Search for the second forbidden β decay of 8B to the ground state of 8Be

A significant decay branch of B-8 to the ground state of Be-8 would extend the solar neutrino spectrum to higher energies than anticipated in the standard solar models. These high-energy neutrinos would affect current neutrino oscillation results and also would be a background to measurements of the hep process. We have measured the delayed alpha particles from the decay of B-8, with the goal of observing the two 46-keV alpha particles arising from the ground-state decay. The B-8 was produced using an in-flight radioactive beam technique. It was implanted in a silicon PIN-diode detector that was capable of identifying the alpha particles from the Be-8 ground state. From this measurement we find an upper limit (at 90% confidence level) of 7.3x10(-5) for the branching ratio to the ground state. In addition to describing this measurement, we present a theoretical calculation for this branching ratio.