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.
In May 2007 the Borexino detector started taking data in the Gran Sasso Underground Laboratory in Italy with the goal of detecting low energy neutrinos emitted by the Sun. Measurements of the 0.86 MeV 7Be neutrinos, which would probe an unexplored part of the solar neutrino spectrum, would provide unique data on the nuclear fusion reactions that power the sun, and explore an untested feature of neutrino oscillations that explained the "solar neutrino problem". Direct detection of solar neutrinos below 1 MeV had never been achieved because of the overwhelming "wall" of background radiation from natural radioactivity -uranium, thorium, and potassium- present at trace levels in all materials. Prior to the start of data acquisition, the Borexino collaboration spent more than a decade developing a low background liquid scintillator that had the required low background and sensitivity to detect low solar energy neutrinos. It was apparent that the effort paid off when the 7Be neutrino signals were observed in the first months of operations. In the five years since the first 7Be neutrino signals became apparent, the Borexino collaboration completed a measurement of the rate of 7Be neutrinos with an uncertainty of 5%. With background from U, Th, and K much lower than anticipated, the collaboration was also able to measure the 1.44 MeV pep neutrinos and the high energy 8B neutrinos. The data also yield an upper limit for neutrinos emitted in the CNO cycle of nuclear fusion and demonstrate from direct observation that the CNO process contributes at most a small fraction of the Sun’s energy. The new solar neutrino results are the most accurate measurements of low energy solar neutrinos to date. The 7Be, 8B, and pep results, together with the upper limit for CNO neutrinos, confirm that the proton-proton fusion cycle is the dominant process that powers the sun, in agreement with the Standard Solar Model. The Borexino data provide further evidence for neutrino oscillations the expected transition from matter-effect to vacuum neutrino oscillations predicted in the theory of Mikheyev-Smirnov-Wolfenstein. Borexino made a contribution to geology through a measurement of "geo-neutrinos", which are anti-neutrinos produced by the beta decays of naturally occurring radioactive isotopes in the Earth. The data on geo-neutrinos give a unique view of the interior composition of the Earth and show that radioactivity contributes a significant fraction of the internal heating within the Earth. The possibility of more accurate measurements, and detectors at a variety of sites over oceanic and continental crust, holds much promise for better understanding the composition of the Earth, and how the Earth operates.
