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

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

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.