Optimization a thickness of GAGG scintillator for detecting plutonium in a field of high beta and gamma background

・線バックグラウンド下におけるプルトニウム検知のためのGAGGシンチレータの厚みの最適化

森下 祐樹   ; 山本 誠一*; 井崎 賢二 ; 金子 順一*

Morishita, Yuki; Yamamoto, Seiichi*; Izaki, Kenji; Kaneko, Junichi*

線のスペクトル測定はプルトニウムとラドンの弁別のため必要である。我々はそのための線検出器を過去に開発し、線に対し良好なエネルギー分解能を有することを確認したが、バックグラウンドである線や線に対しても感度を有していた。そこで線や線が存在する場でもプルトニウムを検知するため、GAGGシンチレータの最適化を行い比較測定した。0.05mm厚のGAGGシンチレータを用いたとき、線に対し最も良いエネルギー分解能を示し、かつ感度が最も低いことが明らかとなった。

Spectroscopy of alpha particles is necessary to distinguish plutonium isotopes from radon progeny. We previously developed an alpha particle imaging detector by combining a 0.1-mm-thick GAGG scintillator with a silicon photomultiplier (SiPM). This detector had better energy resolution than the ZnS(Ag) scintillator-based alpha particle detector. However, it had or sensitivity because the GAGG scintillator was too thick compared with the range of 5.5 MeV particles. Therefore, it is difficult to measure alpha particles from plutonium in a field with high and background. To solve this problem, we optimized the thickness of GAGG scintillator by comparing three GAGG scintillators with thicknesses of 0.05 mm, 0.07 mm, and 0.1 mm. Each of the GAGG scintillators was optically coupled to SiPM, which was used as the photodetector. The peak channels and energy resolutions for 5.5 MeV alpha particles of three GAGG scintillators were compared. Also, the count rates of particles of the GAGG scintillators were evaluated by using a Sr source. The peak channel of the 0.05-mm-thick GAGG scintillator was 1.6x higher than that of the 0.1-mm-thick GAGG scintillator and 3.7x higher than that of the plastic scintillator. The 0.05-mm-thick GAGG scintillator had the best energy resolution of 11.6 % FWHM and the lowest count rate of 0.2 cpm among the three GAGG scintillators.