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Hans A. Bethe prize 2023 from APS to prof Frank Paul Calaprice

Frank Calaprice in Campo Imperatore, Gran Sasso, during a summer school. Credits: Borexino Collaboration

Prof. Emeritus Frank Paul Calaprice, Princeton University, has been awarded the prestigious Bethe prize for 2023.

“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.”

Frank Calaprice earned his Ph.D. from UC Berkeley in 1967 and joined Princeton faculty in 1970. In early 1990’s Frank started working on Borexino with Gianpaolo Bellini, Raju Raghavan, Jay Benziger, and Franz von Feilitzsch. Borexino aimed for the first time to search for sub-MeV solar neutrinos with a massive liquid scintillator at the Gran Sasso Laboratory, Italy. The main goal for Borexino was the measurement of the so-called 7Be solar neutrinos. At that time this was a great challenge. An extreme radio-purity better than 10-16 g/g for uranium and thorium was required. To face this challenge Frank and collaborators designed and built the Counting Test Facility (CTF). The CTF was a 4-ton liquid scintillator detector viewed by 100 8-inch photomultipliers inside a muon veto made of 1 kton of high purity water. The CTF was successful and paved the way for Borexino. Construction started in 1998. Borexino used 1 kton of pseudocumene viewed by 2212 8-inch photomultipliers inside a 14 meters in diameter stainless steel vessel. The ultra-high radio-purity 280 tons liquid scintillator was contained by a 125 mm thick nylon vessel. Borexino took data from May 2007 until October 2021. The radio-purity achieved by Borexino was much better than expected at the level of 10-18 g/g. This outstanding achievement allowed for the first time the measurement of sub-MeV solar neutrinos including pep and pp neutrinos besides 7Be, boosting our understanding of the interior of the sun. In 2020, Borexino observed CNO neutrinos. The CNO cycle, predicted by Bethe, makes only about 1% of the energy produced by the sun. Yet, in more massive stars it is the dominant source of energy. For the first time this energy source has been probed experimentally. CNO neutrinos are also a unique probe for the sun’s metallicity. Metallicity is a key input in the theory which describes the sun. Therefore, understanding experimentally the sun’s metallicity is crucial. The first attempt in this direction has been carried out by Borexino with CNO neutrinos.

Frank played a crucial role in the design and construction of the Borexino detector. In addition to much more he was deeply involved in the purification strategy, where he contributed with many important ideas. His role and involvement in the measurement of CNO neutrinos has been crucial. His recognition with a Bethe prize is quite appropriate, considering that Borexino has probed the basis of energy production in the sun through the pp-chain and the CNO cycle. Furthermore, the technology developed in the framework of Borexino is at the root of next-generation experiments which search for rare events, such as dark matter and neutrinoless double beta decay. In the last two decades Frank gave important contributions to direct search for dark matter through DarkSide-50 and SABRE both at the Gran Sasso Laboratory.