We report a measurement of the neutrino-electron elastic scattering rate of
862 keV 7Be solar neutrinos based on a 165.4 kton-day exposure of KamLAND. The
observed rate is 582 +/- 90 (kton-day)^-1, which corresponds to a 862 keV 7Be
solar neutrino flux of (3.26 +/- 0.50) x 10^9 cm^-2s^-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 +/- 0.14 is determined from the KamLAND data. Utilizing a
global three flavor oscillation analysis, we obtain a total 7Be solar neutrino
flux of (5.82 +/- 0.98) x 10^9 cm^-2s^-1, which is consistent with the standard
solar model predictions.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
