First operation of poly(ethylene naphthalate) enclosures for high-purity germanium detectors in liquid argon for $^{42}$K/$^{42}$Ar mitigation

TL;DR

This work addresses cosmogenic $^{42}$Ar–driven $^{42}$K backgrounds in LEGEND by evaluating poly(ethylene naphthalate) (PEN) enclosures around HPGe detectors operating in $^{42}$Ar–enriched liquid argon. The authors compare bare and PEN-enclosed detectors in a cryogenic setup with a $^{42}$Ar spike to assess energy performance and the time-dependent behavior of $^{42}$K decays, focusing on suppression near the $Q_{etaeta}$ region of $^{76}$Ge. They find no evidence that PEN deteriorates energy stability or resolution, while the beta rate near $Q_{etaeta}$ is significantly reduced and the gamma/beta time evolution reveals enclosure-driven dynamics consistent with ion drift and surface charging. These results support PEN-based mitigation as a practical alternative when underground argon is unavailable, informing LEGEND-1000 design and background-control strategies for $0 uetaeta$ searches.

Abstract

Commercial argon contains cosmogenic $^{42}$Ar whose progeny $^{42}$K is a critical background component for the Large Enriched Germanium Experiment for Neutrinoless $ββ$ Decay (LEGEND). LEGEND operates High-Purity Germanium (HPGe) detectors bare in liquid argon. $^{42}$K is attracted by the HPGe detectors' electric fields, and drifts toward the germanium surface, where it undergoes beta decay. LEGEND-1000 will mitigate $^{42}$K-induced background by using underground-sourced argon, depleted in cosmogenic isotopes. If underground argon is not available, mitigation techniques must be employed. Poly(ethylene naphthalate) (PEN) enclosures were proposed to hinder the ion drift, decrease the beta-particle's energy, and produce scintillation light. In this paper, we report on operating two HPGe detectors, both bare and PEN-enclosed, in $^{42}$Ar-enriched liquid argon, and find no evidence for deterioration of energy stability or resolution due to the enclosures. We monitor the beta and gamma rates of $^{42}$K, find complex time-dependencies extending to roughly 30 days after applying the HPGe detectors' high-voltage, and qualitatively demonstrate the $^{42}$K suppression capabilities of enclosures.

Figures (4)

• Figure 1: Simplified decay scheme of $^{42}\text{Ar}$ and $^{42}\text{K}$. $^{42}\text{K}$ is mainly decaying directly to the ground state of $^{42}\text{Ca}$ and has a higher $Q_\beta$-value than $^{76}\text{Ge}$. Therefore, it contributes to background in the region of interest for LEGEND.
• Figure 2: HPGe detectors mounted in a string. A BEGe detector is mounted on top, and an IC detector is mounted on the bottom. The setup is bare on the left and enclosed in PEN on the right.
• Figure 3: Energy resolution at 2.6 of all $^{228}\text{Th}$ calibration runs in periods 0, 5, 6, and 7. No evidence was found to suggest that pen enclosures deteriorate the hpge detectors' energy resolutions.
• Figure 4: Time evolution of beta and gamma radiation by $^{42}\text{K}$ recorded with the IC detector (top) for all physics runs in p05, p06, and p07. The rates show a complex time-dependence. Partially filled bins are corrected. Energy stability of the 1524.6 gamma line (bottom) for the same dataset, along a projection onto the energy axis (bottom right). The peak position and width are approximately stable in time. The standard dsp is used.