Obituary of Peder Gregers Hansen

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 island of Fyn in Denmark. 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.

At Risø, Gregers was involved in all aspects of nuclear physics experimentation. His rich knowledge 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 work 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. At the same time he became involved in the experimental program at the ISOLDE facility at CERN. Physics of nuclei far from stability, previously an exotic subject with scarce experimental information, became accessible, and this was where Gregers’ main efforts were concentrated for the rest of his life.

As a senior physicist at CERN, Gregers served as the group leader 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 knowledge made him a prominent authority in physics of exotic nuclei, the field undergoing a tempestuous growth, as one can 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). He also participated in important experiments at other European facilities, GANIL in France and GSI in Germany. 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.

In 1995 Gregers moved to the USA, where for the last ten years he was the John A. Hannah Distinguished Professor of Physics at Michigan State University. He brought together and led a team of young physicists that performed a number of pioneering experiments with exotic nuclei. One of the most 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 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). 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. He was instrumental for 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.

A favorite subject of Gregers was the statistics of experimental data, where he was a great expert, with a subtle feeling of possible dangers and misleading obvious appearances. Together with John Hardy and Björn Jonson, he invented the artificial nucleus Pandemonium that served as a testing ground for statistical hypotheses; the resulting approach allows one to extract information on fine structure states not seen directly with poor experimental resolution. Gregers, with his deep knowledge of physical reality, was always a welcome participant of theory meetings and worked directly with many theorists.

In everyday life Gregers combined a fine politeness with firm moral principles. With a broad knowledge of literature and proficiency in several European languages, he could 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.