$^{42}$Ar Production and Injection to a Liquid Argon Environment for Background Mitigation Studies

TL;DR

This study demonstrates the production, injection, and monitoring of $^{42}$Ar and $^{42}$K in liquid argon to facilitate background mitigation research for rare-event physics experiments.

Contribution

It introduces a controlled method to produce and inject $^{42}$Ar into liquid argon, enabling detailed background studies for large-scale neutrino and dark matter detectors.

Findings

Produced $^{42}$Ar with a rate of ~1 million atoms/sec.

Injected and monitored $^{42}$K activity in a 1-ton liquid argon setup.

Modeled mixing and equilibration times of $^{42}$K in liquid argon.

Abstract

Atmosphere-sourced argon contains traces of $^{42}$Ar, whose $\beta^-$-decaying progeny $^{42}$K represents a significant intrinsic background for rare-event experiments using liquid argon (LAr) as detector or shielding medium. Understanding and mitigating this background is crucial for current and future large-scale detectors in neutrino and dark-matter physics. To enable controlled studies of $^{42}$K behavior and suppression techniques, $^{42}$Ar was produced by irradiating natural argon with 34 MeV $^{7}$Li$^{3+}$ ions at the Maier-Leibnitz-Laboratorium tandem accelerator, using beam currents of $101 \pm 5$ nA and $140 \pm 5$ nA, yielding $476 \pm 9$ Bq within two weeks, corresponding to a production rate of $\sim 1 \times 10^{6}$ atoms$\,$s$^{-1}$. The activated argon was injected into the one-ton SCARF cryostat, where two HPGe detectors monitored the subsequent $^{42}$K activity build-up. A time-dependent model describing $^{42}$Ar mixing and $^{42}$K equilibration in LAr yielded characteristic mixing time constants between one and two days. The established production and injection capability provides a reproducible platform for high-statistics $^{42}$K background studies, essential for developing and validating suppression strategies for next-generation LAr-based rare-event experiments such as LEGEND-1000.