Very Low Background Radiation Measurements with GSO Scintillator

本研究的目标是用GSO（Gd_2SiO_5：Ce）闪烁体测量环境γ射线和中子时实现很低的本底。如果能区分来自地球静止轨道辐射源中受污染放射性元素的事件，这是可能的。以下是本研究的结果。(1)用GSO闪烁体进行低本底测量：在非常低的本底条件下获得了Gd-152非常长的半衰期和关于Gd-160的Onubetabeta衰变的知识。从测量污染的这些元素和U-和Th-系列元素进行了估计。(2)用GSO闪烁体进行中子探测：观测了GSO闪烁体对热中子的响应。用低强度中子源证实了热中子俘获Gd-158产生的79.5keV γ射线，并对0.9”φ * 1”GSO晶体获得了14%的高探测效率。应用计划将非常低的环境中子通量的测量将是可能的。(3)地球静止轨道闪烁体闪烁的脉冲形状：为了在背景非常低的情况下区分α射线与地球静止轨道闪烁体中的污染元素，用高速数字示波器测量闪烁脉冲形状。研制了脉冲波形分析系统，测量了不同Ce浓度的GSO晶体闪烁的衰减时间。α射线和γ射线的衰变时间没有很大的差异。本研究提供了有关极低背景条件下地球静止轨道辐射衰减器的知识。

In this research, very low backgrounds are intended to be realized for measurements of environmental gamma-rays and neutrons with GSO (Gd_2SiO_5 : Ce) scintillators. It may be possible if events from contaminated radioactive elements in GSO scintillators are discriminated. Followings are the results from the present research.(1) Low background measurement with a GSO scintillator : The very long half-life of Gd-152 and knowledge on the Onubetabeta decay from Gd-160 were obtained under very low background conditions. From the measurements contamination of these elements and U- and Th-series elements were estimated.(2) Neutron detection with a GSO scintillator : The GSO scintillator response were observed for thermal neutrons. Gamma-rays of 79.5 keV from Gd-158 resulted by the thermal neutron capture were confirmed with a low intensity neutron source, and then a high detection efficiency of 14% was obtained for a 0.9" phi * 1" GSO crystal. Applications will be planed to measurement of very low flux of environmental neutrons will be possible.(3) Pulse shape of scintillation from a GSO scintillator : In order to discriminate alpha-rays from the contaminated elements in GSO scintillators for very low background condition, scintillation pulse shapes were measured with a high speed digital oscilloscope. A pulse shape analyzing system were developed for this purpose, and measured the decay times of scintillation from GSO crystals of different Ce concentrations. No large difference in the decay times of scintillations for alpha-rays and gamma-rays. Further measurements for dependence to applied temperature will be needed.The present research provides the knowledge about very low background condition with GSO scintillators.