P. Gregers Hansen Obituary

Peder Gregers Hansen, an outstanding leader in experiments with nuclei far from stability, died of multiple myeloma on July 20, 2005 in Lansing, Michigan.

Gregers was born on January 11, 1933 in Svendborg on the south of the island of Fyn in Denmark. He had a profound sense of history and could trace his genealogy back for many generations. He received the M.Sc. degree from the Technical University of Denmark in 1955 as chemical engineer and started his work at the Niels Bohr Institute and Risø National Laboratory, first as research scientist and later as group leader.

From the very beginning Gregers was involved in all aspects of nuclear physics experimentation where chemical techniques were used as an auxiliary, though frequently necessary, practical tool, especially in working with rare earth elements. His rich knowledge and permanent interest in chemistry helped Gregers develop a deep understanding of the relationship between atomic and nuclear phenomena, which he preserved during his entire career: twenty years later he and J. Lindhard would write a short letter on atomic effects in tritium beta decay that convincingly dismissed claims for the existence of a 17 keV neutrino. Ten years of experimental work at Risø and dozens of publications on the frontiers of nuclear spectroscopy earned him the degree of Doctor of Science from the University of Copenhagen in 1965.

In 1966 Gregers accepted the position of Professor of Experimental Physics at the University of Aarhus, a chair that he held until 1995. Continuing his active work in nuclear physics, he was especially interested in nuclear beta decay and in precise measurements of transition rates, extraction of nuclear matrix elements, beta-gamma directional correlations and their relation to nuclear structure. At the same time he became directly involved in the experimental program at the newly constructed ISOLDE facility at CERN. Up to then, the physics of nuclei far from stability was an exotic subject with scarce experimental information. Now this field became experimentally accessible, and this was where Gregers' main interests and efforts were concentrated for the rest of his life.

As a senior physicist at CERN, Gregers served as the group leader responsible for the experimental program at ISOLDE (1969-1979). During 1974-1977 he also acted as deputy division leader in the Experimental Physics Division at CERN. Enormous experience and deep theoretical knowledge made him a prominent authority in the physics of nuclei far from the line of stability (now often referred to as rare isotope research). This field was undergoing a tempestuous growth, as one can easily see from the comparison of two review papers by Gregers: "Nuclei far off the stability line" (Montreal, 1969) and "Experiments with beams of rare isotopes: A fifty-year perspective 1951-2001" (Nuclear Physics News, 2001). Gregers' first-hand account on the development of nuclear physics at CERN can be found in History of CERN, vol. III. At the same time he participated in important experiments at other European facilities, GANIL in France and GSI in Germany. Being always socially active, he served as President of the Danish Physical Society (1982-1984) and in 1987 was elected to the Royal Danish Academy of Sciences and Letters. He also served on numerous scientific advisory bodies and program committees, organized schools and conferences, and chaired the Proton Synchrotron and Synchrocyclotron Committee at CERN (1981-1985).

In 1995 Gregers moved to the USA, where for the last ten years of his life he was the John A. Hannah Distinguished Professor of Physics at Michigan State University. Here he brought together and led a team of young physicists that performed a number of important experiments with exotic nuclei. He was instrumental in the development of the experimental program at the National Superconducting Cyclotron Laboratory at MSU and was one of the architects of the scientific case for the future Rare Isotope Accelerator.

One of the most interesting and unusual nuclear species, the exotic isotope 11Li, has been an important part of Gregers' work for many years, from the discovery of two-neutron (1979) and three-neutron (1980) radioactivity to the idea of a quantum halo formed by an extended neutron-matter distribution in the classically forbidden region (1987). This halo-physics was popularized by Gregers in four insightful articles in Nature (1987, 1988, 1993, 1996). At the NSCL, Gregers and his collaborators studied the detailed structure of 11Li and other halo nuclei in a series of pioneering experiments. During his last years, Gregers and his colleagues developed one- and two-particle knockout reactions into an important experimental tool; their results contain much new and still not fully understood information.

A favorite subject of Gregers was the statistics of experimental data, especially in spectroscopy, reactions and decays. He was a great expert in these questions, with a subtle feeling of possible dangers and misleading "obvious appearances". Together with John Hardy and Björn Jonson, he invented the famous artificial nucleus "Pandemonium" that served as an excellent testing ground for various statistical hypotheses. Together with Achim Richter and collaborators, these ideas were combined with those of quantum chaos; the resulting approach allows the experimentalists to extract information on complex fine structure states not seen directly with relatively poor experimental resolution. Gregers, with his deep knowledge of physical reality, was always a welcome participant of theory meetings. Until the end, he worked directly on various subjects with theorists from the NSCL and other institutions.

In everyday life Gregers combined a fine politeness with firm moral principles. With a broad knowledge of literature and proficiency in several European languages, including Russian, he was ready to discuss at the highest level such different subjects as ancient history, Russian poetry, or Michigan mammals. He was a great fan of mountaineering and explored with his family many dangerous passages in Alps. He continued to work until the last days of his untimely illness. He was an exemplary colleague and friend, a scholar and a gentleman, and is greatly missed by those whose lives he touched.

C. Konrad Gelbke
Michigan State University

Björn Jonson
Chalmers University of Technology, Göteborg

Vladimir Zelevinsky
Michigan State University

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