Hans Bichsel

Hans Bichsel died at age 94 on 24 November 2018 in Seattle. He had a long and distinguished career in physics and was the world’s expert on energy loss by charged particles in matter.

Hans was born on 2 September 1924 in Basel, Switzerland. After getting his MA and PhD from Universität Basel in 1951, he became a research assistant at Princeton University. He moved to Rice University as a research associate in 1955, and then in 1957 became assistant and then associate professor at the University of Southern California. In 1969 he went to the University of Washington as professor of radiology until 1978. Following a period of global consultantship at many institutions, he became in 1992 affiliate professor of physics at Washington, where he remained for the rest of his career, active in research until his 90th year. Dementia later deprived him of this capacity. Hans was a member of the Swiss Physical Society and a fellow of the American Physical Society.

Hans spent his career working on the complexities of electronic charged particle stopping power and range (mostly for single-charged particles but also for alpha particles and heavier nuclei): shell corrections, Barkas corrections, and a host of other minor corrections relevant at low energies, oscillator strengths, mean excitation energies, multiple scattering corrections, thin-detector response and straggling, with applications to dosimetry, radiation therapy, and particle identification. Notably, he taught us that the mean energy loss in a single detector is often not a useful concept; it is the most probable that is nearly unaffected by fluctuations. The University of Washington was a center for neutron therapy, and Hans established dose conversion factors for neutron dosimetry. His exquisite attention to detail refined and improved many models used in medical radiotherapy and physics.

Over the years, with many collaborations, he applied his skills to simulation and analysis: optimizing the energy resolution of the proton counter in ISOLDE, simulating the Sudbury Neutrino Detector’s proportional counters, modeling the electron response of the KATRIN detector, and modeling the ATLAS pixel digitization to achieve ultimate reconstruction performance. He was a member of the STAR RHIC collaboration, where worked to achieve optimal particle identification in STAR’s TPC. He did similar work with the ALICE TPC.

The author of more than 280 publications, Hans contributed to several books, and in particular was part of the report committee preparing the definitive ICRU reports 49 (“Stopping powers and ranges for protons and alpha particles”) and 37 (“Stopping powers for electrons and positrons”). These, in turn, are the basis for the NIST “Stopping power and range” tables ESTAR, PSTAR, and ASTAR, and the comparison calculations for the Atomic and Nuclear Properties of Materials . He was a coauthor of “Passage of particles through matter” in the Particle Data Group’s Review of Particle Physics. Most of its approach to stopping power is by him, as are many of the figures.

He is survived by Sue, his wife of 59 years, children Elizabeth and Joseph, and a grandson, Orion.