Richard Guy Helmer

Richard Guy Helmer, an internationally recognized gamma-ray spectroscopist, died on 16 January 2005 in Idaho Falls, Idaho, of complications arising from colon cancer.

Dick was born on 19 February 1934 in Homer, Michigan, and grew up in Ypsilanti, about 8 miles southeast of Ann Arbor. He obtained his BS (1956) and MS (1957), both in physics, from the University of Michigan, Ann Arbor, and his PhD in nuclear physics there in 1961. Dick’s thesis research, on the decay properties of several rare-earth nuclides, was carried out at Argonne National Laboratory under the direction of S. Bradley Bur-son. On receiving his doctorate, Dick joined Russell Heath’s gamma-ray spectroscopy group at the National Reactor Testing Station (now the Idaho National Laboratory), where he spent the rest of his career.

In the 1970s Dick, in collaboration with Reginald Greenwood and others at the Idaho laboratory, began work on germanium-detector-based gammaray spectroscopy, primarily in the measurement of gammaray energies and the precise calibration of germanium detector efficiencies. Throughout his career, Dick made seminal contributions to that field.

That work led to Dick’s involvement with a number of collaborators, both national and international. With Pieter Van Assche of the nuclear research center SCK/CEN in Belgium and Cor van der Leun of the R. J. Van de Graaff Laboratorium of Utrecht University in the Netherlands, for example, Dick presented a set of well-documented and precisely determined gamma-ray energy standards for calibrating Ge-detector energies. Dick and van der Leun subsequently updated the standards, and after van der Leun’s death in 1998, Dick completed the update and published the standards in 2000 in Nuclear Instruments and Methods in Physics Research A.

A large body of published work on the precise measurement of gamma-ray emission probabilities resulted from Dick’s work on the measurement of Ge-detector efficiencies. For example, he and Klaus Debertin of the Physikalisch-Technische Bunde-sanstalt, the national metrology institute in Brunswick, Germany, cowrote the book Gamma- and X-Ray Spectrometry with Semiconductor Detectors (North-Holland, 1988), a standard reference for many practicing gamma-ray spectrometrists. From the mid-1970s to 1990, his work also influenced many areas of science and technology. Examples include coordinated research programs, conducted under the auspices of the International Atomic Energy Agency (IAEA), on the measurement of actinide-nuclide decay data (1977–85), x-ray and gamma-ray standards for detector calibration (1986–90), and an update and extension of those standards—an effort that is still in progress.

Beginning in the late 1990s, Dick and John Hardy of Texas A&M University used both careful measurements and Monte Carlo calculations to precisely calibrate the efficiency of a high-purity Ge detector. Their work resulted in the HPGe detector’s being, as far as presently known, the most precisely calibrated detector in the world.

Dick was also committed to efforts to evaluate nuclear data. Beginning in 1984, he became involved in the international Nuclear Structure and Decay Data Evaluation Network, an international group of nuclear physicists, coordinated by the IAEA, that is charged with producing the Evaluated Nuclear Structure Data File from which the Nuclear Data Sheets are produced. From 1998 to 2002, Dick chaired the network’s Nuclear Structure and Decay Data Working Group.

In 1991 Dick began discussions with members of radiation standards laboratories in Germany and France to establish an international collaboration that would provide carefully documented evaluations of decay data for applications in science and technology. As a result, the Decay Data Evaluation Project was formed four years later and held its first meeting. The group’s participants today include evaluators from national radioactivity standards laboratories in Brazil, France, Germany, and Russia, as well as national laboratories in the UK and the US.

Dick’s achievements, though, were not restricted to standards and evaluations. He contributed significantly to fundamental nuclear physics. In 1968 he and one of us (Reich) reported the discovery of a second isomeric state in hafnium-178. The unique combination of properties—long half-life (31 years), high spin (16+), and high energy (2.45 MeV)—of this four-quasiparticle state continues to attract a lively interest and even some notoriety (see Physics Today, May 2004, page 21). And those two researchers discovered in 1990 that the first excited state of thorium-229 must lie within a few electron volts of the ground state. Following that discovery, Dick carried out an extensive set of energy measurements with sub-eV precision on several gamma rays from the alpha decay of uranium-233, from which the value 3.5 eV was obtained for the ²²⁹Th energy level. That finding has led to numerous experimental and theoretical studies and continued interest in the topic.

Dick’s accomplishments were re-warded well. At the 2001 Annual Winter Meeting of the American Nuclear Society, he was honored at a special session and received the society’s Radiation Science and Technology Award. In 1991 he was elected a member of the International Committee on Radionuclide Metrology, the only US member of that committee not associated with NIST.

An exceptionally kind and caring person, Dick was actively involved in his community and the world. He served for six years as a trustee of the Idaho Falls school board, in lay leadership positions in his church, and on the advisory board of the Good Samaritan Nursing Center in Idaho Falls. He helped establish kinder-gartens for low-income children in his community; engaged in church missions to Brazil, Chile, and Nicaragua; and worked on Habitat for Humanity projects. Perhaps the best description of him is found in Micah 6:8:

He is greatly missed.