We report a measurement of the neutrino-electron elastic scattering rate of 862 keV ^7Be solar neutrinos based on a 165.4 kt d exposure of KamLAND. The observed rate is 582±94(kt d)^(−1), which corresponds to an 862-keV Be7 solar neutrino flux of (3.26±0.52)×10^9cm^(−2)s^(−1), assuming a pure electron-flavor flux. Comparing this flux with the standard solar model prediction and further assuming three-flavor mixing, a ν_e survival probability of 0.66±0.15 is determined from the KamLAND data. Utilizing a global three-flavor oscillation analysis, we obtain a total ^7Be solar neutrino flux of (5.82±1.02)×10^9cm^(−2)s^(−1), which is consistent with the standard solar model predictions.
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