Felix H Boehm 1924–2021

2 June 2021
Felix Boehm

Felix H Boehm, who was William L Valentine Professor of Physics at Caltech, passed away on 25 May in his Altadena home. He was a pioneer in the study of fundamental laws in nuclear- physics experiments. 

Born in Basel, Switzerland, in 1924, Felix studied physics at ETH Zürich, earning a diploma in 1948 and a PhD in 1951 for a measurement of the (p,n) reaction at the ETH cyclotron. In 1951 he moved to the US and joined the group of Chien-Shiung Wu at Columbia University, which was investigating beta decay. He joined Caltech in 1953 and spent the rest of his academic career there. 

Felix worked first with Jesse DuMond, who had developed the bent-crystal spectrometer, an instrument with unrivalled energy resolution in gamma-ray spectrometry. He used it to determine nuclear radii by measuring X-ray isotope shifts in atoms. Later, he installed such devices at LAMPF, SREL and PSI to investigate pionic atoms, which led to a precise determination of the strong-interaction shift in pionic hydrogen. At Caltech, he also became interested in parity violation and time-reversal invariance. In 1957, in an experiment performed with Aaldert Wapstra, he demonstrated that electrons in beta decay have a predominantly negative helicity. 

In the mid 1970s, discussions with Harald Fritzsch and Peter Minkowski convinced Felix that the study of neutrino masses and mixings might provide answers to fundamental questions. From then on, long before it was fashionable, it became his main field of activity. He first looked at neutrino oscillations and initiated an electron–neutrino disappearance experiment with Rudolf Mössbauer. 

Theirs was the first dedicated search for neutrino oscillations, beginning with a short-baseline phase at the ILL reactor in Grenoble. The concept of the experiment was presented at the Neutrino 79 conference in Bergen, at which the Gargamelle collaboration also reported limits on νμ ↔ νe oscillations. Both talks were relegated to a session on exotic phenomena. The ILL experiment was continued at the Gösgen reactor in Switzerland with a longer baseline. No evidence of oscillations was found and stringent limits in a given parameter space were derived, contradicting positive claims made by others. A larger detector was later built at the Palo Verde nuclear power station in Arizona, where again no evidence for oscillations was found. A logical continuation of the effort initiated by Felix was the KamLAND experiment in Japan, which was exposed to several reactors and eventually, in 2002, observed neutrino oscillations in the disappearance mode at a still-longer baseline. 

In parallel, Felix decided to probe neutrino masses by searching for neutrinoless double- beta decay. He led a small collaboration that installed a germanium detector in the Gotthard underground laboratory in Switzerland to probe 76Ge, and then searched for the process using a time-projection chamber (TPC) filled with xenon enriched with 136Xe. The TPC, a novel device at the time, improved the event signature and thus reduced the background, allowing stringent constraints to be placed on the effective neutrino mass. The ongoing EXO experiment can be seen as a continuation of this programme, vastly improving the sensitivity in its first phase (EXO-200 at WIPP, New Mexico) and expected to do even better in the second phase, nEXO.

Felix Boehm had a talent to identify important issues on the theoretical side, and to select the appropriate technical methods on the experimental side. He was always ready to innovate. In particular, he realised very early on the importance of selecting radio-pure materials in low-count-rate, low-background experiments. All those who worked with him appreciated his open mind, his determination, his great culture and his kindness.

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