DUSEL R&D: Depleted Argon from Underground Sources

Princeton University, Princeton NJ

Investigators

Abstract

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.