Borexino Solar Neutrino Experiment

Princeton University, Princeton NJ

Investigators

Abstract

PROPOSAL NUMBER: 0802646 INSTITUTION: Princeton University NSF PROGRAM: PHY - PARTICLE ASTROPHYSICS PRINCIPAL INVESTIGATOR: Calaprice, Frank P. TITLE: Borexino Solar Neutrino Experiment ABSTRACT Borexino is a large underground liquid scintillation detector designed to observe low-energy neutrinos, subatomic particles produced by the sun, to elucidate how the sun produces energy and to better understand the fundamental properties of neutrinos. The detector, in operation since May 2007, contains 300 tons of ultra-pure liquid scintillator and is located in the underground Gran Sasso laboratory in Italy. The proton-proton (pp) cycle, by far the dominant set of nuclear fusion reactions in the sun?s dense, hot core, converts four hydrogen atoms into a helium atom and produces 26.7 MeV of energy per cycle. The energy is released to charged particles, gamma rays, and neutrinos. The charged particles and gamma rays transfer energy by collision processes to other atoms. Energy slowly diffuses to the solar surface, where it is radiated into space mainly as visible light. Neutrinos, on the other hand, interact only weakly with matter. They travel almost unperturbed out of the sun, providing a direct view into its core. Ninety percent (the so-called pp neutrinos) are generated in the first step of the pp cycle; most of the rest are produced in a later step of the cycle involving the isotope 7Be. Neutrinos have a small mass that allows the electron-type neutrinos to oscillate, between sun and Earth, into states that are more difficult to detect. Neutrino oscillations explain all the data, provided one also considers their weak interactions with the matter in the sun ? the matter-enhanced oscillations described by Mikheyev, Smirnov, and Wolfenstein (MSW). The Borexino detector has already acquired a first set of data that so far agrees with MSW predictions for 7Be neutrinos. This award will provide funding for Princeton University to continue to participate in the Borexino project. The scintillator system and the purification plants, both Princeton responsibilities, require ongoing maintenance and they intend to re-purify the scintillator to achieve still lower backgrounds in the detector, further enhancing clarity of the neutrino signals. The Borexino project will continue to provide an excellent opportunity for training graduate and undergraduate students in applications of electronics, chemistry, radiology, materials science, software development, and of course fundamental physics. For the past four years the Borexino project at Princeton has been active in an outreach program that brings high school students from a relatively poor region of Italy to Princeton for a summer program in physics. Technology developed in the process of detector construction and installation includes new low-background techniques and development of a system for the reduction of radon in air. Both have potential commercial or governmental use, e.g., in security applications.