低氡环境超纯水中脱气膜除氡与测试系统研究

In current and near future non-accelerator experiments, such as neutrino and dark matter direct detection experiments, ultrapure water is an attractive material employed as an anti-coincidence detector or target. The preparation of ultrapure water with low levels of radon is critical to maximizing an experiment’s sensitivity. Radon, a radioactive element existing in the gas state, is readily soluble in water. The concentration of radon in natural water is higher than 1 Bq/m^3, thus radon is the most important background in the process of making ultrapure water, which seriously restricts all kinds of low background experiments that use ultrapure water. Among the radon removal methods, the degassing membrane has low cost, is environmentally friendly, has long term stability, is easy to apply in batches, and can reach a flow rate up to 100 tons / hour. We have achieved 0.2 Bq/m^3, which is required by the JUNO experiment, and can save 200 million yuan in experimental funds. Based on this, the radon measurement system can obtain a sensitivity of 0.001 Bq/m^3 by reducing radon exhalation on the surface of stainless steel and by utilizing the low temperature drying method. By reforming the interface of sealed degassing equipment and optimizing the experimental parameters of film degassing, the concentration of radon in water can reach 0.01 Bq/m^3. This would provide a solid underpinning for experiments with higher demand for ultrapure water in the future.

在当前及可见未来的中微子和暗物质直接探测等非加速器实验中，通常用超纯水做反符合探测器或者直接做为靶物质，实验规模大，灵敏度高，对本底要求也更为严格。而以气体形式存在的放射性元素氡极易溶于水中，自然环境水中氡含量大于1Bq/m^3，已成为超纯水生产过程中留存下来的最重要的本底，制约了使用超纯水的低本底实验的物理灵敏度。因此，除去水中氡气是一件非常必要也十分有现实意义的工作。除氡的各种方法中，脱气膜除氡具有造价低廉，无污染，稳定性好，而且容易成批量扩大等特点，适合大体积，流量达100吨/小时以上的实验需求。已完成了水中氡浓度0.2Bq/m^3的江门中微子实验需求，可节约实验经费2亿元。在此基础上，通过降低不锈钢表面氡析出和低温干燥水气法提高水氡测量系统灵敏度到1mBq/m^3；改造密封脱气膜接口和优化脱气膜使用条件达到水中氡浓度10mBq/m^3，为未来对超纯水有更高要求的实验打下坚实的基础。

当前，以大亚湾中微子实验以及正在建造的江门中微子实验为代表的极低本底实验中，直接用超过2米厚的超纯水做防护层和反符合探测器。随着实验灵敏度要求越来越高，超纯水中的氡含量成为影响实验关键本底。本项目基于江门中微子实验水循环模型探测器，利用3级脱气膜，通过微气泡装置往超纯水中增加高纯氮气的方法，使得超纯水中的氡含量由~Bq/m3降低到几个mBq/m3；在水氡测试系统中，通过对不锈钢表面进行精细的电解抛光来降低探测器腔室氡析出本底，同时采用基本无本底的低温干燥方法代替传统的化学试剂干燥水气，使得水氡测试系统也达到了小于10mBq/m3，满足测量需要，实验测试结果为5.47±2.75 mBq/m3，完全达到了本项目的小于10mBq/m3主要目标。具有重要的科学意义，使得在线，大流量低氡超纯水得已实现，满足类似江门实验几万吨低氡超纯水的需求，也为将来更进一步的实验需求，奠定了基础。