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 ${\nu }_{e}e\to {\nu }_{e}e$ 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 $E_e\ge 9.3$ MeV, and 590 days at $E_e\ge 7.5$ MeV relative to calculations of the ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ flux based on the standard solar model are 0.46±0.05(stat)±0.06(syst) times the prediction of Bahcall and Ulrich, and 0.70±0.08(stat)±0.09(syst) times the prediction of Turck-Chièze 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 ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ $\beta$ decay and the cross section for ${\nu }_{e}e\to {\nu }_{e}e$ scattering. Within the statistical error, there is no evidence in the solar-neutrino signal for a significant time variation.