The 8 B neutrino spectrum

Knowledge of the energy spectrum of 8B neutrinos is important for interpreting experiments that detect energetic neutrinos from the Sun. The neutrino spectrum deviates from the allowed β-decay approximation because of the broad α-unstable 8Be final state and recoil order corrections. We have measured the total energy of the α particles emitted following the β decay of 8B. The measured α spectrum is inconsistent with a recent measurement of comparable precision. Theα spectrum is fit using the R-matrix approach, which gives a functional form for the β-decay strength function for the transition from 8B to the accessible excitation energies in 8Be. Both the positron and neutrino energy spectra, corrected for recoil order effects, are constructed from the strength function. The positron spectrum is in good agreement with a previous direct measurement. The neutrino spectrum disagrees with previous experiments.