氡在液闪中的扩散行为研究

In order to find rare physical events interested, the sensitive volume of the detector is gradually increased, and the background requirement of the detector is also more strictly in the field of particle physics experiments. Liquid scintillator (LS) is widely used as targets in the large detectors with excellent performance, such as Jiangmen Underground Neutrino Observatory (JUNO), Taishan Antineutrino Observatory (TAO). In order to meet the needs of physical experiments, it is necessary to strictly control the background of liquid scintillator and source of background. Natural radioactive radon dissolved in liquid scintillator and its natural radioactive daughter lead with a half-life of 22.3 years will be the most important background which decides whether the experiment is successful or not. Radon which are from sources, such as detector leakage, sealed nitrogen and radon emanation for material, can diffuse into liquid scintillator. In addition to restricting these sources, the diffusion coefficient, migration properties and distribution of radon in liquid scintillator need to be studied for providing a reliable basis for future experimental analysis of background caused by radon and lead. For this purpose, a device for measuring radon diffusion coefficient in liquid scintillator is proposed, and the migration properties and distribution of radon in liquid scintillator are studied by using the prototype detector of JUNO central detector. The developed device can also be used as a platform for measuring radon diffusion coefficient in various materials, which can serve various physical experiments.

当前粒子物理实验研究领域中，为寻找感兴趣的稀有物理事例，探测器灵敏体积逐渐加大，对探测器的本底要求也更加严格。液闪是一种性能优良的液体闪烁体，被广泛应用到大型探测器中作为靶物质，如江门中微子实验、台山中微子观测站等。为满足物理实验需求，需要严格控制液闪的自身本底及本底来源。易溶于液闪的天然放射性氡气及其半衰期长达22.3年的天然放射性子体铅将成为实验成败的最重要本底。探测器泄露处、密封氮气、材料自身析出等来源都可以使氡扩散进入液闪中。除需要对这些来源进行控制以外，还需要测量氡在液闪中的扩散系数及迁移性质、分布规律，为未来实验分析氡和铅的本底提供可靠的依据。为此提出研制一套可测量氡在液闪中扩散系数的装置，填补这方面的空白，并利用江门中微子模型探测器对氡在液闪中的迁移性质、分布规律进行相关研究。研制完成的装置也可作为氡在各种材料中扩散系数的测量平台，为各种有此需求的物理实验进行服务。

目前以液闪为探测介质的低本底实验中，氡及其子体是不可忽略的本底来源。为研究液闪中氡及其子体的分布以及迁移规律，需要了解氡在液闪中的扩散系数。为此本项目提出搭建氡在液闪中扩散系数测量的装置，首次在世界上给出氡在液闪中的扩散系数，约为1.4x10-5 cm2/s。此外研制了高灵敏度氡探测器，其灵敏度可以达到0.7mBq/m3。此外进一步提高探测器的灵敏度，针对低本底活性炭在低温下氡的吸附能力进行了研究，得到了在零下120摄氏度下，活性炭对氡气的吸附能力可达到6m3/g。基于此吸附能力，氡探测器的灵敏度可以达到uBq/m3。为证明氡在液闪中扩散系数测量装置的可靠性和测量结果的准确性，在同样的实验条件下对文献中已有测量结果的PE膜进行了测量，得到的扩散系数与文献中是自洽的。同时利用非稳态扩散模型去拟合实验数据时，也可以得到氡在液闪中的溶解度。为此专门搭建了一个氡在液闪中溶解度测量装置，以进一步验证扩散系数测量的正确性。实验结果表明两套实验装置得到的溶解度是自洽的。本项目搭建的实验平台也可以用于氡在其它材料的扩散系数及溶解度测量，为更多低本底实验服务。