Development of a real-time detector for a small amount of radon in degasified water

本研究首先研究了从脱气净水中提取少量氡的方法。我们采用中空膜组件稳定、高效地提取氡。我们制作了一个特殊的膜模块封装在一个丙烯酸容器中，丙烯酸容器附着在氡探测容器的底部。我们还研究了其他一些方法，如以不同的方式安装膜组件，使用气泵，但都不合适。接下来，我们使用700L氡检测仪对脱气水进行了各种校准。我们得到了700L探测器对脱气水中氡的校准因子14.6±2.1（计数/天）/(mBq/m^3)。超级神冈目前使用的70L氡探测器的校准因子约为0.3（计数/天）/(mBq/m^3)。因此，灵敏度提高了约40倍。考虑背景电平，检测限为0.7mBq/m^3。这些结果总结成一篇论文，并提交给NIM a。论文被NIM a接受（截止2002年10月）。在总结了这些结果后，我们制作了一个低背景PIN光电二极管的预生产样品，其中铀-238减少了约50倍。对700L氡探测器的电场进行了优化，使其对空气中氡的收集效率提高了约2倍。我们计划在未来使用优化的电场和低背景光电二极管测试新的700L氡探测器。

In this research, the method to extract a small amount of radon from degasified and purified water was studied at first. We used a hollow membrane module to extract radon stably and efficiently. We made a special membrane module encapsulated in an acrylic vessel, and the acrylic vessel was attached to the bottom of the radon detector vessel. We studied some other methods, for example, mount the membrane module in different way, use an air pump, but, all of them were not appropriate.Next, we have performed various calibrations by using the 700L radon detector for degasified water. We have obtained 14.6 ± 2.1(count/day)/(mBq/m^3) as a calibration factor of the 700L detector against radon in degasified water. The calibration factor for the current 70L radon detector used at Super-Kamiokande was about 0.3(count/day)/(mBq/m^3). Therefore, the sensitivity increased by about factor 40. Considering the background level, the detection limit became 0.7mBq/m^3. These results were summarized in a paper, and submitted to NIM A. The paper was accepted by NIM A (as of Oct. 2002).After summarizing these results in a paper, we made a pre-production sample of low background PIN photodiode in which Uranium-238 was reduced by factor about 50. We also optimized the electric field in the 700L radon detector, then the collection efficiency against radon in air was increased by factor about 2. We are planning to test the new 700L radon detector with the optimized electric field and the low background photodiode in the future.