We report a measurement of the neutrino-electron elastic scattering rate of 862 keV ${}^7\mathrm{Be}$ solar neutrinos based on a 165.4 kt d exposure of KamLAND. The observed rate is $582\pm 94$(kt d)${}^{-1}$, which corresponds to an 862-keV ${}^7\mathrm{Be}$ solar neutrino flux of $(3.26\pm 0.52)\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, assuming a pure electron-flavor flux. Comparing this flux with the standard solar model prediction and further assuming three-flavor mixing, a ${\nu }_e$ survival probability of $0.66\pm 0.15$ is determined from the KamLAND data. Utilizing a global three-flavor oscillation analysis, we obtain a total ${}^7\mathrm{Be}$ solar neutrino flux of $(5.82\pm 1.02)\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, which is consistent with the standard solar model predictions.

• Received 23 May 2014
• Revised 22 July 2015

DOI:https://doi.org/10.1103/PhysRevC.92.055808