Armbruster, Peter

▪ 1997

      On Feb. 9, 1996, German physicist Peter Armbruster and a multinational team of scientists at the Institute for Heavy Ion Research (GSI), Darmstadt, Ger., synthesized element 112, thereby attaining yet another goal in their quest to discover increasingly heavy chemical elements. Armbruster and physicist Sigurd Hofmann led the researchers who made the new element, their third such achievement in less than two years.

      In 1971 Armbruster became chief scientist at GSI, where he worked for more than two decades to synthesize the superheavy elements—i.e., a group of relatively stable elements with atomic numbers (numbers of nuclear protons) around 114 and mass numbers (numbers of nuclear protons and neutrons) around 298. Scientists began creating new elements with atomic numbers higher than that of uranium, element 92, in the early 1940s. As they attempted to make elements heavier than fermium, element 100, the extreme instability of those elements posed increasing challenges. In response, Armbruster and physicists at other accelerators around the world developed more sophisticated synthetic techniques. At GSI the approaches proved quite successful. In the early 1980s Armbruster and co-workers produced elements 107 through 109, and in 1994, within a two-month period, they created elements 110 and 111.

      Element 112, with an atomic mass of 277, was the heaviest yet to be produced in the laboratory. It was created from the fusion of the nuclei of lead and zinc when zinc atoms were raised to high kinetic energies in a heavy-ion accelerator and aimed at a lead target. The two nuclei combined, and element 112 was born. Only one atom of the element was detected in the experiment, and in less than a thousandth of a second it decayed. In spite of its short life span, the new element was expected to provide insight into the nature of nuclear structure.

      The synthesis of increasingly heavy elements allowed physicists to test predictions about the stability of atomic nuclei. Scientists had identified certain, "magic" numbers of protons and neutrons that should confer particular stability to a nucleus. The stability arises because the internal nuclear structure can arrange itself such that the binding energy of the nucleus is increased. Element 112 has 161 neutrons in its nucleus, which is only one short of the predicted magic number of 162 neutrons.

      Armbruster was born in Dachau, Bavaria, on July 25, 1931. He received his doctorate from the Technical University of Munich. Throughout his career he remained intrigued by the reactions between heavy nuclei and applied the results of his studies to understanding atomic and solid-state physics. As 1996 progressed, Armbruster continued his work to extend the periodic table beyond its current limits, hoping in the near future to create the superheavy element predicted to be the most stable of the group—element 114.

      (MARY JANE FRIEDRICH)

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