Determination of theB8Neutrino Spectrum

We have measured the total energy of the alpha particles following the beta decay of ${}^8\mathrm{B}$ by implanting ${}^8\mathrm{B}$ into a planar silicon surface barrier detector. Calibration was performed using alpha particles following the beta decay of ${}^{20}\mathrm{N}\mathrm{a}$, similarly implanted. The alpha spectrum is used to infer the ${}^8\mathrm{B}$ neutrino spectrum which is an important input in the interpretation of experiments that detect energetic neutrinos from the Sun. The alpha spectrum reported here is in disagreement with the previous best measurement which used two detectors in coincidence.