Development of the dosimetry device with a magnetic levitation electrode ionization chamber which can measure a low level radioactive gas dose and neutron dose

本研究的目的是研制一种能够测量低水平放射性气体剂量和低水平中子剂量的高灵敏度磁悬浮电极电离室。研究时间为2008年，从2006年开始。首先，我们研制了一台大型磁悬浮电极电离室，并测试了该电离室的一项性能特性，该电离室的体积为8升。我们证实了电离室绝对可以测量低水平辐射剂量。我们测量了在密闭容器中放置设备的条件下的本底剂量。结果表明，暗室快门的打开不会影响背景的测量。用1升、8升和14升不同体积的电离室同时测量了本底剂量率。然后，即使电离体积不同，我们也得到了相同的剂量当量率大小和时间涨落的实验结果。在10min、30min和60min的时间间隔下测量8L电离室，其检测下限的可靠性为68%，分别为0.0054、0.0027和0.0012μSv/h。该装置可以测量10-16A级的空间电荷。用这个装置，我们测量了非常微弱的氚气体。此外，通过改进这个装置，我们测量了热中子的剂量。我们在日本原子能学会的会议上宣布了这些发现。此外，我们的文章还发表在《放射性同位素》(第55卷，第7期)和IEEE核科学会刊(第53卷，第4期)上。

A purpose of this study is to develop a high sensitivity magnetic levitation electrode ionization chamber which can measure a low level radioactive gas dose and a low level neutron dose. The study period was 2008 years from 2006.At first, we developed a large-scale magnetically levitated electrode ionization chamber and examined a performance characteristic of the chamber with the volume of 8 liters. We confirmed that the ionization chamber can definitely measure low level radiation dose. We measured background dose under the condition that a device was put in an airtight container. We obtained the result that the opening of the chamber's shutter did not influence measurement of background. We measured also dose rate of background by the ionization chamber with the different volumes of 1, 8 and 14 liters simultaneously. Then, we obtained the same experimental results about magnitude and time fluctuation of the dose equivalent rate; even if ionization volumes were different. In measurement of 8 liters ionization chamber under time intervals 10, 30 and 60 minutes, a detection limit with reliability, 68% were 0.0054, 0.0027 and 0.0012 μSv/h, respectively.At the next stage, we developed a differential type dosimetry device used a magnetic levitated electrode ionization chamber. This device is able to measure the space charge of 10-16A level. With this device, we measured very feeble tritium gas. Furthermore, by improving this device, we measured doses of a thermal neutron.We announced these findings to Meeting of Atomic Energy Society of Japan. In addition, our articles was published by the RADIOISOTOPES(Vol.55, No.7) and the IEEE Transaction on Nuclear Science(Vol.53,No.4).