Development of Ultra-high Sensitivity Analysis Methods for Very Small Amount of Radio-activitie Materials in Liquid-scintillator

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The aim of the study is to develop analysis methods for very small amount of radio-impurities in the liquid scintillator which is used in a neutrino experiment KamLAND carried out by Tohoku University as a main institute in the underground laboratory in Kamioka mine in Japan. The study has also a scope of removing radio-impurities to realize real-time detection of low energy neutrinos below 1MeV region. ICP-Mass Spectrometry (ICP-MS) and Neutron Activation Analysis (NAA) methods were applied to the liquid scintillator purified by water extraction and nitrogen purge methods. As a result 10^<-13>g/g and 10^<-15>g/g sensitivities were achieved for ^<238>U/^<232>Th and ^<40>K, respectively. Removing contamination in the sample preparation was found critical to improve the analysis sensitivity. KamLAND reactor neutrino experiment was started as the required purity of the liquid scintillator was confirmed by the ICP-MS and NAA. The Bi-Po decay chains in the KamLAND data was found to be less than 1μBq/m^3 sufficient for doing ^7Be solar neutrino detection. These results showed that the purification methods are quite excellent and made good contribution for the discovery of the reactor anti-neutrino deficit by KamLAND. In sub-MeV region, a large amount of backgrounds from radio-activities mainly from ^<210>Pb and ^<85>Kr decays with long lives are found crucial for ^7Be solar neutrino experiment. The main purpose of this program was moved toward finding efficient removal methods of these elements and various efforts were made ; nitrogen purge by an upgraded test device for removing radioactive rare gases, and adsorption, filtration, adsorption as well as water extraction for removing ^<210>Pb. Precious data for further R&D studies have been accumulated.