Nuclear response of water Cherenkov detectors to supernova and solar neutrinos

Abstract

I present a careful examination of the nuclear cross sections of low-energy neutrinos and antineutrinos in water Cherenkov detectors, and consider the implications for neutrinos from SN1987A. For thermal sources with T≳4–5 MeV (depending on the detector threshold) the reaction O(${\nu }_e$,$e^{\mathrm{-}}$)F becomes the dominant mechanism for ${\nu }_e$ interactions, and at T≳7–9 MeV its rate exceeds that for (${\nu }_e$,e) elastic scattering by at least an order of magnitude. This has important implications for proposed mechanisms for prompt emission of energetic ${\nu }_e$’s following neutronization. The O(ν${¯}_e$,$e^{+}$)N contribution remains a small correction to the dominant ν${¯}_e$+p→$e^{+}$+n rate, accounting for less than 10% of the events even at T=10 MeV. Thus delayed ${\beta }^{\mathrm{-}}$ emission by ${}_{}{}^{16}{}_{}{}^{}({\tau }_{1/2}$=7.1 sec) cannot distort event timing. Observable neutral-current nuclear excitations are absent in water, but can be strongly excited in carbon-bearing liquid-scintillation detectors. Arguments are given that a burst poor in electron neutrinos and antineutrinos, but otherwise similar to those derived from standard supernovae theory, might produce comparable signals in the Mt. Blanc, Kamioka, and Irvine-Michigan-Brookhaven detectors. For solar neutrinos generated by ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ β decay, reactions with trace quantities of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ (two atoms per ${10}^4$ electrons in natural water) account for approximately 10% of electron events. If it proves feasible to enhance the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ content of a water Cherenkov detector, the hard spectrum of produced electrons would make an attractive charged-current signal.