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

该研究的目的是开发液体闪烁体中极少量放射性杂质的分析方法，该闪烁体用于由东北大学作为主要研究所在日本神冈矿山地下实验室进行的中微子实验KamLAND。该研究还具有去除放射性杂质的范围，以实现对1MeV以下区域的低能中微子的实时检测。采用ICP-MS（质谱分析）和NAA（中子活化分析）方法对水提法和氮气吹扫法纯化的液体闪烁体进行分析。结果，对于^<238>U/^<232>Th和^<40>K，灵敏度分别达到了10^<-13>g/g和10^<-15>g/g。在样品制备过程中去除污染对提高分析灵敏度至关重要。通过ICP-MS和NAA确认液体闪烁体纯度达到要求后，开始进行KamLAND反应堆中微子实验。发现KamLAND数据中的Bi-Po衰变链小于1μBq/m^3，足以进行^7Be太阳中微子探测。这些结果表明，这些净化方法是非常优秀的，为KamLAND发现反应堆反中微子赤字做出了很好的贡献。在亚mev区域，发现了大量的放射性活动背景，主要来自^<210>Pb和^<85>Kr的长寿命衰变，对^7Be太阳中微子实验至关重要。该计划的主要目的是寻找这些元素的有效去除方法，并做出了各种努力；升级后的氮气吹扫装置用于去除放射性稀有气体，吸附、过滤、吸附和水萃取用于去除^<210>Pb。为进一步的研究积累了宝贵的数据。

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.