DUSEL R&D: Depleted Argon from Underground Sources

暗物质存在的证据现在是令人信服的，但它的性质仍然是一个根本的谜。尤其耐人寻味的是，暗物质可能是由弱相互作用的大质量粒子组成的。可以通过它们与原子核的碰撞来探测到它们，但这种碰撞的预期低速率和反冲核的低能量需要具有极低本底速率的大质量探测器。液态Ar是闪烁和电离探测器的极佳靶材，具有独特的特性，使其对暗物质的探测特别有吸引力。氩暗物质探测器的大小和灵敏度的可能限制来自Argon-39，这是一种宇宙射线的放射性产物，对大气中的Ar的污染水平为1贝克勒尔/公斤。在Argon-39中耗尽的Ar将是非常可取的，以提高目前探测器的灵敏度，并允许建造未来的多吨Ar暗物质探测器。离心法和差热扩散法是Argon-39/Argon-40同位素分离的既定技术，但考虑到它们的成本和全球生产能力，这些选择是不现实的。天然气井是一个很有希望的来源，因为宇宙射线产生的Argon-39在地下受到了强烈的抑制。投资促进机构正在对地下油井中的Ar-39/Ar-40比率进行调查，迄今已查明两个含有Ar-39中耗尽的气体的来源。该奖项提供资金，用于部署一个小型系统，以收集和提纯产量约为1公斤/天的Ar。此外，该合同还将用于开发一种低本底、重1公斤的液态Ar探测器，其对Argon-39的灵敏度是大气中Ar水平的0.1%。它将用于监测地下气体的生产，并测试来自其他气源的1公斤气体样本，如果它们可用的话。对于更广泛的影响，这项工作将促进研究生和本科生的教育。耗尽的Ar还可能使研究反应堆和高强度停止介子中微子源的中微子，以及开发非常灵敏的中子探测器成为可能。

Evidence for the existence of dark matter is now compelling, but its nature remains a fundamental mystery. Particularly intriguing is the possibility that dark matter is made of Weakly Interacting Massive Particles. These may be detected by their collisions with nuclei, but the expected low rate of such collisions and low energy of the recoil nuclei require massive detectors with extremely low background rates. Liquid argon is an excellent target for scintillation and ionization detectors and has unique features that make it particularly attractive for the detection of dark matter. Possible limits to size and sensitivity of argon dark matter detectors come from Argon-39, a radioactive product of cosmic rays that pollutes atmospheric argon at the level of one Becquerel/kilogram. Argon depleted in Argon-39 would be highly desirable to improve the sensitivity of the current detectors and to permit construction of future multi-ton argon dark matter detectors. Centrifugation and differential thermal diffusion are established techniques for Argon-39/Argon-40 isotopic separation, but, with their cost and the global production capacity, these options are not practical. Natural gas wells are a promising source because Argon-39 production by cosmic rays is strongly suppressed underground. The PIs are conducting an investigation of the Argon-39/Argon-40 ratio in underground wells and have so far identified two sources of gas containing argon depleted in Argon-39. This award provides funding to deploy a small-scale system to collect and purify argon with a production rate of about 1 kilogram/day. In addition, this award will be used to develop a low-background 1-kilogram liquid argon detector with a sensitivity to Argon-39 that is 0.1% of the level in atmospheric argon. It will be used to monitor the production of the underground argon gas and to test 1-kilogram gas samples from other gas sources, should they become available. For Broader Impacts, this work will advance the education of graduate and undergraduate students. Depleted argon may also enable studies on neutrinos from reactors and from high-intensity stopped-pion neutrino sources, and the development of very sensitive neutron detectors.