Measurement of the 8B solar neutrino flux with the KamLAND liquid scintillator detector

We report a measurement of the neutrino-electron elastic scattering rate from B-8 solar neutrinos based on a 123 kton-day exposure of KamLAND. The background-subtracted electron recoil rate, above a 5.5-MeV analysis threshold is 1.49 +/- 0.14(stat) +/- 0.17(syst) events per kton-day. Interpreted as due to a pure electron flavor flux with a B-8 neutrino spectrum, this corresponds to a spectrum integrated flux of 2.77 +/- 0.26(stat) +/- 0.32(syst) x 10(6) cm(-2)s(-1). The analysis threshold is driven by Tl-208 present in the liquid scintillator, and the main source of systematic uncertainty is due to background from cosmogenic Be-11. The measured rate is consistent with existing measurements and with standard solar model predictions which include matter-enhanced neutrino oscillation.