Subsurface cosmogenic and radiogenic production of ^{42}Ar

TL;DR

This paper estimates underground production rates of ^{42}Ar, a background source in liquid-argon detectors, showing it is significantly lower than ^{39}Ar, thus supporting the use of underground argon for rare event searches.

Contribution

The study provides the first detailed estimate of underground ^{42}Ar production, demonstrating its minimal presence compared to ^{39}Ar, which enhances the feasibility of using underground argon in experiments.

Findings

^{42}Ar production rate is 1.8E-3 atoms per ton per year at 3000 mwe.

^{42}Ar activity is less than 2 decays per ton per year in underground argon.

^{42}Ar production is seven orders of magnitude lower than ^{39}Ar at similar depths.

Abstract

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