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

Mario Schwarz, Christoph Vogl, Niko N. P. N. Lay, Tommaso Comellato, Gunther Korschinek, Moritz Neuberger, Oskar Moras, Patrick Krause, Stefan Schönert

Abstract

Atmosphere-sourced argon contains traces of $^{42}$Ar, whose $β^-$-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.

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

Abstract

Atmosphere-sourced argon contains traces of Ar, whose -decaying progeny 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 K behavior and suppression techniques, Ar was produced by irradiating natural argon with 34 MeV Li ions at the Maier-Leibnitz-Laboratorium tandem accelerator, using beam currents of nA and nA, yielding Bq within two weeks, corresponding to a production rate of atomss. The activated argon was injected into the one-ton SCARF cryostat, where two HPGe detectors monitored the subsequent K activity build-up. A time-dependent model describing Ar mixing and 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 K background studies, essential for developing and validating suppression strategies for next-generation LAr-based rare-event experiments such as LEGEND-1000.

Paper Structure

This paper contains 13 sections, 12 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Ar-42 production at MLL
  3. Ar-42 production
  4. Screening of irradiated argon gas
  5. Ar-42 production rate
  6. Injection of Ar-42 into SCARF
  7. Experimental setup
  8. Injection of Ar-42 into Scarf
  9. Energy spectrum
  10. K-42 build-up
  11. K-42 build-up
  12. Conclusion and outlook
  13. Details of K-42 build-up modeling

Figures (8)

  • Figure 1: Simplified $^{42}$Ar/$^{42}$K decay scheme. Cosmogenic $^{42}$Ar $\beta^-$ decays to $^{42}$K. $^{42}$K $\beta^-$ decays to the $^{42}$Ca ground state in 82 of the cases, and to the first excited state in 18, which is followed by a 1525$\gamma$ emission.
  • Figure 2: Setup for $^{42}$Ar production at MLL, showing the irradiation cell mounted at the end of the beamline. The $^7$Li$^{3+}$ beam is entering from the left, and irradiating the 40 long cell. After irradiation, the activated argon gas is condensed into a LN$_2$-cooled transfer bottle (not shown) below the beamline.
  • Figure 3: P&ID of the setup used for producing $^{42}$Ar. The gate valve which separates the beam from the irradiation cell is shown alongside the $^{\mathrm{nat}}$Ar gas cylinder, the $^{42}$Ar transfer bottle and the LN$_2$ dewar used to extract the irradiated argon gas from the cell. A vacuum pump is employed to remove air, and several valves define the gas flow.
  • Figure 4: A single transfer bottle in the "GEM" screening station at TUM. All 10 $^{42}$Ar bottles were measured in this station. The first six bottles, filled in 2018, were screened a second time in 2023.
  • Figure 5: $^{42}$Ar and $^{39}$Ar production cross sections for irradiation of $^{40}$Ar with $^7$Li$^{3+}$, predicted by PACE4 PhysRevC.21.230PACE4.
  • ...and 3 more figures