PNNL

Abstract

Radioactive decays from 42Ar and its progeny 42K are potential background sources in large-scale liquid-argon-based neutrino and dark matter experiments. In the atmosphere, 42Ar is produced primarily by cosmogenic activation on 40Ar. The use of low radioactivity argon from cosmogenically shielded underground sources can expand the reach and sensitivity of liquid-argon-based rare event searches. We estimate 42Ar production underground by nuclear reactions induced by natural radioactivity and cosmic-ray muon-induced interactions. At 3,000 mwe, 42Ar production rate is 1.8×10-3 atoms per ton of crust per year, 7 orders of magnitude smaller than the 39Ar production rate at a similar depth in the crust. By comparing the calculated production rate of 42Ar to that of 39Ar for which the concentration has been measured in an underground gas sample, we estimate the activity of 42Ar in gas extracted from 3,000 mwe depth to be less than 2 decays per ton of argon per year.