Jack Sam Greenberg, professor emeritus of physics at Yale University and an experimental physicist who made exceptionally broad fundamental contributions to atomic, nuclear, and high-energy physics, died on 30 March 2005 in New Haven, Connecticut, from complications following an accidental fall at his home.
Jack was born on 23 May 1927 in Warsaw, Poland, but was fortunate in that he and his immediate family im-migrated to Canada in 1934 and thus escaped the catastrophes of World War II that swallowed up virtually all other members of his family. He earned a bachelor’s degree in engineering in 1950 and an MS in engineering physics a year later from McGill University. For his doctorate, Jack moved to MIT and worked on positron emission from beta decay. He received his PhD in physics in 1955 under the tutelage of Martin Deutsch, who had discovered positronium in 1951.
Jack next spent a postdoctoral year at ETH Zürich, where he attended lectures by Wolfgang Pauli. He joined Yale in 1956 as an instructor in physics. That choice was motivated partly by the arrival at Yale of Vernon Hughes, who was setting out to test quantum electrodynamics (QED) with precision experiments on positronium and other simple atomic systems.
After construction of the Yale tandem accelerator during the 1960s, Jack joined the Wright Nuclear Structure Laboratory, which was then directed by D. Allan Bromley. There Jack used Coulomb excitation to study the structure of deformed nuclei. He produced the first Yale tandem PhD graduate—Richard F. Casten—who now directs the lab.
Jack’s experiments evolved into studies of the emission dynamics of × rays produced during nuclear collisions of systems such as nickel + nickel. Those results led to speculations that a new phenomenon would occur during heavy-ion collisions as ephemeral “atoms” are created with nuclear charge Z u > 1/α ≈ 137; the result would be a spontaneous emission of positrons, dubbed “sparking of the vacuum.” That tantalizing possibility inspired Jack to conduct experiments at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, in the late 1970s. In those studies, he and his group, and a competing group, led by Paul Kienlie and also using the GSI heavy ion beam, observed the broad spectrum of such positrons, much as predicted by QED.
During GSI experiments that involved a variety of species, the two groups also observed a narrow positron line superimposed on the broad spectrum at approximately 320 keV. However, if the line was due to sparking of the vacuum, it was predicted to have an energy varying as Z u 20. The absence of that dependence suggested that a new particle had been created, although the line was possibly due to a not yet understood background process. If the 320-keV line was due to a new particle, it should also have been observable in electron—positron collisions.
With a positron beam at Brookhaven National Laboratory, Jack and colleagues carried out experiments in 1992 that yielded negative results for a range of properties of such a new particle but still left open some possibilities. A new bound state of the electron—positron system would have presented a serious problem for QED, so the interpretation of the line had important theoretical implications. Unfortunately, a subsequent experiment at Argonne National Laboratory took place in 1995 when Jack’s health was declining and he was unable to fully participate. Moreover, it was uncertain whether the conditions of Jack’s GSI experiment were properly re-created at Argonne. So the definitive status of the discrete line remains unclear.
Jack’s long association at Yale, until his retirement in 1999, involved promotions through the ranks to professor in 1976. During his distinguished career, he was director of graduate studies from 1967 to 1969, held a senior faculty fellowship from 1969 to 1970 at the Weizmann Institute in Rehovot, Israel, and won a Senior US Scientist Award from the Alexander von Humbolt Foundation in 1976.
Jack’s many admirers consistently noted his exercise of unusual caution before reaching conclusions based on experimental data, and his untiring desire for additional experiments to reduce statistical effects. His meticulous search for perfection in physics was reflected in his nonscientific endeavors, which included his taste for exotic motor cars, his quest for the state-of-the-art home reproduction of the grand opera repertory, and his exceptional collection of antique oriental carpets. His maxim—as in physics—was that no activity (or commodity) was worth doing (or acquiring) if it risked compromising perfection. Jack’s warm, engaging personality and willingness to share valuable insights with colleagues and students are sorely missed by those at Yale and by his loving family, who were privileged beneficiaries of his remarkable gifts.
