The ⁸B Neutrino Spectrum

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Knowledge of the energy spectrum of ⁸B neutrinos is an important ingredient for interpreting experiments that detect energetic neutrinos from the Sun. The neutrino spectrum deviates from the allowed approximation because of the broad alpha-unstable ⁸Be final state and recoil order corrections to the beta decay. We have measured the total energy of the alpha particles emitted following the beta decay of ⁸B. The measured spectrum is inconsistent with some previous measurements, in particular with a recent experiment of comparable precision. The beta decay strength function for the transition from ⁸B to the accessible excitation energies in ⁸Be is fit to the alpha energy spectrum using the R-matrix approach. 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, particularly for neutrino energies above 12 MeV.