| KamLAND is an anti-neutrino detector installed in the Japanese Kamioka underground laboratory. It is used to measure anti-neutrino oscillations in disappearance mode using Japanese nuclear power reactors as anti-neutrino sources at an average distance of 180 km. To make the low rate measurement possible, random background had to be reduced to unprecedented levels by selecting all detector materials with the lowest possible activity content. A novel technique based on neutron activation has been developed for an extremely high sensitivity analysis of trace elements in the KamLAND detector media. Detection limits of <1.3 x 10-15 g 40K/g LS, <2.8 x 10 -15 g Th/g LS, and <8 x 10 -15 g U/g LS have been achieved. When the KamLAND detector became operational, data were used for the direct determination of the radio-nuclide concentrations in the LS. Studies of Bi--Po sequential decays indicate that the effective equilibrium concentrations of 238U and 232Th in LS are (2.7 +/- 0.5) x 10-18 g/g and (5.5 +/- 1.0) x 10-17 g/g, respectively. To evaluate the reactor associated systematic error, the uncertainties in the thermal power and fuel composition in commercial power plants and their impact on the calculated interaction rates are examined. The number of observed events with nu e energies above 3.4 MeV in a 414.4 day live-time data set was 203, compared to 295.5 events expected in the absence of neutrino oscillations. The number of estimated background events in the data set is 12.4 +/- 8.8, resulting in an average nue survival probability 0.645 +/- 0.048(stat) +/- 0.059(syst). The nue disappearance is observed at 99.99% level. The best-fit values for Deltam2 and sin2 2theta from the two-neutrino oscillation analysis are (8.06 +/- 0.6) x 10-5eV 2 and 0.84 respectively. The observed spectral distortion provides clean evidence for the existence of neutrino oscillations. Thermal power fluctuations indicate that the detected anti-neutrinos come from the distant nuclear reactors. It is shown that neutrino decay can only play a sub-dominant role in the nue disappearance. |
