Calaprice to receive Bethe Prize for experimental nuclear physics

By Liz Fuller-Wright, Office of Communications

Physicist Frank Calaprice will receive the American Physical Society’s 2023 Hans A. Bethe Prize, which recognizes outstanding work in theory, experiment or observation in the areas of astrophysics, nuclear physics, nuclear astrophysics, or closely related fields.

Calaprice, an emeritus professor of physics at Princeton University, will receive the award “for pioneering work on large-scale ultra-low-background detectors, specifically Borexino, measuring the complete spectroscopy of solar neutrinos, culminating in observation of CNO neutrinos, thus experimentally proving operation of all the nuclear energy driving reactions of stellar evolution.” The prize, established in 1998, honors Hans Albrecht Bethe (1906-2005) for his outstanding and numerous accomplishments in both astrophysics and nuclear physics.

Calaprice joined the Princeton faculty in 1970. He served as the director of the Princeton Cyclotron, and starting in the 1990s, Calaprice led the Borexino solar neutrino experiment at Laboratori Nazionali del Gran Sasso with University of Milan colleague Gianpaolo Bellini.

“This prize is a very well-deserved and fitting recognition of a lifetime of work,” said Herman Verlinde, chair of the physics department and Princeton’s Class of 1909 Professor of Physics. “Frank is widely recognized as one of the few authorities in the field of very low background, low counting rate experiments, and Borexino’s success would not have been possible without his essential contributions. He has been an inspiring mentor to a diverse group of researchers, many of whom have become successful leaders themselves.”

“Frank has been a wonderful friend and colleague, starting in the 1980s when I was a professor in the Princeton physics department and co-director of the cyclotron with him,” said Art McDonald, the 2015 Nobel Physics Laureate and an emeritus physics professor at Queen’s University in Canada who was on the Princeton faculty from 1982 to 1989. “The Bethe Prize is a well-deserved recognition for a truly excellent scientist and mentor of many generations of young scientists.”

Borexino, a hyper-sensitive instrument deep underground in Italy, operated from 2007 to 2020. It was built with an onion-like structure to create layers of protection around a radioactively pure core that could detect the smallest known particles, neutrinos, which are produced by the sun’s fusion. Known as “ghost particles” because they pass through most matter without leaving a trace, neutrinos are common — more than 400 billion hit every square inch of the Earth’s surface each second — but virtually all of these ghost particles pass through the entire planet without interacting with anything, forcing scientists to go to great lengths to detect them.

Calaprice was responsible for countless crucial contributions to Borexino, including the design and construction of the containment system, and the design, construction and operation of the purification system.

In 2018, Calaprice and the Borexino team reported the first complete observation of proton-proton neutrinos, which are formed when hydrogen atoms fuse into helium in the sun. Three years later, the team announced that they had detected CNO neutrinos — tiny particles pointing to the presence of carbon, nitrogen and oxygen (CNO) in our sun’s core. The CNO ghost particle detection confirmed a 1939 prediction by Bethe (his prize’s namesake) that some of our sun’s energy is generated by a chain of reactions involving carbon, nitrogen and oxygen.

“Frank Calaprice played a monumental role in designing and building the Borexino detector and in leading the collaboration through the successful execution of the project,” said Cristian Galbiati, a professor of physics and one of Calaprice’s collaborators on the Borexino experiment. “His recognition with a Bethe prize is quite appropriate: the nearly complete measurement of neutrinos from the proton-proton and CNO cycles provides the fullest validation of Bethe’s landmark work on the nuclear fusion cycles fueling all stars. The extraordinary experimental success of Borexino would simply not have been possible absent Frank’s ingenuity and drive for innovation and background reduction.”

Many of the ideas Calaprice introduced laid the groundwork for subsequent generations of neutrino and dark matter experiments; in addition, he personally mentored many students and postdoctoral researchers.

“One cannot help but notice that at least three generation of his mentees are proving themselves successful leaders in the field, validating Frank’s excellent record in science education and making his impact on the field even more significant,” Galbiati said.

Calaprice’s early work had focused on the study of time reversal invariance in decays of polarized noble gas atoms, leading to results that represented the most sensitive test for T-invariance in nuclear beta decay for more than 20 years. His later work focused on finding more and more ways to improve the neutrino-detecting capabilities of the Borexino, which he continued working with after he transferred to emeritus status in 2018.

The Bethe Prize consists of $10,000 and a certificate citing the contributions made by the recipient. Calaprice will receive the certificate and present a talk at an upcoming meeting of the American Physical Society.