Experimental determination of the boron-8 neutrino spectrum

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. I have measured the total energy of the alpha particles emitted following the beta decay of 8B. The spectrum measured here is inconsistent with a previous measurement of comparable precision, but is in good agreement with a subsequent measurement. The alpha energy spectrum is fit using the R-matrix approach, which gives a functional form for the beta decay strength function for the transition from 8B to the 8Be resonance. The positron and neutrino energy spectra, corrected for recoil order effects, are constructed from the strength function. The inferred positron spectrum is in excellent agreement with a previous direct measurement. The neutrino spectrum disagrees with some previous experiments, particularly for neutrino energies above 12 MeV.