TheB8neutrino spectrum

Knowledge of the energy spectrum of ${}^8\mathrm{B}$ neutrinos is important for interpreting experiments that detect energetic neutrinos from the Sun. The neutrino spectrum deviates from the allowed $\beta$-decay approximation because of the broad $\alpha$-unstable ${}^8\mathrm{Be}$ final state and recoil order corrections. We have measured the total energy of the $\alpha$ particles emitted following the $\beta$ decay of ${}^8\mathrm{B}$. The measured $\alpha$ spectrum is inconsistent with a recent measurement of comparable precision. The $\alpha$ spectrum is fit using the R-matrix approach, which gives a functional form for the $\beta$-decay strength function for the transition from ${}^8\mathrm{B}$ to the accessible excitation energies in ${}^8\mathrm{Be}$. 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.