Measurement of Neutron Radiation Doses by Accelerator Mass Spectrometry

由于宇宙射线在上层大气中产生的长寿命放射性核素^<36>Cl（半衰期：30 x 10^4 y）只发射低能β射线，因此在环境样品中测量其辐射存在困难。近年来，加速器质谱法（AMS）在低水平计数方面得到了发展。在筑波大学加速器中心，自1999年以来一直在建造AMS的^<36>Cl测量系统。在这项研究中，我们尝试用AMS1测量^<36>Cl/Cl来确定中子辐射剂量。通过测量JCO事故辐照土壤样品中的^<36>Cl/Cl，估算了中子辐射剂量。这一结果与对事故产生的中子剂量的估计是一致的。通过测量广岛原子弹辐照样品中的^<36>Cl/Cl，重新评估了中子辐射剂量。从已知的原子弹爆炸地区的墓石中采集花岗岩样本并进行分析。测量到的^<36>Cl/Cl1比值与新计算的辐射剂量估计系统DS023一致。通过测量各种加速器设施屏蔽混凝土中的^<36>Cl/Cl来估计中子辐射剂量。对东京大学核研究中心的SF回旋加速器、东北大学核科学实验室的300 MeV电子直线加速器和高能加速器研究组织的12GeV质子同步加速器等三种加速器设备，分析了束流线附近混凝土样品中^<36>Cl/^<35>Cl的深度分布，成功估算了上述三种情况下的中子辐射剂量。AMS测量的^<36>Cl/Cl比值是评价中子剂量的一个很好的工具

Since long-lived radionuclide ^<36>Cl (half life : 30 x 10^4 y) produced by cosmic-ray in upper atmosphere emits only low energy β-ray, its radiation measurement in the environmental samples makes difficult. Recently, Accelerator Mass Spectrometry (AMS) has been developed in low level counting. In Accelerator Center, University of Tsukuba, ^<36>Cl measurement system by AMS has been constructed since 1999. In this study we tried to determine the neutron radiation dose by ^<36>Cl/Cl measurement using the AMS1. Neutron radiation dose has been estimated by measuring ^<36>Cl/Cl in the irradiated soil samples by JCO accident. This result is agreed with estimation of neutron dose emitted from the Accident2. Neutron radiation dose has been re-evaluated by measuring ^<36>Cl/Cl in the irradiated samples by atomic bomb dropped to Hiroshima. Granite samples quarried from tomb stones were collected in the known places of atomic bombed area and analyzed. The measured ^<36>Cl/Cl1 ratios were agreed with newly calculated radiation dose estimate system DS023. Neutron radiation dose has been estimated by measuring ^<36>Cl/Cl in the shielding concrete of various accelerator facilities. For three kinds of accelerator facilities, SF cyclotron (Center for Nuclear Study, The University of Tokyo), 300 MeV electron LINAC (Laboratory of Nuclear Science, Tohoku University) and 12GeV proton synchrotron (High Energy Accelerator Research, Organization), the depth profiles of ^<36>Cl/^<35>Cl ratios in concrete samples near the beam lines were analyzedIn the three cases mentioned above the neutron radiation dose were successfully estimated. The ratios of ^<36>Cl/Cl measured by AMS is a good tool to evaluate the neutron dose