/ak tiv"i tee/, n., pl. activities.1. the state or quality of being active: There was not much activity in the stock market today. He doesn't have enough physical activity in his life.2. a specific deed, action, function, or sphere of action: social activities.3. work, esp. in elementary grades at school, that involves direct experience by the student rather than textbook study.4. energetic activity; animation; liveliness.5. a use of energy or force; an active movement or operation.6. normal mental or bodily power, function, or process.7. Physical Chem. the capacity of a substance to react, corrected for the loss of reactivity due to the interaction of its constituents.8. Physics.a. the number of atoms of a radioactive substance that disintegrate per unit of time, usually expressed in curies.b. radioactivity.9. an organizational unit or the function it performs.[1520-30; ( < MF) < ML activitas. See ACTIVE, -ITY]
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* * *in radioactive-decay processes, the number of disintegrations per second, or the number of unstable atomic nuclei that decay per second in a given sample. Activity is determined by counting, with the aid of radiation detectors and electronic circuits, the number of particles and photons (pulses of electromagnetic energy) ejected from a radioactive material during a convenient time interval. This experimental count, however, must be interpreted in the light of a thorough knowledge of the particular manner of radioactive decay in the sample material, because some sources emit more than one particle or photon per disintegration.Activity is expressed in the International System of Units by the becquerel (abbreviated Bq), which is exactly equal to one disintegration per second. The old standard unit was the curie (abbreviated Ci), which is equal to 3.7 × 1010 Bq.
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