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化水中提取微量氡的方法。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组件封装在丙烯酸容器中，丙烯酸容器连接到氡探测器容器的底部。我们还研究了其它一些方法，如膜组件的不同安装方式、使用气泵等，但这些方法都不合适。接下来，我们用700 L脱气水测氡仪进行了各种标定。700 L探测器对脱气水中氡的刻度因子为14.6 ± 2.1（计数/天）/（mBq/m^3）。目前在超级神冈所使用的70升氡探测器的校准因子约为0.3（计数/天）/（mBq/m^3）。因此，灵敏度增加了约40倍。考虑到背景水平，检测限为0.7mBq/m^3。这些结果被总结在一篇论文中，并提交给NIM A。该论文被NIM A（截至2002年10月）接受。在总结了这些结果之后，我们在一篇论文中制作了低本底PIN光电二极管的预生产样品，其中铀-238减少了约50倍。对700 L测氡仪的电场进行了优化，使测氡仪对空气中氡的收集效率提高了2倍左右。我们计划在未来测试具有优化电场和低本底光电二极管的新700 L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.