O-, etc.) as well. Virtual negative ions are almost invariably in a high vibrational state—i.e., they are vibrationally hot.

      The important point to note from this limited discussion of primary physical effects and their consequences in radiation chemistry is that in general each such effect is the progenitor of many ionizations and excitations, the distribution of which in space depends on the energy of the particle involved as well as on the system traversed. There is no single resultant primary process corresponding to the result of absorption of a single optical photon and thus no analogue to the concept of quantum yield in photochemistry.

      In radiation chemistry, yields are conventionally reported on the purely empirical basis of the number of molecules of a particular kind produced (or destroyed) per 100 eV' input of a particular type of radiation. In the radiolysis (radiation-induced decomposition) of cyclohexane, for example, by cobalt-60 gamma radiation or by electrons of about 2,000,000 eV of energy, the overall yield of hydrogen per 100 eV' input is frequently given as approximately 5.6 or G(H2) ≃ 5.6, in which the symbol G is read as “the 100-electron-volt yield of.” Sometimes a small g is used to denote the 100-electron-volt yield of a postulated intermediate not directly determinable by measurement.

Symbolism of radiation chemistry
      The symbolism of radiation chemistry differs from that of photochemistry. The chemistry is somewhat more complicated, and the establishment of the variety of initial chemical processes is somewhat more of a chore. For the action of high-energy radiation on water, the variety of early products is typically indicated by the relation

      in which  is read “acted upon by high-energy radiation, gives” and eaq- is the symbol for the hydrated electron. Particular note is addressed to the species eaq- (i.e., an electron solvated by water) indicated in the same reaction. For many years there was an awareness in the radiation chemistry of water of the anomalous behaviour of the hydrogen atom, H, as compared with the same atom produced in established chemical processes. The anomaly was resolved on the one hand by John W. Boag and E.J. Hart, who spectroscopically observed the species eaq- in the spectral region predicted by Platzman, and on the other hand by Harold A. Schwarz and Gideon Czapski, who showed the existence of the ionic reducing species with charge of minus unity.

Time scales in radiation chemistry
      The time scale characteristic of radiation chemistry ranges from the extraordinarily short time required for a fast electron to traverse a molecule (about 10-18 second) to the time required for essential completion of some neutralization processes in very viscous media (about three hours). In between, there can be a variety of reactions involving intermediate formation and disappearance of the collections of the various species already discussed. The time-scale spread is so great that a pt scale (in which pt is defined as minus the logarithm, to the base 10, of the time [in seconds] t [i.e., -log10 t]) is conveniently employed. Actual observances in the long time-scale region follow fairly well-established chemical practice. The short-time region, on the other hand, presents interesting challenges. The Van de Graaff generator and the linear accelerator both made possible irradiations by electrons and X rays in the microsecond (10-6 second) region, and spectroscopic devices were quickly devised to make observations in that region. Improvements in irradiation technique (with X rays by Herbert Dreeskamp and Milton Burton, and with ultraviolet by Paul K. Ludwig and Juan d'Alessio) and in observation techniques in the study of luminescence extended precision of observation to 5 × 10-10 second in the work of William P. Helman. John K. Thomas combined use of a fast linac (linear accelerator) with Cherenkov radiation as a marker to extend chemical studies into the same region. Use of the same radiation as a light source for spectroscopic observation of the chemistry produced by a traveling electron front (from a linac) made possible actual observations in the time range of (2 to 4) × 10-11 second.

Tertiary effects of radiation on materials
      The electrons liberated by high-energy irradiation that have sufficient energy cause further ionizations in which additional electrons are produced. Some of these second generation electrons also cause additional ionizations, and this process continues until their remaining energy becomes inadequate. Even though this process goes through several generations of events, it actually takes little time and thus appears as an impact phenomenon as far as radiation-induced chemical changes are concerned. For this purpose, then, they may be considered as primary. Fast chemical changes induced by radiation may take time on the order of nanoseconds (a nanosecond is 10-9 second) or less to complete. Slower reactions involving relatively less reactive scavengers (reagents that eliminate residues) in dilute concentrations may require a time span of approximately 10-4 second.

      This section is concerned with radiation effects measurable on much longer time scales, arbitrarily greater than about one minute. Attention is here addressed to physical changes in the solid state, about which there is a wealth of experimental information. It should again be emphasized that little chemical change is expected in an atomic medium in which the absorption of ionizing radiation also results ultimately in structural changes and induced imperfections. With neutron irradiation, in addition to specific nuclear interactions, one gets “knocked off” atoms or ions (note the discussion of the Wigner effect in Neutrons (radiation) above). These ions quickly capture electrons and the resulting neutral atoms then travel on. Even though a small effect occurring in ionization and electronic excitation attributable to knocked off ions cannot be denied, it is believed that this effect is small compared with that brought about by the neutral knock offs in the form of structural changes.

Heating (heat) effects
      The simplest ultimate effect of absorption of radiation is heating. It can be argued that, for ionizing radiation of low linear energy transfer, the heating effect is negligible. A spur created by such low-LET radiation is a small spherical region in which the energy deposit is localized in isolation. The temperature rise, ΔT, of the spur above the surrounding temperature has a space-time dependence that, by hypothesis, has a statistical distribution, called Gaussian, because of random superposition of events leading to the heating process. The temperature rise at a point located a distance r away from the spur centre at time t is given by the equation

      in which a is the initial spur-size parameter, the Greek letter gamma, γ, is the thermal diffusivity of the medium (equal to heat conductivity divided by the product of density and specific heat at constant volume), and ΔTmax is the initial maximum temperature rise at the centre of the spur. Taking reasonable values for energy deposition (30 eV) and spur size a (20 angstroms, or 2 × 10-7 centimetre) and using (for water) density equal to one gram per cubic centimetre and specific heat 4 × 107 ergs per gram-degree, ΔTmax may be estimated to be 30° C (54° F). The time required for the central temperature to drop to half its initial value (i.e., t1/2) is given by (1 + 4γt1/2/a2)3/2 = 2. With the thermal diffusivity, symbolized by the Greek letter gamma, γ, equal to 10-3 centimetre squared per second for water, t1/2 = 6 × 10-12 second. The conclusion is that for low-LET radiation the local temperature rise is too small and too brief to have any appreciable chemical (or physical) effect. It is particularly notable that the actual temperature rise is smaller than that estimated here because part of the deposited energy is invariably utilized in ionization, dissociation, and similar processes. This part of the energy resides in the potential form and is not completely available for heating. With high-LET ionizing radiations (such as fission fragments, stripped nuclei, and α-particles), the situation is somewhat different. In such a case, a large amount of energy is deposited per unit path length, resulting in cylindrical tracks (rather than spherical spurs). The equation for temperature rise in this case is written in a form much like the equation for spur geometry; that is,

      except that in this case a is the initial size parameter for the track cylinder and ΔTmax is the maximum initial temperature rise on its axis. For fission fragments with LET of 500 eV per angstrom (i.e., 10-8 centimetre) and a equal to 20 angstroms, ΔTmax for water is 1.6 × 104 K. Admittedly, this figure is an overestimate for reasons similar to those that apply for low-LET radiations, but it is believed that the temperature rise is high and may approach 10,000 K. The time for this temperature to drop to half the initial value is given by t1/2 = a2/4γ, which in this case is estimated to be about 10-11 second. This time is not much larger than the corresponding time for survival of an isolated spur. Because of the high local temperature, however, the reaction time of the radiation-produced intermediates is also very small. In an intermediate + substrate (i.e., solvent) reaction, for example, with an activation energy of approximately eight kilocalories per mole, the rate constant, k, for such a typical pseudo-first-order reaction may be written, according to the simple usage of chemical kinetics, in the form

      in which R is the gas constant in units of two calories per degree and T is the absolute temperature. For a substrate concentration of 1022 molecules per cubic centimetre and a temperature of 10,000 K, -d(ln ν)/dt = 6.6 × 1010 reciprocal seconds, in which the Greek letter nu, ν, here denotes the concentration of intermediates still existent at time t. Therefore, the time required for the intermediate concentration to drop to an “e-folding” value (i.e., to fraction 1/e) is approximately 1.5 × 10-11 second, a time that compares favourably with the duration of the temperature pulse. The conclusion is that for very high LET radiation there is indeed a high degree of local heating, and, even though the heat pulse survives only a short time, that time still is long enough to bring about the acceleration of reactions between the short-lived intermediates and the ambient substrate.

Crystal-lattice effects
      In neutron irradiation of a solid, atoms are dislodged from normal lattice positions and set in motion (the Wigner effect). The fractional amount of energy transfer depends, as in any elastic collision, on the mass ratio of the neutron to that of the recoil atom. Thus, in graphite a carbon atom, on first collision with a neutron of 1,000,000-eV (produced, say, in a fission process), receives a kinetic energy of approximately 105 eV, which is large compared with its binding energy in the lattice (about 10 eV). It is estimated that the 1,000,000-electron-volt neutron strikes about 60 carbon atoms before it is thermalized or its speed is so much reduced that it cannot knock off other carbon atoms. Much of the structural damage caused by radiation is attributable to these (relatively heavy) carbon atoms rather than to the original neutron. In this sense, radiation damage by fast neutrons may be viewed as an indirect action. Slowing of the fast carbon (or other dislodged) atoms is basically governed by interaction time. This fact means that the stopping is light in the beginning of the journey of a dislodged atom and results only in occasional displacement of atoms. Toward the end of its career a large number of atoms are displaced in quick succession along a row, and finally a large amount of residual energy is dumped locally into a relatively small group of atoms. This process generates the displacement (or thermal) spike; the local temperature rise is estimated to be about 1,000 K. Even though the temperature rise lasts only about 10-11 second before the track is cooled down, this duration is enough for permanent structural damage. At least a part of the swelling of graphite under reactor-neutron irradiation is the result of this local heating; another part originates in change in lattice dimension under irradiation.

      The high temperature rise in a thermal spike probably results in local melting of the solid. Evidence in that direction has been obtained from a study of beta-brass (an alloy consisting of equal numbers of atoms of copper and zinc) under neutron bombardment at low temperature. Before irradiation, the alloy structure is ordered: each copper atom is surrounded by eight zinc atoms as nearest neighbours and vice versa. After irradiation, a general random rearrangement of the atoms can be detected, presumably the result of melting and refreezing.

      Long-term effects of radiation on crystals are numerous, and the magnitudes of these effects depend on the crystal structure and previous history. Only some general features of these effects are recounted here.

      1. Radiation damage may be thought to consist of pairs of interstitial atoms ejected from their normal lattice sites and the corresponding vacancies left behind. A vacancy-interstitial pair is called a Frenkel defect.

      2. A solid has a tendency to recover spontaneously from radiation damage. If it were not for this property, it would indeed be extremely difficult to operate nuclear reactors that are permitted to heat up periodically to remove the effect in the graphite core. The healing (or so-called annealing) is presumably attributable to the recombination of interstitial atoms and vacancies, thereby removing Frenkel defects. It is not necessary that an interstitial atom always recombine with its corresponding vacancy. Often it may recombine with a vacancy that resembles the one that it left; the result is approximate restoration of the original properties of the crystal. Such annealing is facilitated by the increased mobility of the vacancies and interstitials at higher temperature. At a particular temperature called the annealing temperature, the healing becomes fast and essentially complete. The same substance may have somewhat different annealing temperatures depending on the particular property under study. Many experiments on radiation damage must be carried out at low temperatures to freeze in the defects produced. Pure metals are the most easily annealed substances. Annealing temperatures in such cases are relatively low. Accordingly, the annealing temperature for the increase of electrical resistance in pure copper is only around 40 K. On the other hand, changes in elastic modulus and hardness, such as are required to produce tuning-fork characteristics, persist up to room temperature—namely, 293 K. Quick annealing in pure metals is directly attributable to the high mobility of atoms in perfectly ordered structures. At the other extreme are organic solids, particularly polymers, that are composed of large molecules. In this case, the damage originates in the breaking of bonds that ordinarily do not rejoin in the original manner but instead produce chemically different material.

      3. In simple metals irradiation decreases conductivity for both heat (thermal conduction) and electricity. Conduction of both in metallic crystals is attributable to their ordered structure. The more perfect the structure, the better is the conduction. Frenkel defects, generated by irradiation, therefore decrease both conductivities. In extreme cases conductivity decrease of orders of magnitude has been observed. With moderate irradiation, however, both thermal and electrical conductivities decrease usually by half. The thermal conductivity of graphite falls to roughly half the unirradiated value with an exposure of 3 × 1020 neutrons per square centimetre at room temperature. Like other property changes, this effect also can be annealed at elevated temperatures with concomitant release of stored energy. Energy storage in graphite amounts to about 200 calories per gram per 1020 neutrons per square centimetre total flux. Interstitial carbon atoms produced in the irradiation scatter electrons and thus decrease electrical conductivity. The pattern of conductivity decrease and increase depends on the nature of the graphite and the duration of exposure in a reactor. With ceramic materials, loss of thermal conductivity by a factor of about 3 to 5 may be observed under conditions in which the decrease is about one-half in graphite. In mica, on the other hand, the change is somewhat less than in graphite.

      4. Hardness and ductility depend on perfection of the crystal structure. It is thus found that irradiation results in a loss of ductility and an increase in hardness. Such effects are attributed to glide-plane obstruction in the crystal. Most structured materials become harder, less ductile, and sometimes more brittle as the result of neutron irradiation. Similarly, most polymers also lose ductility on irradiation. In a certain sense radiation-induced damage to the crystal structure is qualitatively similar to that produced by cold-working (for example, by hammering). Neutron irradiation of pure copper, which is naturally soft at room temperature, makes it so hard that it can be made to sing like a tuning fork. Graphite experiences increase in strength and hardness upon irradiation. Annealing is faster at elevated temperatures; also, damage is less when the irradiation is at a higher temperature. A similar effect is seen for the compressive stress-strain curve. Studies of dynamic properties in ceramics indicate a saturation effect at large doses.

      5. As was discussed above, irradiation causes expansion and lattice distortion in most cases. A perfect crystal of graphite consists of planes of carbon atoms layer upon layer. When irradiated by neutrons, graphite expands perpendicular to the base plane and contracts slightly parallel to it. After moderate exposure in a nuclear reactor, the expansion is approximately 1 percent for a flux of 1020 neutrons per square centimetre. The actual amount of expansion, of course, depends on the fabrication history and operating temperature of the graphite. Expansion of moderator materials such as graphite is of considerable importance in the design of nuclear reactors. Even a small percentage change in dimension can result in large total change in the reactor structure; if this change is not allowed for in the engineering design of the reactor, it may well create strained operating conditions eventually leading to failure.

Milton Burton Asokendu Mozumder Myron Luntz

Surface effects
      A surface is distinct from bulk matter in that it constitutes the physical interface with the environment. Whether or not a metal will corrode in salt water, for example, or how much resistance to wear is inherent in the design of a bearing are concerns that relate primarily to the physical condition of surfaces. The latter, in turn, may be selectively modified by the application of coatings or by the action of radiation, or by both. Three of the most common examples of surface modification by radiation—ultraviolet curing, ion implantation, and sputtering—are considered here.

      Ultraviolet curing is a process in which polymers, generally employed as coatings, are irradiated by ultraviolet light. Such action produces electronic excitation and ionization of the long chain molecules that make up the polymer, either directly or through the mediation of imbedded, light-sensitive “activators.” This results in intermolecular bonding, a process called cross-linking. The entire polymeric coating, typically on the order of tenths of millimetres thick (depending on the application), becomes so highly cross-linked as to take on the character of a single giant molecule. The major effects of ultraviolet irradiation of polymers include reduction of friction, increased resistance to wear, increased hardness, and increased resistance to attack by acids and other corrosive agents. Ultraviolet curing is employed for diverse purposes ranging from the formation of “no-wax” coatings on floor tiles to application in the photolithographic process integral to the fabrication of solid-state electronic devices.

      Ion implantation involves the irradiation of solids by beams of energetic ions emanating from particle accelerators. Typical energies employed are on the order of 100 keV (100,000 electron volts). Typical depths of penetration are on the order of several thousand angstroms, depending on energy, ion type, and target material. In ion implantation, virtually any atomic species can be embedded to predetermined depths and with predetermined concentration profiles in any target material so as to modify the surface characteristics without affecting desirable bulk properties. A typical example is the implantation of titanium in iron alloys to reduce wear of bearings and gears. A particularly promising technique was developed by physicists Michael W. Ferralli and Luntz, in which vacuum deposition of polymeric coatings on metallic substrates and simultaneous ion-beam irradiation act to produce implanted hydrocarbon films. The latter can be made to vary in carbon-to-hydrogen ratio from very high values—with the implanted region having some characteristics of diamond—to values on the order of unity and corresponding polymeric characteristics. This is accomplished by a process called preferential sputtering (see below). The films so produced are highly resistant to corrosion and appear to possess important bio-compatibility properties, making them suitable for applications in, for example, the treatment of the surfaces of surgical implants such as artificial hip joints. Such effects of ion implantation result in part from structural changes induced by radiation damage (e.g., implantation of boron or phosphorus in steel can render the surface amorphous so as to eliminate grain boundaries and other corrosion-sensitive sites), and in part from chemical changes arising from bonding of the implanted species with constituents of the substrate.

      Sputtering is a process in which atoms, ions, and molecular species in the surface of a target material are ejected under the action of ion-beam irradiation. Energies typical of ion implantation are employed and, while any ion type may be used, noble (or rare) gases such as argon and neon are most common. The latter avoid unwanted chemical interactions between the ions of the beam and the substrate. Sputtering results from several interaction mechanisms. Conceptually, the simplest is rebound sputtering, in which an incident ion strikes an atom on the surface, causing it to recoil into the target. The recoiling atom promptly collides with a neighbouring atom in the target, rebounds elastically, and is ejected from the surface. A similar but somewhat more complex mechanism is recoil sputtering, in which a struck, recoiling surface atom undergoes a random sequence of elastic scatterings in the target material, ultimately migrating back to, and through, the surface. Yet another mechanism is prompt thermal sputtering, in which energized atoms in thermal spikes created close to the surface escape through the surface before annealing occurs. Certain materials (e.g., crystalline alkali halides) are prone to electronic sputtering, in which energy associated with electronic excitations induced by the incident ion is transformed into atomic recoil kinetic energy, often sufficient to cause the ejection of ions through the surface. By means of any of these various mechanisms, several atoms may be sputtered for each ion incident on the target. The number of atoms sputtered per incident ion is called the sputtering yield.

      Surface modifications caused by sputtering are characterized as structural (e.g., phase conversion from crystalline to amorphous and vice versa), topographical (e.g., alteration of the shape of surface protrusions such as grain boundaries, development of facets, and the removal of surface contaminants), electronic (e.g., radiation-induced chemical changes), and compositional (e.g., preferential sputtering of a particular atomic species resulting in changes in the composition of alloys).

Myron Luntz

Biologic effects of ionizing radiation
      The biomedical effects of ionizing radiation have been investigated more thoroughly than those of any other environmental agent. Evidence that harmful effects may result from small amounts of such radiation has prompted growing concern about the hazards that may be associated with low-level irradiation from the fallout of nuclear weapons, medical radiography (X-ray), nuclear power plants, and other sources.

      Assessment of the health impact of ionizing radiation requires an understanding of the interactions of radiation with living cells and the subsequent reactions that lead to injury. These subjects are surveyed in the following sections, with particular reference to the principal sources and levels of radiation in the environment and the different types of biologic effects that may be associated with them.

Historical background
      Within weeks after Röntgen revealed the first X-ray photographs in January 1896, news of the discovery spread throughout the world. Soon afterward, the penetrating properties of the rays began to be exploited for medical purposes, with no inkling that such radiation might have deleterious effects.

      The first reports of X-ray injury to human tissue came later in 1896. Elihu Thomson (Thomson, Elihu), an American electrical engineer, deliberately exposed one of his fingers to X rays and provided accurate observations on the burns produced. That same year, Thomas Alva Edison (Edison, Thomas Alva) was engaged in developing a fluorescent X-ray lamp when he noticed that his assistant, Clarence Dally, was so “poisonously affected” by the new rays that his hair fell out and his scalp became inflamed and ulcerated. By 1904 Dally had developed severe ulcers on both hands and arms, which soon became cancerous and caused his early death.

      During the next few decades, many investigators and physicians developed radiation burns and cancer, and more than 100 of them died as a result of their exposure to X rays. These unfortunate early experiences eventually led to an awareness of radiation hazards for professional workers and stimulated the development of a new branch of science—namely, radiobiology.

      Radiations from radioactive materials were not immediately recognized as being related to X rays. In 1906 Henri Becquerel (Becquerel, Henri), the French physicist who discovered radioactivity, accidentally burned himself by carrying radioactive materials in his pocket. Noting that, Pierre Curie (Curie, Pierre), the co-discoverer of radium, deliberately produced a similar burn on himself. Beginning about 1925, a number of women employed in applying luminescent paint that contained radium to clock and instrument dials became ill with anemia and lesions of the jawbones and mouth; some of them subsequently developed bone cancer.

      In 1933 Ernest O. Lawrence (Lawrence, Ernest Orlando) and his collaborators completed the first full-scale cyclotron at the University of California at Berkeley. This type of particle accelerator was a copious source of neutrons, which had recently been discovered by Sir James Chadwick in England. Lawrence and his associates exposed laboratory rats to fast neutrons produced with the cyclotron and found that such radiation was about two and a half times more effective in killing power for rats than were X rays.

      Considerably more knowledge about the biologic effects of neutrons had been acquired by the time the first nuclear reactor was built in 1942 in Chicago. The nuclear reactor, which has become a prime source of energy for the world, produces an enormous amount of neutrons as well as other forms of radiation. The widespread use of nuclear reactors and the development of high-energy particle accelerators, another prolific source of ionizing radiation, have given rise to health physics. This field of study deals with the hazards of radiation and protection against such hazards. Moreover, since the advent of spaceflight in the late 1950s, certain kinds of radiation from space and their effects on human health have attracted much attention. The protons in the Van Allen radiation belts (two doughnut-shaped zones of high-energy particles trapped in the Earth's magnetic field), the protons and heavier ions ejected in solar flares, and similar particles near the top of the atmosphere are particularly important.

Units for measuring ionizing radiation
      Ionizing radiation is measured in various units. The oldest unit, the roentgen (R), denotes the amount of radiation that is required to produce 1 electrostatic unit of charge in 1 cubic centimetre of air under standard conditions of pressure, temperature, and humidity. For expressing the dose of radiation (rad) absorbed in living tissue, the principal units are the gray (Gy; 1 Gy = 1 joule of radiation energy absorbed per kilogram of tissue) and the rad (1 rad = 100 ergs per gram of tissue = 0.01 Gy). The sievert (Sv) and the rem make it possible to normalize doses of different types of radiation in terms of relative biologic effectiveness (RBE), since particulate radiations tend to cause greater injury for a given absorbed dose than do X rays or gamma rays. The dose equivalent of a given type of radiation (in Sv) is the dose of the radiation in Gy multiplied by a quality factor that is based on the RBE of the radiation. Hence, one sievert, defined loosely, is that amount of radiation roughly equivalent in biologic effectiveness to one gray of gamma rays (1 Sv = 100 rem). Because the sievert and the rem are inconveniently large units for certain applications, the milligray (mGy; 1 mGy = 1/1000 Gy) and millisievert (mSv; 1 mSv = 1/1000 Sv) are often substituted.

      For expressing the collective dose to a population, the person-Sv and person-rem are the units used. These units represent the product of the average dose per person times the number of people exposed (e.g., 1 Sv to each of 100 persons = 100 person-Sv = 10,000 person-rem).

      The units employed for measuring the amount of radioactivity contained in a given sample of matter are the becquerel (Bq) and the curie (Ci). One becquerel is that quantity of a radioactive element in which there is one atomic disintegration per second; one curie is that quantity in which there are 3.7 × 1010 atomic disintegrations per second (1 Bq = 2.7 × 10-11 Ci). The dose that will accumulate over a given period (say, 50 years) from exposure to a given source of radiation is called the committed dose, or dose commitment.

Sources and levels of radiation in the environment
Natural sources
       Cosmic-radiation exposure External dose due to natural radioactivity in soil or rock Average dose due to natural radioactivity deposited internally Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population Cosmic-radiation exposure External dose due to natural radioactivity in soil or rock Average dose due to natural radioactivity deposited internallyFrom the beginning, life has evolved in the presence of natural background ionizing radiation. The principal types and sources of such radiation are: (1) cosmic rays, which impinge on the Earth from outer space (Table 3 (Cosmic-radiation exposure); Figure 4); (2) terrestrial radiations, which are released by the disintegration of radium, thorium, uranium, and other radioactive minerals in the Earth's crust (Table 4 (External dose due to natural radioactivity in soil or rock); Figure 4); and (3) internal radiations, which are emitted by the disintegration of potassium-40, carbon-14, and other radioactive isotopes that are normally present within living cells (Table 5 (Average dose due to natural radioactivity deposited internally); Figure 4). The average total dose received from all three sources by a person residing at sea level is approximately 0.91 mSv per year (Table 6 (Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population)); however, a dose twice this size may be received by a person residing at a higher elevation such as Denver, Colo., where cosmic rays are more intense (Table 3 (Cosmic-radiation exposure)), or by a person residing in a geographic region where the radium content of the soil is relatively high (Table 4 (External dose due to natural radioactivity in soil or rock)). In the latter type of region, the radioactive gas radon, which is formed in the decay of radium, may enter a dwelling through its floor or basement walls and accumulate in the indoor air unless the dwelling is well ventilated periodically; occupants of such a dwelling may therefore receive a dose as high as 100 mSv per year in their lungs from inhalation of the entrapped radon and its disintegration products (Table 5 (Average dose due to natural radioactivity deposited internally); Figure 4).

Artificial sources
       Typical doses to exposed tissue received in routine X-ray diagnosis Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population Worldwide dose commitment from radioactive fallout from nuclear tests prior to 1970 Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. populationIn addition to natural background radiation, people are exposed to radiation from various man-made sources, the largest of which is the application of X rays in medical diagnosis. Although the doses delivered in different types of X-ray examinations vary from a small fraction of a mGy to tens of mGy (Table 7 (Typical doses to exposed tissue received in routine X-ray diagnosis)), the average annual dose per capita from medical and dental irradiation in developed countries of the world now approaches in magnitude the dose received from natural background radiation (Table 6 (Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population)). Less significant artificial sources of radiation include radioactive minerals in crushed rock, building materials, and phosphate fertilizers; radiation-emitting components of television sets, smoke detectors, and various other consumer products; radioactive fallout from nuclear weapons (Table 8 (Worldwide dose commitment from radioactive fallout from nuclear tests prior to 1970)); and radiation released in nuclear power production (Table 6 (Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population)).

      Most of the radioactivity produced in nuclear power reactors is safely contained; however, a small percentage escapes as stack gas or liquid effluent and eventually may contaminate the atmosphere and water supply. (There are similar releases from nuclear-fuel reprocessing plants.) Though nuclear plants are basically clean sources of energy, they thus contribute to the worldwide background radiation level. This problem cannot be entirely avoided by using coal instead of nuclear fuel for power production, since many sources of coal contain natural radioactivity (e.g., radium) that is released in stack gases, along with chemical pollutants.

       Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. populationFrom Table 6 (Estimates of average annual dose equivalent to the whole body from various sources of irradiation received by members of the U.S. population) it is evident that the human population is now exposed to about twice as much radiation from all sources combined as it receives from natural sources alone. Hence, it is important to understand the possible consequences, if any, that may result from the additional exposure to radiation.

      In comparison with the relatively small amounts of radiation described above, the dose typically administered to a patient in the treatment of cancer is thousands of times larger; i.e., a total dose of 50 Sv or more is usually delivered to a tumour in daily exposures over a period of four to six weeks. To protect the normal tissues of the patient against injury from such a large dose, as well as to protect medical personnel against excessive occupational exposure to stray radiation, precautions are taken to restrict exposure to the tumour itself insofar as possible. Comparable safeguards are utilized to minimize the exposure of workers employed in other activities involving radiation or radioactive material. Similarly, elaborate safety measures are required for disposal of radioactive wastes from nuclear reactors, due in part to the slow rate at which certain fission products decay. A given amount of plutonium-239, for example, still retains about one-half of its radioactivity after 25,000 years, so that reactor wastes containing this long-lived radionuclide must be safely isolated for centuries.

      In the event of an atmospheric nuclear bomb explosion, large quantities of radioactivity are released, the dispersal of which depends on the prevailing weather conditions as well as on the height and nature of the blast. Although the level of contamination resulting from such an explosion or from a nuclear-power plant accident is generally highest in the immediate vicinity of the event itself, both radioactive gas and dust may be transported via air or water for many hundreds of kilometres and eventually contaminate the entire globe.

Mechanism of biologic action
      As ionizing radiation penetrates living matter, it gives up its energy through random interactions with atoms and molecules in its path, leading to the formation of reactive ions and free radicals. It is the molecular alterations resulting from these ionizations and, in turn, the resultant biochemical changes that give rise to various types of injury. X rays and gamma rays, for example, impart their energy to “planetary” atomic electrons, which are thereby ejected from their orbits. Such an ejection of a planetary electron results in an ion pair consisting of a free electron and the electrically charged atom from which it was ejected. The ejected electron may give rise to a highly reactive free radical, which in turn may diffuse far enough to attack a biologically important target molecule in its vicinity. This so-called indirect action process, through which radiation causes damage via radiation-induced free radicals, may be envisioned as follows:

      While the initial steps in the above process occur almost instantaneously, expression of the biologic effect may take years or decades, depending on the type of injury involved. The indirect action of radiation is more important in the biologic effects of low-LET radiations than in those of high-LET radiations (see above The passage of matter rays: Linear energy transfer and track structure (radiation)), but the latter have a greater capacity to cause injury through direct interaction with biologic targets.

      Direct biologic actions, studied in detail between 1927 and 1947, gave rise to a target theory of radiobiology that has provided a quantitative treatment of many of the biologic effects of radiation, particularly in the field of genetics. According to this theory, a tissue or cell undergoing irradiation is likened to a field traversed by machine-gun fire, in which the production of a given effect requires one or more hits by an ionized track on a sensitive target. The probability of obtaining the effect is thus dependent on the probability of obtaining the requisite number of hits on the appropriate target or targets.

      The distribution of ionizing atomic interactions along the path of an impinging radiation depends on the energy, mass, and charge of the radiation. The ionizations caused by neutrons, protons, and alpha particles are characteristically clustered more closely together than are those caused by X rays or gamma rays. Thus, because the probability of injury depends on the concentration of molecular damage produced at a critical site, or target, in the cell (e.g., a gene or a chromosome), charged particles generally cause greater injury for a given total dose to the cell than do X rays or gamma rays; i.e., they have a high RBE. At the same time, however, charged particles usually penetrate such a short distance in tissue that they pose relatively little hazard to tissues within the body unless they are emitted by a radionuclide, or radioactive isotope, that has been deposited internally.

Radionuclides and radioactive fallout
      Radionuclides emit various ionizing radiations (e.g., electrons, positrons, alpha particles, gamma rays, or even characteristic X rays), the precise types of which depend on the radionuclide in question. Exposure to a radionuclide and its emissions may be external, in which case the penetrating power of the radiation is an important factor in determining the probability of injury. Alpha particles, for example, do not penetrate deeply enough into the skin to cause damage, whereas energetic beta particles or X rays can be hazardous to the skin and deeper tissues.

Accumulation in critical organs
      Radionuclides can enter the body by ingestion, inhalation, or injection. Once taken into the body, their radiation effects depend on their anatomic distribution, duration of retention in the body, and rate of radioactive decay, as well as on the energies of their emitted radiations. An internally deposited radioactive element may concentrate in, and thus irradiate, certain organs more than others. Radioiodine, for example, collects in the thyroid gland, whereas radium and strontium accumulate chiefly in the bones. Different radioelements also vary in their rates of removal. Radioiodine, for instance, is normally eliminated from the thyroid rapidly enough so that its concentration is halved within days. Strontium-90, on the other hand, is retained in high concentrations in the skeleton for years.

       Values for the maximum permissible concentration (MPC) of certain radionuclidesThe term critical organ refers to the part of the body most vulnerable to a given isotope. The critical organ for plutonium, radium, strontium, and many other fission products is bone and the adjacent bone marrow. For iodine, the critical organ is the thyroid gland. Insoluble airborne radioactive dust often settles in the alveoli of the lungs, while small colloidal particles may become deposited in the bone marrow, liver, or spleen. Table 9 (Values for the maximum permissible concentration (MPC) of certain radionuclides) gives an abbreviated list of the maximum permissible concentrations (U.S. recommendations) of some radionuclides for humans. (The maximum permissible concentration is the largest amount of a radionuclide that can be accumulated in the body without producing undue risk of injury.)

      Since a radionuclide delivers radiation continuously to the surrounding tissue, the effect of such protracted continuous exposure must be distinguished from that of a single exposure or of periodically repeated exposures. From experiments with divided doses of gamma radiation or X radiation, it has been found that up to about 60 percent of the radiation effect from a single brief exposure is repaired within several hours. The body therefore is able to tolerate a larger total dose when the dose is accumulated slowly or when part of it is absorbed at a later time. There is less recovery with neutron and alpha radiation, however. (Neutrons are generally more effective agents of mutation than are X rays: for a single brief exposure, by a factor 1 to 8; for chronic irradiation, by a factor up to 100.)

       fallout is the deposition of airborne radioactive contaminants on Earth. Radioisotopes are produced naturally in the air by cosmic radiation, and they may enter the air in stack gases from nuclear power plants or be released through industrial accidents or nuclear explosions. After 1954, nuclear bomb tests carried out by several nations produced measurable fallout on the surface of the entire Earth, arousing great concern and controversy with respect to the resultant health effects. While much of the hazard from the detonation of a nuclear weapon is due to blast waves and heat, the radiation dose from fission products can be so intense that only persons remaining in underground shelters for some weeks could hope to survive. Usually the most prominent isotopes in fallout are fission products; however, all materials exposed to nuclear blasts may become radioactive.

The hazards of long-lived radioisotopes
      Several of the radioisotopes contained in fallout are especially hazardous because they remain radioactive for relatively long periods. Cesium-137, strontium-90, and plutonium-239 may be the most significant among these. Fallout material can cover external surfaces and foliage and later be washed into the soil, from which plants may absorb strontium-90, along with the chemically similar calcium, and cesium-137 with potassium. Humans take in these radioactive materials chiefly from drinking water and from plant and animal foods, including milk. Many fallout isotopes that reach the sea and inland waterways eventually end up in concentrated form in the bodies of waterborne animals and plants, becoming a source of concern when they are part of the human food chain.

      The most easily detectable fallout product in humans and other animals is iodine-131, an isotope that emits beta and gamma rays and is enriched about 100 times in the thyroid gland through selective accumulation. Because of its relatively short half-life (eight days), iodine-131 is probably not the most hazardous fallout isotope; yet, excessive amounts of radiation from this isotope can lead to metabolic disturbances and an increased incidence of thyroid cancer, especially in children.

      A mixture of radioactive gases is discharged into the atmosphere in small amounts by nuclear power reactors. Reactors are thus generally placed at sites where atmospheric mixing and transport are such that the short-lived gases decay and are diluted before they can be inhaled in appreciable amounts by human populations.

      Methods that have been developed for biologic protection against fallout range from measures designed to keep radioisotopes out of the body to biochemical means for rapidly eliminating such isotopes from tissues. At times of nuclear emergencies, airborne radioactive particles may be kept from the lungs by staying indoors or by wearing masks with suitable filtration. Absorption of ingested isotopes via the intestinal tract may be inhibited by certain mucoprotein substances that possess great surface affinity for adsorption of strontium and other substances; sodium alginate prepared from seaweed kelp is such a substance. It is possible with appropriate chemicals to remove virtually all radioactive strontium from cow's milk without affecting its essential nutritive components. Certain chelates—for example, EDTA (ethylenediaminetetraacetic acid)—will react with strontium and “cover” this atom. As a result, the presence of EDTA in the blood reduces the deposition of strontium in bones (elimination of already deposited isotopes also is somewhat accelerated). Unfortunately, however, EDTA and most other chelating agents are not specific for strontium; they also chelate the closely related and important element calcium. Consequently, their use requires expert medical supervision and is limited in effectiveness. On the other hand, the uptake of radioactive iodine by the thyroid gland may be reduced by the ingestion of large amounts of stable iodine, which is relatively nontoxic except to those with special sensitivity.

Major types of radiation injury
      Any living organism can be killed by radiation if exposed to a large enough dose, but the lethal dose varies greatly from species to species. Mammals can be killed by less than 10 Gy, but fruit flies may survive 1,000 Gy. Many bacteria and viruses may survive even higher doses. In general, humans are among the most radiosensitive of all living organisms, but the effects of a given dose in a person depend on the organ irradiated, the dose, and the conditions of exposure.

      The biologic effects of radiation in humans and other mammals are generally subdivided into (1) those that affect the body of the exposed individual—somatic effects—and (2) those that affect the offspring of the exposed individual—genetic, or heritable, effects. Among the somatic effects, there are those that occur within a short period of time (e.g., inhibition of cell division) and those that may not occur until years or decades after irradiation (e.g., radiation-induced cancer). In addition, there are those, called non-stochastic effects, that occur only in response to a considerable dose of radiation (e.g., ulceration of the skin) and those, termed stochastic, for which no threshold dose is known to exist (e.g., radiation-induced cancer).

      Every type of biologic effect of radiation, irrespective of its precise nature, results from injury to the cell, the microscopic building block of which all living organisms are composed. It therefore seems useful to open a review of such effects with a discussion of the action of radiation on the cell.

Effects on the cell
      The effects of radiation on the cell include interference with cell division, damage to chromosomes, damage to genes (mutations), neoplastic transformation (a change analogous to the induction of cancer), and cell death. The mechanisms through which these changes are produced are not yet fully understood, but each change is thought to be the end result of chemical alterations that are initiated by radiation as it randomly traverses the cell.

      Any type of molecule in the cell can be altered by irradiation, but the DNA of the genetic material is thought to be the cell's most critical target, since damage to a single gene may be sufficient to kill or profoundly alter the cell. A dose that can kill the average dividing cell (say, 1–2 Sv) produces dozens of lesions in the cell's DNA molecules. Although most such lesions are normally reparable through the action of intracellular DNA repair processes, those that remain unrepaired or are misrepaired may give rise to permanent changes in the affected genes (i.e., mutations) or in the chromosomes on which the genes are carried, as discussed below.

      In general, dividing cells (such as cancer cells) are more radiosensitive than nondividing cells. As noted above, a dose of 1–2 Sv is sufficient to kill the average dividing cell, whereas nondividing cells can usually withstand many times as much radiation without overt signs of injury. It is when cells attempt to divide for the first time after irradiation that they are most apt to die as a result of radiation injury to their genes or chromosomes.

      The percentage of human cells retaining the ability to multiply generally decreases exponentially with increasing radiation dose, depending on the type of cell exposed and the conditions of irradiation. With X rays and gamma rays, traversal by two or more radiation tracks in swift succession are usually required to kill the cell. Hence, the survival curve is typically shallower at low doses and low dose rates than at high doses and high dose rates. The reduced killing effectiveness of a given dose when it is delivered in two or more widely spaced fractions is attributed to the repair of sublethal damage between successive exposures. With densely ionizing particulate radiations, on the other hand, the survival curve is characteristically steeper than with X rays or gamma rays, and its slope is relatively unaffected by the dose or the dose rate, implying that the death of the cell usually results from a single densely ionizing particle track and that the injury produced by such a track is of a relatively irreparable type.

Damage to genes (gene) (mutations (mutation))
      Gene mutations resulting from radiation-induced damage to DNA have been produced experimentally in many types of organisms. In general, the frequency of a given mutation increases in proportion to the dose of radiation in the low-to-intermediate dose range. At higher doses, however, the frequency of mutations induced by a given dose may be dependent on the rate at which the dose is accumulated, tending to be lower if the dose is accumulated over a long period of time.

      In human white blood cells (lymphocytes), as in mouse spermatogonia and oocytes, the frequency of radiation-induced mutations approximates 1 mutation per 100,000 cells per genetic locus per Sv. This rate of increase is not large enough to detect with existing methodology in the children of the atomic-bomb survivors of Hiroshima and Nagasaki, owing to their limited numbers and the comparatively small average dose of radiation received by their parents. Accordingly, it is not surprising that heritable effects of irradiation have not been observable thus far in this population or in any other irradiated human population, in spite of exhaustive efforts to detect them.

      The observed proportionality between the frequency of induced mutations and the radiation dose has important health implications for the human population, since it implies that even a small dose of radiation given to a large number of individuals may introduce mutant genes into the population, provided that the individuals are below reproductive age at the time of irradiation. The effect on a population of a rise in its mutation rate depends, however, on the role played by mutation in determining the characteristics of the population. Although deleterious genes enter the population through mutations, they tend to be eliminated because they reduce the fitness of their carriers. Thus, a genetic equilibrium is reached at the point where the entry of deleterious genes into the population through mutation is counterbalanced by their loss through reduction in fitness. At the point of equilibrium, an increase of the mutation rate by a given percentage causes a proportionate increase in the gene-handicapped fraction in the population. The full increase is not manifested immediately, however, but only when genetic equilibrium is again established, which requires several generations.

      The capacity of radiation to increase the frequency of mutations is often expressed in terms of the mutation-rate doubling dose, which is the dose that induces as large an additional rate of mutations as that which occurs spontaneously in each generation. The more sensitive the genes are to radiation, the lower is the doubling dose. The doubling dose for high-intensity exposure in several different organisms has been found experimentally to lie between about 0.3 and 1.5 Gy. For seven specific genes in the mouse, the doubling dose of gamma radiation for spermatogonia is about 0.3 Gy for high-intensity exposure and about 1.0 Gy for low-intensity exposure. Little is known about the doubling dose for human genes, but most geneticists assume that it is about the same as the doubling dose for those of mice. Studies of the children of atomic-bomb survivors are consistent with this view, as noted above.

 From the results of experiments with mice and other laboratory animals, the dose required to double the human mutation rate is estimated to lie in the range of 0.2–2.5 Sv, implying that less than 1 percent of all genetically related diseases in the human population is attributable to natural background irradiation (Table 10—>). Although natural background irradiation therefore appears to make only a relatively small contribution to the overall burden of genetic illness in the world's population, millions of individuals may be thus affected in each generation.

      Notwithstanding the fact that the vast majority of mutations are decidedly harmful, those induced by irradiation in seeds are of interest to horticulturists as a means of producing new and improved varieties of plants. Mutations produced in this manner can affect such properties of the plant as early ripening and resistance to disease, with the result that economically important varieties of a number of species have been produced by irradiation. In their effects on plants, fast neutrons and heavy particles have been found to be up to about 100 times more mutagenic than X rays. Radioactive elements taken up by plants also can be strongly mutagenic. In the choice of a suitable dose for the production of mutations, a compromise has to be made between the mutagenic effects and damaging effects of the radiation. As the number of mutations increases, so also does the extent of damage to the plants. In the irradiation of dry seeds by X rays, a dose of 10 to 20 Gy is usually given.

Damage to chromosomes
      By breaking both strands of the DNA molecule, radiation also can break the chromosome fibre and interfere with the normal segregation of duplicate sets of chromosomes to daughter cells at the time of cell division, thereby altering the structure and number of chromosomes in the cell. Chromosomal changes of this kind may cause the affected cell to die when it attempts to divide, or they may alter its properties in various other ways.

      Chromosome breaks often heal spontaneously, but a break that fails to heal may cause the loss of an essential part of the gene complement; this loss of genetic material is called gene deletion. A germ cell thus affected may be capable of taking part in the fertilization process, but the resulting zygote may be incapable of full development and may therefore die in an embryonic state.

      When adjoining chromosome fibres in the same nucleus are broken, the broken ends may join together in such a way that the sequence of genes on the chromosomes is changed. For example, one of the broken ends of chromosome A may join onto a broken end of chromosome B, and vice versa in a process termed translocation. A germ cell carrying such a chromosome structural change may be capable of producing a zygote that can develop into an adult individual, but the germ cells produced by the resulting individual may include many that lack the normal chromosome complement and so yield zygotes that are incapable of full development; an individual affected in this way is termed semisterile. Because the number of his descendants is correspondingly lower than normal, such chromosome structural changes tend to die out in successive generations.

      As would be expected from target theory considerations, X rays and gamma rays given at high doses and high dose rates induce more two-break chromosome aberrations per unit dose than are produced at low doses and low dose rates. With densely ionizing radiation, by comparison, the yield of two-break aberrations for a given dose is higher than with sparsely ionizing radiation and is proportional to the dose irrespective of the dose rate. From these comparative dose-response relationships, it is inferred that a single X-ray track rarely deposits enough energy at any one point to break two adjoining chromosomes simultaneously, whereas the two-break aberrations that are induced by high-LET irradiation result preponderantly from single particle tracks.

      In irradiated human lymphocytes, the frequency of chromosome aberrations varies so predictably with the dose of radiation that it is used as a crude biologic dosimeter of exposure in radiation workers and other exposed persons. What effect, if any, an increase in the frequency of chromosome aberrations may have on the health of an affected individual is uncertain. Only a small percentage of all chromosome aberrations is attributable to natural background radiation; the majority result from other causes, including certain viruses, chemicals, and drugs.

Effects on organs of the body (somatic effects)
      A wide variety of reactions occur in response to irradiation in the different organs and tissues of the body. Some of the reactions occur quickly, while others occur slowly. The killing of cells in affected tissues, for example, may be detectable within minutes after exposure, whereas degenerative changes such as scarring and tissue breakdown may not appear until months or years afterward.

      In general, dividing cells are more radiosensitive than nondividing cells (see above Effects on the cell (radiation)), with the result that radiation injury tends to appear soonest in those organs and tissues in which cells proliferate rapidly. Such tissues include the skin, the lining of the gastrointestinal tract, and the bone marrow, where progenitor cells multiply continually in order to replace the mature cells that are constantly being lost through normal aging. The early effects of radiation on these organs result largely from the destruction of the progenitor cells and the consequent interference with the replacement of the mature cells, a process essential for the maintenance of normal tissue structure and function. The damaging effects of radiation on an organ are generally limited to that part of the organ directly exposed. Accordingly, irradiation of only a part of an organ generally causes less impairment in the function of the organ than does irradiation of the whole organ.

      Radiation can cause various types of injury to the skin, depending on the dose and conditions of exposure. The earliest outward reaction of the skin is transitory reddening (erythema) of the exposed area, which may appear within hours after a dose of 6 Gy or more. This reaction typically lasts only a few hours and is followed two to four weeks later by one or more waves of deeper and more prolonged reddening in the same area. A larger dose may cause subsequent blistering and ulceration of the skin and loss of hair, followed by abnormal pigmentation months or years later.

      The blood-forming cells of the bone marrow are among the most radiosensitive cells in the body. If a large percentage of such cells are killed, as can happen when intensive irradiation of the whole body occurs, the normal replacement of circulating blood cells is impaired. As a result, the blood cell count may become depressed and, ultimately, infection, hemorrhage, or both may ensue. A dose below 0.5–1 Sv generally causes only a mild, transitory depletion of blood-forming cells; however, a dose above 8 Sv delivered rapidly to the whole body usually causes a fatal depression of blood-cell formation.

      The response of the gastrointestinal tract is comparable in many respects to that of the skin. Proliferating cells in the mucous membrane that lines the tract are easily killed by irradiation, resulting in the denudation and ulceration of the mucous membrane. If a substantial portion of the small intestine is exposed rapidly to a dose in excess of 10 Gy, as may occur in a radiation accident, a fatal dysentery-like reaction results within a very short period of time.

Reproductive organs
      Although mature spermatozoa are relatively resistant to radiation, immature sperm-forming cells (spermatogonia) are among the most radiosensitive cells in the body. Hence, rapid exposure of both testes to a dose as low as 0.15 Sv may interrupt sperm-production temporarily, and a dose in excess of 4 Sv may be sufficient to cause permanent sterility in a certain percentage of men.

      In the human ovary (reproductive system disease), oocytes of intermediate maturity are more radiosensitive than those of greater or lesser maturity. A dose of 1.5–2.0 Sv delivered rapidly to both ovaries may thus cause only temporary sterility, whereas a dose exceeding 2–3 Sv is likely to cause permanent sterility in an appreciable percentage of women.

lens of the eye
      Irradiation can cause opacification (cataract) of the lens, the severity of which increases with the dose. The effect may not become evident, however, until many months after exposure. During the 1940s, some physicists who worked with the early cyclotrons developed cataracts as a result of occupational neutron irradiation, indicating for the first time the high relative biologic effectiveness of neutrons for causing lens damage. The threshold for a progressive, vision-impairing opacity, or cataract, varies from 5 Sv delivered to the lens in a single exposure to as much as 14 Sv delivered in multiple exposures over a period of months.

brain and sensory organs
      Generally speaking, humans do not sense a moderate radiation field; however, small doses of radiation (less than 0.01 Gy) can produce phosphene, a light sensation on the dark-adapted retina. American astronauts on the first spacecraft that landed on the Moon (Apollo 11, July 20, 1969) observed irregular light flashes and streaks during their flight, which probably resulted from single heavy cosmic-ray particles striking the retina. In various food-preference tests, rats, when given the choice, avoid radiation fields of even a few mGy. A dose of 0.03 Gy is sufficient to arouse a slumbering rat, probably through effects on the olfactory system, and a dose of the same order of magnitude can accelerate seizures in genetically susceptible mice. The mature brain and nervous system are relatively resistant to radiation injury, but the developing brain is radiosensitive to damage (see below).

       Symptoms of acute radiation sickness (hematopoietic form)The signs and symptoms resulting from intensive irradiation of a large portion of the bone marrow or gastrointestinal tract constitute a clinical picture known as radiation sickness, or the acute radiation syndrome. Early manifestations of this condition typically include loss of appetite, nausea, and vomiting within the first few hours after irradiation, followed by a symptom-free interval that lasts until the main phase of the illness (Table 11 (Symptoms of acute radiation sickness (hematopoietic form))).

      The main phase of the intestinal form of the illness typically begins two to three days after irradiation, with abdominal pain, fever, and diarrhea, which progress rapidly in severity and lead within several days to dehydration, prostration, and a fatal, shocklike state. The main phase of the hematopoietic form of the illness characteristically begins in the second or third week after irradiation, with fever, weakness, infection, and hemorrhage. If damage to the bone marrow is severe, death from overwhelming infection or hemorrhage may ensue four to six weeks after exposure unless corrected by transplantation of compatible unirradiated bone marrow cells.

       Symptoms of acute radiation sickness (hematopoietic form)The higher the dose received, the sooner and more profound are the radiation effects. Following a single dose of more than 5 Gy to the whole body, survival is improbable (Table 11 (Symptoms of acute radiation sickness (hematopoietic form))). A dose of 50 Gy or more to the head may cause immediate and discernible effects on the central nervous system, followed by intermittent stupor and incoherence alternating with hyperexcitability, epileptiform seizures, and death within several days (the cerebral form of the acute radiation syndrome).

       Symptoms of acute radiation sickness (hematopoietic form)When the dose to the whole body is between 6 and 10 Gy, the earliest symptoms are loss of appetite, nausea, and vomiting, followed by prostration, watery and bloody diarrhea, abhorrence of food, and fever (Table 11 (Symptoms of acute radiation sickness (hematopoietic form))). The blood-forming tissues are profoundly injured, and the white blood cell count may decrease within 15–30 days from about 8,000 per cubic millimetre to as low as 200. As a result of these effects, the body loses its defenses against microbial infection, and the mucous membranes lining the gastrointestinal tract may become inflamed. Furthermore, internal or external bleeding may occur because of a reduction in blood platelets. Return of the early symptoms, frequently accompanied by delirium or coma, presage death; however, symptoms may vary significantly from individual to individual. Complete loss of hair within 10 days has been taken as an indication of a lethally severe exposure.

      In the dose range of 1.5–5.0 Gy, survival is possible (though in the upper range improbable), and the symptoms appear as described above but in milder form and generally following some delay. Nausea, vomiting, and malaise may begin on the first day and then disappear, and a latent period of relative well-being follows. Anemia and leukopenia set in gradually. After three weeks, internal hemorrhages may occur in almost any part of the body, but particularly in mucous membranes. Susceptibility to infection remains high, and some loss of hair occurs. Lassitude, emaciation, and fever may persist for many weeks before recovery or death occurs.

      Moderate doses of radiation can severely depress the immunologic defense mechanisms, resulting in enhanced sensitivity to bacterial toxins, greatly decreased fixation of antigens, and reduced efficiency of antibody formation. Antibiotics, unfortunately, are of limited effectiveness in combating postirradiation infections. Hence, of considerable value are plastic isolators that allow antiseptic isolation of a person from his environment; they provide protection against infection from external sources during the period critical for recovery.

      Below a dose of 1.5 Gy, an irradiated person is generally able to survive intensive whole-body irradiation. The symptoms following exposure in this dose range are similar to those already described but milder and delayed. With a dose under 1 Gy, the symptoms may be so mild that the exposed person is able to continue his normal occupation in spite of measurable depression of his bone marrow. Some persons, however, suffer subjective discomfort from doses as low as 0.3 Gy. Although such doses may cause no immediate reactions, they may produce delayed effects that appear years later.

Effects on the growth and development of the embryo
      The tissues of the embryo, like others composed of rapidly proliferating cells, are highly radiosensitive. The types and frequencies of radiation effects, however, depend heavily on the stage of development of the embryo or fetus at the time it is exposed. For example, when exposure occurs while an organ is forming, malformation of the organ may result. Exposure earlier in embryonic life is more likely to kill the embryo than cause a congenital malformation, whereas exposure at a later stage is more likely to produce a functional abnormality in the offspring than a lethal effect or a malformation.

      A wide variety of radiation-induced malformations have been observed in experimentally irradiated rodents. Many of these are malformations of the nervous system, including microcephaly (reduced size of brain), exencephaly (part of the brain formed outside the skull), hydrocephalus (enlargement of the head due to excessive fluid), and anophthalmia (failure of the eyes to develop). Such effects may follow a dose of 1–2 Gy given at an appropriate stage of development. Functional abnormalities produced in laboratory animals by prenatal irradiation include abnormal reflexes, restlessness, and hyperactivity, impaired learning ability, and susceptibility to externally induced seizures. The abnormalities induced by radiation are similar to those that can be caused by certain virus infections, neurotropic drugs, pesticides, and mutagens.

      Abnormalities of the nervous system, which occur in 1–2 percent of human infants, were found with greater frequency among children born to women who were pregnant and residing in Hiroshima or Nagasaki at the time of the atomic explosions. The incidence of reduced head size and mental retardation in such children was increased by about 40 percent per Gy when exposure occurred between the eighth and 15th week of gestation, the age of greatest susceptibility to radiation.

      The period of maximal sensitivity for each developing organ is sharply circumscribed in time, with the result that the risk of malformation in a particular organ depends heavily on the precise stage of development at which the embryo is irradiated. The risk that a given dose will produce a particular malformation is thus much smaller if the dose is spread out over many days or weeks than if it is received during the few hours of the critical period itself. It also would appear that the induction of a malformation generally requires injury to many cells in a developing organ, so that there is little likelihood of such an effect resulting from the low doses and dose rates characteristic of natural background radiation.

Effects on the incidence of cancer
      Atomic-bomb survivors, certain groups of patients exposed to radiation for medical purposes, and some groups of radiation workers have shown dose-dependent increases in the incidence of certain types of cancer. The induced cancers have not appeared until years after exposure, however, and they have shown no distinguishing features by which they can be identified individually as having resulted from radiation, as opposed to some other cause. With few exceptions, moreover, the incidence of cancer has not been increased detectably by doses of less than 0.01 Sv.

      Because the carcinogenic effects of radiation have not been documented over a wide enough range of doses and dose rates to define the shape of the dose-incidence curve precisely, the risk of radiation-induced cancer at low levels of exposure can be estimated only by extrapolation from observations at higher dose levels, based on assumptions about the relation between cancer incidence and dose. For most types of cancer, information about the dose-incidence relationship is rather meagre. The most extensive data available are for leukemia and cancer of the female breast.

      The overall incidence of all forms of leukemia other than the chronic lymphatic type has been observed to increase roughly in proportion to dose during the first 25 years after irradiation. Different types of leukemia, however, vary in the magnitude of the radiation-induced increase for a given dose, the age at which irradiation occurs, and the time after exposure. The total excess of all types besides chronic lymphatic leukemia, averaged over all ages, amounts to approximately one to three additional cases of leukemia per year per 10,000 persons at risk per sievert to the bone marrow.

      Cancer of the female breast (breast cancer) also appears to increase in incidence in proportion to the radiation dose. Furthermore, the magnitude of the increase for a given dose appears to be essentially the same in women whose breasts were irradiated in a single, brief exposure (e.g., atomic-bomb survivors), as in those who were irradiated over a period of years (e.g., patients subjected to multiple fluoroscopic examinations of the chest or workers assigned to coating watch and clock dials with paint containing radium), implying that even small exposures widely separated in time exert carcinogenic effects on the breast that are fully additive and cumulative. Although susceptibility decreases sharply with age at the time of irradiation, the excess of breast cancer averaged over all ages amounts to three to six cases per 10,000 women per sievert each year.

      Additional evidence that carcinogenic effects can be produced by a relatively small dose of radiation is provided by the increase in the incidence of thyroid tumours that has been observed to result from a dose of 0.06–2.0 Gy of X rays delivered to the thyroid gland during infancy or childhood, and by the association between prenatal diagnostic X irradiation and childhood leukemia. The latter association implies that exposure to as little as 10–50 mGy of X radiation during intrauterine development may increase the subsequent risk of leukemia in the exposed child by as much as 40–50 percent.

       Estimated lifetime cancer risks attributed to low-level irradiationAlthough some, but not all, other types of cancer have been observed to occur with greater frequency in irradiated populations (Table 12 (Estimated lifetime cancer risks attributed to low-level irradiation)), the data do not suffice to indicate whether the risks extend to low doses. It is apparent, however, that the dose-incidence relationship varies from one type of cancer to another. From the existing evidence, the overall excess of all types of cancer combined may be inferred to approximate 0.6–1.8 cases per 1,000 persons per sievert per year when the whole body is exposed to radiation, beginning two to 10 years after irradiation. This increase corresponds to a cumulative lifetime excess of roughly 20–100 additional cases of cancer per 1,000 persons per sievert, or to an 8–40 percent per sievert increase in the natural lifetime risk of cancer.

      The above-cited risk estimates imply that no more than 1–3 percent of all cancers in the general population result from natural background ionizing radiation. At the same time, however, the data suggest that up to 20 percent of lung cancers in nonsmokers may be attributable to inhalation of radon and other naturally occurring radionuclides present in air.

Shortening of the life span
      Laboratory animals whose entire bodies are exposed to radiation in the first half of life suffer a reduction in longevity that increases in magnitude with increasing dose. This effect was mistakenly interpreted by early investigators as a manifestation of accelerated or premature aging. The shortening of life in irradiated animals, however, has since been observed to be attributable largely, if not entirely, to the induction of benign and malignant growths. In keeping with this observation is the finding that mortality from diseases other than cancer has not been increased detectably by irradiation among atomic-bomb survivors.

Protection against external radiation
       Some chemicals that exert radioprotective effects in laboratory animalsA growing number of substances have been found to provide some protection against radiation injury when administered prior to irradiation (Table 13 (Some chemicals that exert radioprotective effects in laboratory animals)). Many of them apparently act by producing anoxia or by competing for oxygen with normal cell constituents and radiation-produced radicals. All of the protective compounds tried thus far, however, are toxic, and anoxia itself is hazardous. As a consequence, their administration to humans is not yet practical.

      Diurnal changes in the radiosensitivity of rodents indicate that the factors responsible for daily biologic rhythms may also alter the responses of tissues to radiation. Such factors include the hormone thyroxine, a normal secretion of the thyroid gland. Other sensitizers at the cellular level include nucleic-acid analogues (e.g., 5-fluorouracil) as well as certain compounds that selectively radiosensitize hypoxic cells such as metronidazole.

      Radiosensitivity is also under genetic control to some degree, susceptibility varying among different inbred mouse strains and increasing in the presence of inherited deficiencies in capacity for repairing radiation-induced damage to DNA. Germ-free mice, which spend their entire lives in a sterile environment, also exhibit greater resistance to radiation than do animals in a normal microbial environment owing to elimination of the risk of infection.

      For many years it was thought that radiation disease was irreversible once a lethal dose had been received. It has since been found that bone-marrow cells administered (transplant) soon after irradiation may enable an individual to survive an otherwise lethal dose of X rays, because these cells migrate to the marrow of the irradiated recipient, where they proliferate and repopulate the blood-forming tissues. Under these conditions bone-marrow transplantation is feasible even between histo-incompatible individuals, because the irradiated recipient has lost the ability to develop antibodies against the injected “foreign” cells. After a period of some months, however, the transplanted tissue may eventually be rejected, or it may develop an immune reaction against the irradiated host, which also can be fatal. The transplantation of bone-marrow cells has been helpful in preventing radiation deaths among the victims of reactor accidents, as, for example, those injured in 1986 at the Chernobyl nuclear power plant in Ukraine, then in the Soviet Union. It should be noted, however, that cultured or stored marrow cells cannot yet be used for this purpose.

Control of radiation risks
       Values for the maximum permissible concentration (MPC) of certain radionuclides Estimated lifetime cancer risks attributed to low-level irradiation Recommended Exposure Limits*, TableIn view of the fact that radiation is now assumed to play a role in mutagenic or carcinogenic activity, any procedure involving radiation exposure is considered to entail some degree of risk. At the same time, however, the radiation-induced risks associated with many activities are negligibly small in comparison with other risks commonly encountered in daily life. Nevertheless, such risks are not necessarily acceptable if they can be easily avoided or if no measurable benefit is to be gained from the activities with which they are associated. Consequently, systematic efforts are made to avoid unnecessary exposure to ionizing radiation in medicine, science, and industry. Toward this end, limits have been placed on the amounts of radioactivity (Tables 9 (Values for the maximum permissible concentration (MPC) of certain radionuclides) and 12 (Estimated lifetime cancer risks attributed to low-level irradiation)) and on the radiation doses (Table 14 (Recommended Exposure Limits*, Table)) that the different tissues of the body are permitted to accumulate in radiation workers or members of the public at large.

      Although most activities involving exposure to radiation for medical purposes are highly beneficial, the benefits cannot be assumed to outweigh the risks in situations where radiation is used to screen large segments of the population for the purpose of detecting an occasional person with an asymptomatic disease. Examples of such applications include the “annual” chest X-ray examination and routine mammography. Each use of radiation in medicine (and dentistry) is now evaluated for its merits on a case-by-case basis.

      Other activities involving radiation also are assessed with care in order to assure that unnecessary exposure is avoided and that their presumed benefits outweigh their calculated risks. In operating nuclear power plants, for example, much care is taken to minimize the risk to surrounding populations. Because of such precautions, the total impact on health of generating a given amount of electricity from nuclear power is usually estimated to be smaller than that resulting from the use of coal for the same purpose, even after allowances for severe reactor accidents such as the one at Chernobyl.

Cornelius A. Tobias Arthur Canfield Upton

Biologic effects of non-ionizing radiation

Effects of Hertzian waves and infrared rays
      The effects of Hertzian waves (electromagnetic waves in the radar and radio range) and of infrared rays usually are regarded as equivalent to the effect produced by heating. The longer radio waves induce chiefly thermal agitation of molecules and excitation of molecular rotations, while infrared rays excite vibrational modes of large molecules and release fluorescent emission as well as heat. Both of these types of radiation are preferentially absorbed by fats containing unsaturated carbon chains.

      The fact that heat production resulted from bombardment of tissue with high-frequency alternating current (wavelengths somewhat longer than the longest radio waves) was discovered in 1891, and the possibility of its utilization for medical purposes was realized in 1909, under the term diathermy. This method of internal heating is beneficial for relieving muscle soreness and sprain (see also below). Diathermy can be harmful, however, if so much internal heat is given that the normal cells of the body suffer irreversible damage. Since humans have heat receptors primarily in their skin, they cannot be forewarned by pain when they receive a deep burn from diathermy. Sensitive regions easily damaged by diathermy are those having reduced blood circulation. Cataracts of the eye lens have been produced in animals by microwave radiation applied in sufficient intensity to cause thermal denaturation of the lens protein.

      Microwave ovens have found widespread use in commercial kitchens and private homes. These can heat and cook very rapidly and, if used properly, constitute no hazard to operators. In the radio-television industry and in the radar division of the military, persons are sometimes exposed to high densities of microwave radiation. The hazard is particularly pronounced with exposure to masers, capable of generating very high intensities of microwaves (e.g., carbon dioxide masers). The biologic effects depend on the absorbency of tissues. At frequencies higher than 150 megahertz, significant absorption takes place. The lens of the human eye is most susceptible to frequencies around 3,000 megahertz, which can produce cataracts. At still higher frequencies, microwaves interact with superficial tissues and skin, in much the same manner as infrared rays.

      Acute effects of microwaves become significant if a considerable temperature rise occurs. Cells and tissues eventually die at temperatures of about 43° C. Microwave heating is minimized if the heat that results from energy absorption is dissipated by radiation, evaporation, and heat conduction. Normally one-hundredth of a watt (10 milliwatts) can be so dissipated, and this power limit generally has been set as the permissible dose. Studies with animals have indicated that, below the permissible levels, there are negligible effects to various organ systems. Microwaves or heat applied to testes tend strongly to decrease the viability of sperm. This effect, however, is not significant at the “safe” levels.

      In the late 1980s, some investigators in the Soviet Union documented a variety of nonthermal effects of microwaves and recommended about 1,000 times lower safe occupational dose levels than are still in force in the United States today. Most prominent among the nonthermal effects appear to be those on the nervous system. Such effects have resulted in untimely tiring, excitability, and insomnia registered by persons handling high-frequency radio equipment. Nonthermal effects have been observed on the electroencephalogram of rabbits. These effects may be due to changes in the properties of neural membranes or to denaturation of macromolecules.

      A significant part of solar energy reaches the Earth in the form of infrared rays. Absorption and emission by the human body of these rays play an important part in temperature exchange and regulation of the body. The principles of infrared emission and absorption must be considered in the design of air conditioning and clothing.

      Overdosage of infrared radiation, usually resulting from direct exposure to a hot object (including heating lamps) or flame, can cause severe burns (burn). While infrared exposure is a hazard near any fire, it is particularly dangerous in the course of nuclear chain reactions. In the course of a nuclear detonation, a brief but very intense emission of infrared occurs, together with visible and ultraviolet light emitted from the fireball (flash burns). Of the total energy of nuclear explosion, as much as one-third may be in the form of thermal radiation, moving with the velocity of light. The rays will arrive almost instantaneously at regions removed from the source by only a few kilometres. Smoke or fog can effectively scatter or absorb the infrared components, and even thin clothing can greatly reduce the severity of burn effects.

Effects of visible and ultraviolet (ultraviolet radiation) light
      Life could not exist on Earth without light from the Sun. Plants utilize the energy of the Sun's rays in the process of photosynthesis to produce carbohydrates and proteins, which serve as basic organic sources of food and energy for animals. Light has a powerful regulating influence on many biologic systems. Most of the strong ultraviolet rays of the Sun, which are hazardous, are effectively absorbed by the upper atmosphere. At high altitudes and near the Equator, the ultraviolet intensity is greater than at sea level or at northern latitudes.

      Ultraviolet light of very short wavelength, below 2200 angstroms, is highly toxic for cells; in the intermediate range, the greatest killing effectiveness on cells is at about 2600 angstroms. The nucleic acids of the cell, of which genetic material is composed, strongly absorb rays in this region. This wavelength, readily available in mercury vapour, xenon, or hydrogen arc lamps, has great effectiveness for germicidal purification of the air.

      Since penetration of visible and ultraviolet light in body tissues is small, only the effects of light on skin and on the visual apparatus are of consequence. When incident light exerts its action on the skin without additional external predisposing factors, scientists speak of intrinsic action. In contrast, a number of chemical or biologic agents may condition the skin for action of light; these latter phenomena are grouped under photodynamic action. Visible light, when administered following lethal doses of ultraviolet, is capable of causing recovery of the cells exposed. This phenomenon, referred to as photorecovery, has led to the discovery of various enzyme systems that are capable of restoring damaged nucleic acids in genes to their normal form. It is probable that photorecovery mechanisms are continually operative in some plants exposed to the direct action of sunlight.

      The surface of the Earth is protected from the lethal ultraviolet rays of the Sun by the top layers of the atmosphere, which absorb far ultraviolet, and by ozone molecules in the stratosphere, which absorb most of the near ultraviolet. Even so, it is believed that an enzymatic mechanism operating in the skin cells of individuals continually repairs the damage caused by ultraviolet rays to the nucleic acids of the genes. Many scientists believe that chlorofluorocarbons used in aerosol spray products and in various technical applications are depleting the stratospheric ozone layer, thus exposing persons to more intense ultraviolet radiation at ground level.

      There is some evidence to indicate that not only overall light intensity but also special compositions have differential effects on organisms. For example, in pumpkins, red light favours the production of pistillate flowers, and blue light leads to development of staminate flowers. The ratio of females to males in guppies is increased by red light. Red light also appears to accelerate the rate of proliferation of some tumours in special strains of mice. The intensity of incident light has an influence on the development of light-sensing organs; the eyes of primates reared in complete darkness, for instance, are much retarded in development.

Intrinsic action
      Light is essential to the human body because of its biosynthetic action. Ultraviolet light induces the conversion of ergosterol and other vitamin precursors present in normal skin to vitamin D, an essential factor for normal calcium deposition in growing bones. While some ultraviolet light appears desirable for the formation of vitamin D, an excess amount is deleterious. Humans have a delicate adaptive mechanism that regulates light exposure of the more sensitive deeper layers of the skin. The transmission of light depends on the thickness of the upper layers of the skin and on the degree of skin pigmentation. All persons, with the exception of albinos, are born with varying amounts of melanin pigment in their skin. Exposure to light further enhances the pigmentation already present and can induce production of new pigment granules. The therapeutic possibilities of sunlight and ultraviolet light became apparent around 1900, with popularization of the idea that exposure of the whole body to sunlight promotes health.

      By that time, it was already known that large doses of ultraviolet radiation cause sunburn, the wavelength of about 2800 angstroms being most effective. It induces reddening and swelling of the skin (skin disease) (owing to dilation of the blood vessels), usually accompanied by pain. In the course of recovery, epidermal cells are proliferated, melanin is secreted, and the outer corneal layer of dead cells is thickened. In 1928 it was first shown clearly that prolonged or repeated exposure to ultraviolet light leads to the delayed development of skin cancer. The fact that ultraviolet light, like X radiation, is mutagenic may explain its ability to cause skin cancer, but the detailed mechanism of cancer induction is not yet completely understood. There seems very little doubt, however, that skin cancer in humans is in some cases correlated with prolonged exposure to large doses of sunlight. Among blacks who are protected by rich melanin formation and thickened corneal structure of the skin, incidence of cancer of the skin is several times less frequent than it is among whites living at the same latitude.

      There are a number of diseases in humans and other animals in which light sensitivity is involved; for example, hydroa, which manifests itself in blisters on parts of the body exposed to sunlight. It has been suggested that this disease results from a light-sensitive porphyrin compound found in the blood.

      Actually there are many organic substances and various materials of biologic origin that make cells sensitive to light. When eosin is added to a suspension of human red blood corpuscles exposed to light, the red corpuscles will break up in a process called hemolysis. Other typical photodynamic substances are rose bengal, hematoporphyrin, and phylloerythrin—all are dyes capable of fluorescence. Their toxicity manifests itself only in the presence of light and oxygen.

      Some diseases in domestic animals result from ingestion of plants having photodynamic pigments. For example, St. Johnswort's disease is caused by the plant Hypericum. Fagopyrism results from eating buckwheat. In geeldikopp (“yellow thick head”), the photodynamic agent is produced in the animal's own intestinal tract from chlorophyll derived from plants. In humans the heritable condition of porphyria frequently is associated with light sensitivity, as are a number of somewhat ill-defined dermatologic conditions that result from exposure to sunlight. The recessively inherited rare disease xeroderma pigmentosum also is associated with light exposure; it usually results in death at an early age from tumours of the skin that develop on exposed areas. The cells of such individuals possess a serious genetic defect: they lack the ability to repair nucleic-acid lesions caused by ultraviolet light.

      Certain drugs (e.g., sulfanilamide) sensitize some persons to sunlight. Many cases are known in which ingestion of or skin contact with a photodynamic substance was followed by increased light sensitivity.

Effects on development and biologic rhythms
      In addition to its photosynthetic effect, light exerts an influence on growth and spatial orientation of plants. This phototropism is associated with yellow pigments and is particularly marked in blue light. The presence of illumination is a profound modifier of the cellular activities in plants as well. For example, while some species of blue-green algae carry out photosynthesis in the presence of light, they do not undergo cell division.

      Diffuse sensitivity to light also exists in several phyla of animals. Many protozoans react to light. Chameleons, frogs, and octopuses change colour under the influence of light. Such changes are ascribed to special organs known as chromatophores (chromatophore), which are under the influence of the nervous system or endocrine system. The breeding habits and migration of some birds are set in motion by small consecutive changes in the daily cycle of light.

      Light is an important controlling agent of recurrent daily physiological alterations (biological rhythm) (circadian rhythms (circadian rhythm)) in many animals, including humans in all likelihood. Lighting cycles have been shown to be important in regulating several types of endocrine (endocrine system) function: the daily variation in light intensity keeps the secretion of adrenal steroids in synchrony; the annual breeding cycles in many mammals and birds appear to be regulated by light. Ambient light somehow influences the secretions of a tiny gland, the pineal body (pineal gland), located near the cerebellum. The pineal body, under the action of enzymes, produces melanotonin, which in higher concentrations slows down the estrous cycle; low levels of melanotonin, caused by exposure of animals to light, accelerates estrus. It is believed that light stimulates the retina, and information is then transmitted by sympathetic nerves to the pineal body.

Effects on the eyes (eye, human)
      The wavelength of light that produces sunburn also can cause inflammation of the cornea of the eye. This is what occurs in snow blindness or after exposure to strong ultraviolet light sources. Unusual sensitivities have been reported. Ultraviolet light, like infrared or penetrating radiations, can also cause cataract of the eye lens, a condition characterized by denatured protein in the fibrous cells forming the lens (see above Major types of radiation injury: Lens of the eye (radiation)). The retina usually is not reached by ultraviolet light, but large doses of visible and infrared light can irreversibly bleach the visual pigments, as in sun blindness. Numerous pathological conditions of the eye are accompanied by abnormal light sensitivity and pain, a condition that is known as photophobia. The pain appears to be associated with reflex movements of the iris and reflex dilation of the blood vessels of the conjunctiva. Workers exposed to ultraviolet-light sources or to atomic flashes need to wear protective glasses.

Cornelius A. Tobias

Applications of radiation

Medical applications
      The uses of radiation in diagnosis and treatment have multiplied so rapidly in recent years that one or another form of radiation is now indispensable in virtually every branch of medicine. The many forms of radiation that are used include electromagnetic waves of widely differing wavelengths (e.g., radio waves, visible light, ultraviolet radiation, X rays, and gamma rays), as well as particulate radiations of various types (e.g., electrons, fast neutrons, protons, alpha particles, and pi-mesons).

Imaging techniques
      Advances in techniques for obtaining images of the body's interior have greatly improved medical diagnosis. New imaging methods include various X-ray systems, positron emission tomography, and nuclear magnetic resonance imaging.

X-ray systems
      In all such systems, a beam of X radiation is shot through the patient's body, and the rays that pass through are recorded by a detection device. An image is produced by the differential absorption of the X-ray photons by the various structures of the body. For example, the bones absorb more photons than soft tissues; they thus cast the sharpest shadows, with the other body components (organs, muscles, etc.) producing shadows of varying intensity.

      The conventional X-ray system produces an image of all structures in the path of the X-ray beam, so that a radiograph of, say, the lungs shows the ribs located in front and as well as in back. Such extraneous details often make it difficult for the physician examining the X-ray image to identify tumours or other abnormalities on the lungs. This problem has been largely eliminated by computerized tomographic (CT (computerized axial tomography)) scanning, which provides a cross-sectional image of the body part being scrutinized. Since its introduction in the 1970s, CT scanning, also called computerized axial tomography (CAT), has come to play a key role in the diagnosis and monitoring of many kinds of diseases and abnormalities.

      In CT scanning a narrow beam of X rays is rotated around the patient, who is surrounded by several hundred X-ray photon detectors that measure the strength of the penetrating photons from many different angles. The X-ray data are analyzed, integrated, and reconstructed by a computer to produce images of plane sections through the body onto the screen of a television-like monitor. Computerized tomography enables more precise and rapid visualization and location of anatomic structures than has been possible with ordinary X-ray techniques. In many cases, lesions can be detected without resorting to exploratory surgery.

      This imaging technique permits physicians to determine patterns of blood flow, blood volume, oxygen perfusion, and various other physiological, metabolic, and immunologic parameters. It is used increasingly in diagnosis and research, especially of brain and heart functions.

      PET involves the use of chemical compounds “labeled” with short-lived positron-emitting isotopes such as carbon-11 and nitrogen-13, positron cameras consisting of photomultiplier-scintillator detectors, and computerized tomographic reconstruction techniques. After an appropriately labeled compound has been injected into the body, quantitative measurements of its activity are made throughout the sections of the body being scanned by the detectors. As the radioisotope disintegrates, positrons are annihilated by electrons, giving rise to gamma rays that are detected simultaneously by the photomultiplier-scintillator combinations positioned on opposite sides of the patient.

      This method, also referred to as magnetic resonance imaging (MRI), involves the beaming of high-frequency radio waves into the patient's body while it is subjected to a strong magnetic field. The nuclei of different atoms in the body absorb radio waves at different frequencies under the influence of the magnetic field. The NMR technique makes use of the fact that hydrogen nuclei (protons) respond to an applied radio frequency by reemitting radio waves of the same frequency. A computer analyzes the emissions from the hydrogen nuclei of water molecules in body tissues and constructs images of anatomic structures based on the concentrations of such nuclei. This use of proton density makes it possible to produce images of tissues that are comparable, and in some cases superior, in resolution and contrast to those obtained with CT scanning. Moreover, since macroscopic movement affects NMR signals, the method can be adapted to measure blood flow. The ability to image atoms of fluorine-19, phosphorus-31, and other elements besides hydrogen permit physicians and researchers to use the technique for various tracer studies as well. (For information on tracer studies, see radioactivity: Applications of radioactivity (radioactivity).)

Other radiation-based medical procedures

Radionuclides in diagnosis
      Radionuclides have come to play a key role in certain diagnostic procedures. These procedures may be divided into two general types: (1) radiographic imaging techniques for visualizing the distribution of an injected radionuclide within a given organ as a means of studying the anatomic structure of the organ; and (2) quantitative assay techniques for measuring the absorption and retention of a radionuclide within an organ as a means of studying the metabolism of the organ.

      Notable among the radionuclides used for imaging purposes is technetium-99m, a gamma-ray emitter with a six-hour half-life, which diffuses throughout the tissues of the body after its administration. Among the radionuclides suitable for metabolic studies, iodine-131 is one of the most widely used. This gamma-ray emitter has a half-life of eight days and concentrates in the thyroid gland, and so provides a measure of thyroid function.

Treating cancer and other diseases with highly energetic forms of ionizing radiation
      In addition to X rays and gamma rays, densely ionizing particles—neutrons, protons, mesons, alpha particles, and heavy ions, for example—have been used increasingly to treat cancer and other lesions. Such high-LET radiations (see above The passage of matter rays: Linear energy transfer and track structure (radiation)) offer potential advantages over conventional X rays and gamma rays in that they have per given dose greater capacity to damage tumours, particularly deep-seated ones, and can be applied more precisely to the lesion under treatment, causing less injury to surrounding tissue. The results of these radiations in cancer treatment, though preliminary, are promising.

      Ultraviolet radiation (“Wood's” light) is used diagnostically to detect fluorescent materials that are present in certain disorders—e.g., some fungal diseases of the skin. It is also widely employed in combination with a radiosensitizing agent such as 8-methoxypsoralen to treat psoriasis. In this approach, known as PUVA therapy, the entire surface of the skin is bathed repeatedly with ultraviolet radiation.

      Intense visible light is used in treating newborns' jaundice, a disease characterized by the accumulation of the pigment bilirubin in the bloodstream during the first few days of life. Since wavelengths of 420–480 nanometres absorbed in the skin expedite detoxification and elimination of the pigment, the affected infant is bathed in visible light for 12–24 hours in treating the disorder.

Treatment with lasers (laser)
      The laser is used increasingly for surgery, as it has proved to be a finely controlled and relatively bloodless means of dissecting and destroying tissue. By “tuning” the laser to different wavelengths, one can vary the extent to which its light is absorbed in particular cells or cellular inclusions. Certain types of lesions, such as birthmarks of the “port-wine stain” variety, can thus be destroyed more or less selectively, with minimal damage to surrounding tissues.

      The laser also is well-suited for treating lesions of the inner eye, since a beam of laser light can pass through the intact cornea and lens without harming them. In addition, lasers are used together with optical fibres to treat lesions inside blood vessels and in other locations that are not readily accessible to standard surgical intervention. In this procedure, a fibre-optic probe is inserted into a vessel or body cavity by means of cannulas.

      Microwave radiation has long been used for warming internal parts of the body in treating deep-seated inflammations and various other disorders. This approach, termed diathermy, is also being explored as a means of inducing hyperthermia in tumour tissue as an adjunct to radiation therapy (or chemotherapy) in the treatment of certain types of cancer.

Arthur Canfield Upton

Applications in science and industry
      The principal applications of photochemistry (including photography) are in the initiation of reactions by light that can pass through glass or quartz windows. Such light has a wavelength of not less than about 185 nanometres. Light of shorter wavelength is also effective, but the windows required (sapphire, lithium fluoride, or extraordinarily thin aluminum) and the associated mechanical difficulties seriously limit application of photochemical methods in the range from 185 nanometres down to a conceivable lower limit of about 85 nanometres. Photochemical techniques are particularly applicable when a specific initial process (the breakage of a particular bond in a molecule of a particular substance, for example) is required. For such purposes, high-intensity ultraviolet lamps are generally employed, the window is either glass or quartz, and the initiation reaction is limited to the relatively thin layers in which the light is absorbed. The processes include the photochlorination of aromatic compounds (such as benzene, toluene, and xylene), sulfhydration of olefins, production of cyclohexanone oxime, photopolymerization (principally in surface-curing processes), sulfoxidation, and vitamin-D synthesis. Tunable lasers provide a potential means of initiating photochemical processes of practical interest, one such example being the separation of isotopes.

High-energy radiation
      The large-scale use of such ionizing radiation for modifying and synthesizing materials, known as radiation processing, represents a minor yet significant technology. It involves irradiating materials either with a beam of electrons produced by a high-voltage particle accelerator or with gamma rays emitted by the radioisotope cobalt-16 or, in a few cases, cesium-137. The electrons are generally accelerated to an energy range of 0.15–10 MeV. (By comparison, the electron energy in a typical television set is only 0.025 MeV.) The gamma rays given off by cobalt-16 have an energy of 1.25 MeV, while those emitted by cesium-137 have approximately half that amount.

      Exposure to such electrons and gamma rays does not induce radioactivity in the materials irradiated, and so the technique can be used in the manufacture or processing of many kinds of consumer and industrial products. Moreover, radiation processing has several major advantages over conventional technologies. It consumes far less energy than thermally and chemically initiated processes and at the same time causes less environmental pollution. Paints and certain other coatings, for example, can be cured at room temperature with one-tenth of the energy required in heat curing. Radiation preservation of food (food processing) involves substantially less energy expenditure than that associated with either refrigeration or canning. A radiation source that releases 1 kilowatt of gamma energy (roughly equivalent to the electrical requirements of a toaster) can irradiate 10 tons of potatoes per hour; the exposure to a small dose of ionizing radiation inhibits sprouting and thereby delays spoilage.

      Because of such advantages, radiation processing has found increasingly wider application. It has proved particularly valuable in the processing of plastics (plastic). Chemical reactions induced by electron-beam irradiation permit the cross-linking of polymers that make up the foamed plastic used for sound and thermal insulation. A large fraction of the wire and cable employed in high-temperature applications and much of the wiring in telecommunications equipment are covered by insulation cross-linked by electron irradiation. The heat-shrinkable polyethylene packaging for hams and turkeys and various other poultry products is manufactured by the same process. The coating of certain audio and video recording tape is cured by exposure to electron beams, as is the rubber in a large percentage of automobile tires. Sterilization of disposable medical supplies, such as syringes, blood transfusion kits, and hospital gowns, is usually done with gamma rays. Other potential applications of radiation processing include the treatment of a wide assortment of food products so as to reduce the amount of chemical preservatives employed, the treatment of sewage for pathogen reduction, and the precipitation of sulfur dioxide and nitrogen oxide (the primary source materials for acid rain) from the stack gases of electric power plants and smelting facilities that burn fossil fuels.

      Radiation source technology has developed to a point where reliable, safe, and inexpensive sources are readily available. When electron accelerators are used, radioactivity is not involved in any aspect of the process and there is no conceivable hazard to the surrounding community. In processing facilities that use gamma radiation, the source is encapsulated in a double layer of stainless steel to prevent the escape of radioactivity to the environment. Other safeguards minimize the possibility of accidental exposure of either the plant personnel or the population at large.

Lasers (laser)
      As noted above, lasers have become a valuable tool in medicine. They also have important uses in a number of other areas, as, for example, communications. Laser light can carry voice messages and digitally encoded information and can do so in large amounts because of its high frequency. Except in satellite-to-satellite communications, laser beams are transmitted via optical fibres. The speed with which the focal spot of a narrow laser beam can be controlled makes it suitable for a variety of applications in information processing—e.g., use in optical scanners, optical disc storage systems, and certain types of computer printers.

      A highly intense laser beam can instantly vaporize the surface of a target. When laser pulses are concentrated on frozen deuterium-tritium pellets, they can initiate nuclear fusion (see nuclear fusion (nuclear fission)). High-powered lasers can be used as space weapons to destroy reconnaissance and communications satellites and perhaps even ballistic missiles. These same capabilities have led to the use of lasers in research as well as in surgery. The laser microprobe is used for microanalysis of surface composition. Laser beams have been found to have a selective effect on cellular components, or organelles: (organelle) those components that absorb light of the wavelength of the beam are destroyed, whereas transparent parts of the cells remain unaffected. Organelles such as mitochondria, which are responsible for cell respiration, or chloroplasts, which are involved in plant-cell photosynthesis, can be separately studied in this manner.

      An intense beam of laser light can be used for small-scale cutting, scribing, and welding in certain industrial processes. Laser “pens” capable of producing such high-intensity light beams have proved useful in the assembly of various electronic components, such as computer memory and logic units consisting of integrated arrays of microcircuit elements.

      The use of special dyes can alter laser action. The availability of high-pulse-intensity laser beams is also revolutionizing microscopy. It is possible to photograph microaction in a small fraction of a second and to use holography for image synthesis.

Cornelius A. Tobias Joseph Silverman Ed.

Additional Reading

Historical works include Max Planck, Introduction to Theoretical Physics, vol. 4, Theory of Light (1932, reprinted 1957; originally published in German, 1927), the classic work on the subject of light and quanta. Other forms of electromagnetic radiation are covered in Otto Glasser, Wilhelm Conrad Röntgen and the Early History of the Roentgen Rays (1933; originally published in German, 1931). See also R.W. Ditchburn, Light, 3rd ed., 2 vol. (1976), a well-presented text on physical optics that, though not too mathematical, does require understanding of the use of differential equations.

Interaction of radiation with matter
Gerhard K. Rollefson and Milton Burton, Photochemistry and the Mechanism of Chemical Reactions (1939, reprinted 1946); William Albert Noyes and Philip Albert Leighton, The Photochemistry of Gases (1941, reprinted 1966), are both classic works that include material on internal conversion and predissociation. The language of intersystem crossing is discussed in detail in a comprehensive text, Jack G. Calvert and James N. Pitts, Photochemistry (1966). The actual effects of radiation on solids are thoroughly summarized in Hans A. Bethe and Julius Ashkin, “Passage of Radiations Through Matter,” in Emilio Segrè (ed.), Experimental Nuclear Physics, vol. 1 (1953), pp. 166–357; G.J. Dienes and G.H. Vineyard, Radiation Effects in Solids (1957); and Douglas S. Billington and James H. Crawford, Jr., Radiation Damage in Solids (1961). For a survey of radiation effects on aqueous solutions and organic compounds, see J.W.T. Spinks and R.J. Woods, An Introduction to Radiation Chemistry, 2nd ed. (1976); Actions chimiques et biologiques des radiations (annual 1955–71), the first survey on a large variety of subjects in radiation chemistry written by scientists largely about their own work—some volumes have been translated into English with the title, The Chemical and Biological Action of Radiations; and Max S. Matheson and Leon M. Dorfman, Pulse Radiolysis (1969), an excellent book on techniques in radiation chemistry. Advances in Photochemistry (irregular) is concerned mainly with surveys of advances in the field. See also J.F. Ziegler (ed.), Ion Implantation: Science and Technology (1984), a treatment of ion implantation mechanisms, techniques, effects, and practical applications; and Orlando Auciello and Roger Kelly (eds.), Ion Bombardment Modification of Surfaces: Fundamentals and Applications (1984), covering surface alteration mechanisms with major emphasis on topographical effects.Milton Burton Asokendu Mozumder Myron Luntz

Radiological units and measurements
For descriptions, see Ralph E. Lapp and Howard L. Andrews, Nuclear Radiation Physics, 4th ed. (1972); and International Commission on Radiation Units and Measurements, Radiation Quantities and Units (1980).

Biologic effects of radiation
General information is given in Charles Wesley Shilling (ed.), Atomic Energy Encyclopedia in the Life Sciences (1964). Introductory information on radiation biology is given in J.E. Coggle, Biological Effects of Radiation, 2nd ed. (1983); John W. Gofman, Radiation and Human Health (1981); Daniel S. Grosch and Larry E. Hopwood, Biological Effects of Radiations, 2nd ed. (1979); and Eric J. Hall, Radiation and Life, 2nd ed. (1984). More specialized topics are covered in Assembly on Life Sciences (U.S.) Committee on the Biological Effects of Ionizing Radiations, The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, 1980 (1980); Merrill Eisenbud, Environmental Radioactivity: From Natural, Industrial, and Military Sources, 3rd ed. (1987); Donald J. Pizzarello and Richard L. Witcofski, Medical Radiation Biology, 2nd ed. (1982); United Nations Scientific Committee on the Effects of Atomic Radiation, Ionizing Radiation: Sources and Biological Effects (1982), and Genetic and Somatic Effects of Ionizing Radiation (1986); Arthur C. Upton, Radiation Injury: Effects, Principles, and Perspectives (1969); and Arthur C. Upton et al. (eds.), Radiation Carcinogenesis (1986).

Microwave radiation is treated in National Council of Radiation Protection and Measurements, Biological Effects of Ultrasound: Mechanisms and Clinical Implications (1983), and Biological Effects and Exposure Criteria for Radiofrequency and Electromagnetic Fields (1986); and R.C. Petersen, “Bioeffects of Microwaves: A Review of Current Knowledge,” Journal of Occupational Medicine, 25(2):103–110 (February 1983). Visible and ultraviolet radiations are the subject of Walter Harm, Biological Effects of Ultraviolet Radiation (1980); A. Jarret (ed.), The Photobiology of the Skin: Lasers and the Skin (1984); Kendric C. Smith (ed.), Topics in Photomedicine (1984); and Richard J. Wurtman, Michael J. Baum, and John T. Potts, Jr. (eds.), The Medical and Biological Effects of Light (1985).

Nuclear war
Radiation effects of a nuclear war are discussed in Samuel Glasstone and Philip J. Dolan (eds.), The Effects of Nuclear Weapons, 3rd ed. (1977); Jean Petersen and Don Hinrichsen (eds.), Nuclear War: The Aftermath (1982); Julius London and Gilbert F. White (eds.), The Environmental Effects of Nuclear War (1984); and Fredric Solomon and Robert Q. Marston (eds.), The Medical Implications of Nuclear War (1986).

Radiation protection and safety
Procedures and recommendations for protection are analyzed in International Commission on Radiological Protection, Recommendations of the International Commission on Radiological Protection (1977, reprinted with supplements, 1987), and Nonstochastic Effects of Ionizing Radiation (1984); National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States (1987); and Marilyn E. Noz and Gerald Q. Maguire, Jr., Radiation Protection in the Radiologic and Health Sciences, 2nd ed. (1985).

Applications of radiation
Radiological imaging techniques are explored in R.P. Clark and M.P. Goff (eds.), Recent Developments in Medical and Physiological Imaging (1986); W.-D. Heiss and M.F. Phelps (eds.), Positron Emission Tomography of the Brain (1983); Alexander R. Magulis and Charles A. Gooding (eds.), Diagnostic Radiology, 1987 (1987); and Albert A. Moss, Ernest J. Ring, and Charles B. Higgins (eds.), NMR, CT, and Interventional Radiology (1984). Radiation therapy is addressed by Gilbert H. Fletcher, Textbook of Radiotherapy, 3rd ed. (1980); and Ernest J. Ring and Gordon K. McLean, Interventional Radiology: Principles and Techniques (1981). Specific uses of phototherapy are outlined in Audrey K. Brown and Jane Showacre (eds.), Phototherapy for Neonatal Hyperbilirubinemia: Long-Term Implications (1977); Wayne F. March (ed.), Ophthalmic Lasers: Current Clinical Uses (1984); and Warwick L. Morison, Phototherapy and Photochemotherapy of Skin Disease (1983).Arthur Canfield Upton

Scientific and industrial
A review of ionizing radiation processing in medicine and industrial manufacturing is found in Vitomir Markovic, “Modern Tools of the Trade,” International Atomic Energy Agency Bulletin, 27(1):33–39 (Spring 1985). Industrial uses are explored in Joseph Silverman, “Radiation Processing: The Industrial Applications of Radiation Chemistry,” Journal of Chemical Education, 58(2):168–173 (Feb. 1981). International Atomic Energy Agency, Industrial Application of Radioisotopes and Radiation Technology (1982), is a collection of conference papers.Joseph Silverman

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