A “founder” of astroparticle physics retires from SNOLAB

By Stephen Sekula

The end of October marked a retirement from SNOLAB and a new moment in the long and distinguished career of Dr. Bruce Cleveland.

Bruce earned a research staff position on the SNO experiment in 1996 and has been with SNO and SNOLAB ever since. He worked in the SNO water group and on the counting of very low-background processes, with an emphasis on the measurement of radon. But Bruce was a leader in the emerging filed of astroparticle physics long before he came to SNOLAB.

Bruce earned his PhD in solid-state physics from Johns Hopkins University in 1970 working with Dr. James Calvin (“Cal”) Walker, followed by post-doctoral positions at the State University of New York (SUNY) Buffalo (now known as the University at Buffalo SUNY) and then Columbia University.

These post-doctoral years allowed Bruce to work with several remarkable people, including Dr. Chien-Shiung (C.S.) Wu at Columbia. Dr. Wu was well-known for her 1956 discovery of parity violation in the decay of cobalt-60’s nucleus. A seminal research activity in this period was Bruce’s work on the question of lepton number conservation in the double beta decay of selenium-82. In a 1975 paper on the subject (Phys.Rev.Lett. 35 (1975), 757-760), they put a constraint on the neutrinoless double beta decay of this isotope and used this to place a physical constraint on the violation of lepton number in nature. This measurement set a world record at the time for sensitivity to this ultra-rare process.

Our field, and specifically SNOLAB itself, is still following in the footsteps of this subject that were laid down by Bruce and others in the mid-1970s.

Bruce went on to research positions at Brookhaven National Laboratory, Los Alamos National Laboratory, and the University of Pennsylvania. During this period, he was engaged in the “solar neutrino problem” that had been kicked off earlier by the underground experimental work of Ray Davis and the theoretical work of John Bahcall, to name some of the most notable historical figures from that era.

This period was later recognized as incredibly transformative for our field, and Bruce was right at the heart of it. He worked with Ray Davis on the famous Homestake solar neutrino experiment. He signed many of the papers that would become part of the Nobel Prize committee’s cited body of work when Davis, Figure 1: Bruce Cleveland, posing in the SNOLAB surface clean lab. Masatoshi Koshiba, and Riccardo Giacconi earned the Nobel Prize in Physics in 2002.

Bruce is highly regarded for his development of the data analysis of low-statistics experiments. One of his notable publications was “The Analysis of Radioactive Decay with a Small Number of Counts by the Method of Maximum Likelihood” (Nucl.Instrum.Meth. 214 (1983) 451-458). Another highly significant paper was noted in the background information on the 2002 Nobel Prize issued by the Royal Swedish Academy of Sciences in their announcement of the prize. This was “Measurement of the solar electron neutrino flux with the Homestake chlorine detector” (Bruce T. Cleveland et al 1998 ApJ 496 505), a publication on which Bruce was the lead author and which has been cited over 3,000 times! Bruce developed over his career into an expert on data analysis and that resulted in the analysis used in the Homestake experiment papers.

Bruce co-authored a seminal paper in 1978 with Ray Davis, John Bahcall, Israel Dostrovsky, and Chris Evans that proposed a new solar neutrino experiment utilizing the isotope gallium-71 (Phys.Rev.Lett. 40 (1978) 1351- 1354). This paper would become the foundation for the SAGE and GALLEX experiments, both of which would break new ground in the study of solar neutrinos.

Bruce would go on to collaborate with others on the construction and operation of SAGE (the Soviet-American Gallium Experiment, later known as the Russian-American Gallium solar neutrino Experiment). The experiment consisted of about 50 tonnes of liquid gallium metal located (you guessed it!) at a deep underground site. The site was 2100 meters underground and located at the Baksan Neutrino Observatory in Russia. It was Figure 2: (Left-to-Right) John Galvin, Raymond Davis, and Bruce Cleveland (Solar Neutrino group, 1977). On the wall is a photo of the Homestake detector tank. Photo courtesy Brookhaven National Laboratory and the AIP Archives. during his work at Baksan that Bruce met his spouse, Tanya, who is remarkable in her own right. Friends recount that his ability to speak Russian was “perfect”.

SAGE would make many observations of the neutrino flux and determined, using data from 1990-2007, that indeed the solar neutrino flux is just about 60% of what was predicted from (what was then known as) the standard solar model. This was part of a large effort over decades to strengthen, with independent approaches, the observation of a neutrino flux from the sun that was repeatedly inconsistent with the original neutrino models.

It is valuable to put this incredible period in perspective. This is summarized by the title of an article for the CERN Courier, co-authored by Ray Davis and John Bahcall: “The beginning of a new science”. They recount that period of uncertainty and exploration, both in theoretical and experimental labours, that led to what we now call “astroparticle physics”. This was a new area of science, in part born from confusion over solar neutrinos. Davis and Bahcall, the former an experimentalist and the latter a theorist, noted in their essay that “Very few people worked on solar neutrinos during 1968-1988. The [Homestake] chlorine experiment was the only solar neutrino experiment to provide data in these two decades. It is not easy for us to explain why this was the case; we certainly tried hard to interest others in doing different experiments and we gave many joint presentations. Each of us had one principal collaborator during this long period – Bruce Cleveland (experimental) and Roger Ulrich (solar models).” (CERN Courier. 27 June 2000)

Bruce earned a research staff position on the SNO experiment (which became SNOLAB much later) in 1996 and has been with our research organization ever since. He worked in the SNO water group and on the counting of very low background processes, with an emphasis on the measurement of radon.

He would go on to develop the gamma radiation counting program here at the laboratory. When SNO+ and DEAP were both proposed, he spearheaded the effort to secure two new gamma counters for installation in the underground laboratory. As this community well knows, the SNO experiment’s observations were essential to the modern understanding that neutrinos have small, non-zero masses. This resulted in the 2015 Nobel Prize in Physics awarded jointly to Dr. Takaaki Kajita and Dr. Art McDonald “…for the discovery of neutrino oscillations, which shows that neutrinos have mass.” It also resulted in the awarding of the 2016 Breakthrough Prize awarded to Dr. Art McDonald and the entire SNO team.

Bruce has also been a talented mentor over the course of his career. One highlight was MSc student Corina Nantais (“Radiopurity Measurement of Acrylic for the DEAP-3600 Dark Matter Experiment.” Queen’s University. MSc Thesis. 2014), whose thesis work in partnership with Bruce resulted in measuring the lead-210 contamination of DEAP acrylic to one part in 1018! Corina would thank Bruce in her MSc thesis, noting “… his patience in listening to me think out loud … “

The entire field was changed by the career of Dr. Bruce Cleveland. Indeed, Bruce was one of the founders of what we now call astroparticle physics. We are excited to remain his colleagues and friends, even as he formally retires. Bruce continues as a member of this community, a light for all of those looking to measure things deeply and change the field doing so. When asked why he chose physics as a career path, Bruce recounted that “I had some mathematical ability, liked to build things, and wanted to understand how the world worked, so physics seemed to be the right choice.”

It was definitely the right choice. Thank you, Bruce, for your brilliance, your mentorship, and your achievements.

Stephen Sekula is Research Group Manager for SNOLAB.