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.

Project Report

Research supported by Grant NSF PHY-0811186 accomplished two main goals: 1) Underground Argon Extraction. Argon is an excellent choice as a dark matter target thanks to its excellent pulse shape discriminatio, which allows to separate β/γ events from nuclear recoils, the expected signature of dark matter interactions. Atmospheric argon contains a radioactive isotope, 39Ar, produced by cosmic rays in the outer layers of the atmosphere. The activity of 39Ar in atmospheric argon is 1 Bq/kg (one decay per second per kilogram of argon!), and limits the size and sensitivity of argon-based dark matter detectors. 39Ar can be separated effectively by use of isotopic separation techniques (i.e., centrifuges), but supply is very limited and costs enormous (several tens of thousands of US Dollars per kilogram of purified argon). Researchers supported by Grant NSF PHY-0811186 developed a novel method to collect argon gas mined from very deep wells. This underground argon is shielded from cosmic rays and was found to be naturally depleted from the radioactive isotope 39Ar. Reserchers have extracted to date 169 kg of underground argon. The purification of the underground argon is ongoing with the use of a cryogenic distillation plant funded by the US National Science Foundation and installed and operated at Fermilab. 2) Measurement of 39Ar in uderground argon. Researchers supported by Grant NSF PHY-0811186 built a low-background 1-kg liquid argon scintillation detector to measure the residual concentration of 39Ar in underground argon. The detector was operated on the surface at Princeton University and then underground at the Kimballton Underground Research Facility (KURF). No trace of 39Ar decay was observed in underground argon, resulting in a limit of 5 mBq/kg (one decay every 200 seconds per kilogram of argon!). This novel method achieved the best sensitivity ever reached for measurements of 39Ar contaminations in argon, improving previous limits by a factor ten. The research activities supported by Grant NSF PHY-0811186 enabled the construction of the dark matter exepriment DarkSide-50, currently under commissioning at Laboratori Nazionali del Gran Sasso, Italy. DarkSide-50 is set to start operations in 2013. They also enable consideration of future, larger and more powerful dark matter detectors with underground argon as target.

National Science Foundation (NSF)
Division of Physics (PHY)
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James J. Whitmore
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Princeton University
United States
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