Real-time, directional measurement ofB8solar neutrinos in the Kamiokande II detector
The method of $^{8}\mathrm{B}$ solar-neutrino measurement by means of the reaction ${\ensuremath{\nu}}_{e}e\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}e$ in the Kamiokande II detector is described in detail. A data sample of 1040 live detector days in the time period January 1987 through April 1990 yields a clear directional correlation of the solar-neutrino-induced electron events with respect to the Sun and a measurement of the differential electron energy distribution. The measured flux of $^{8}\mathrm{B}$ solar neutrinos from the subsamples of 450 days at electron energy threshold ${E}_{e}\ensuremath{\ge}9.3$ MeV, and 590 days at ${E}_{e}\ensuremath{\ge}7.5$ MeV relative to calculations of the $^{8}\mathrm{B}$ flux based on the standard solar model are 0.46\ifmmode\pm\else\textpm\fi{}0.05(stat)\ifmmode\pm\else\textpm\fi{}0.06(syst) times the prediction of Bahcall and Ulrich, and 0.70\ifmmode\pm\else\textpm\fi{}0.08(stat)\ifmmode\pm\else\textpm\fi{}0.09(syst) times the prediction of Turck-Chi\`eze et al. The shape of the recoil electron energy distribution is consistent with that expected from the product of the known shape of the neutrino flux from $^{8}\mathrm{B}$ $\ensuremath{\beta}$ decay and the cross section for ${\ensuremath{\nu}}_{e}e\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}e$ scattering. Within the statistical error, there is no evidence in the solar-neutrino signal for a significant time variation.