Development of a Hermetic Gaseous Xenon Detector for Suppressing External Radon Background

TL;DR

Radon-induced backgrounds threaten the sensitivity of next-generation liquid xenon detectors. The authors demonstrate a compact hermetic gaseous xenon detector with flange-based mechanical sealing (2 mm $ePTFE$ gasket, tightened to $5.0\ \mathrm{N\cdot m}$) to physically isolate the active volume from external radon, and quantify leakage via a dual-loop GXe system. Over a 670-hour radon-injection campaign, two independent channels (electrostatic radon detectors and PMTs) yield consistent leakage-flow estimates of $\sim(2.9-2.6)\times10^{-11}$ m$^{3}$ s$^{-1}$ and steady-state interior/exterior radon ratios of $\sim1\times10^{-2}$, confirming effective radon suppression by about a factor of 100. Extrapolations to a 60-tonne XLZD LXe TPC indicate a leakage of $\sim1.2\times10^{-2}$ mBq, negligible compared with an expected internal emanation of ~3 mBq, implying strong potential for hermetic designs to meet neutrino-fog background goals. The work supports practical implementation of hermetic, low-radioactivity inner detectors in large-scale LXe experiments and sets the stage for LXe-condition tests and TPC integration.

Abstract

Radon-induced backgrounds, particularly from $^{222}$Rn and its beta-emitting progeny, present a critical challenge for next-generation liquid xenon (LXe) detectors aimed at probing dark matter down to the neutrino fog. To address this, we developed a compact hermetic gaseous xenon (GXe) detector. This device physically isolates the active volume from external radon sources by using a PTFE vessel sealed between two quartz flanges with mechanically compressed ePTFE gaskets. To quantify radon sealing performance, we implemented a dual-loop GXe circulation system and conducted a 670-hour radon-injection measurement campaign. Radon ingress into the hermetic detector was monitored using electrostatic radon detectors and photomultiplier tubes (PMTs). From these two independent measurements, the steady-state ratios of the radon concentrations inside the hermetic detector to those outside were estimated to be $ (1.1 \pm 0.1) \times 10^{-2} $ and $ (1.1 \pm 0.2) \times 10^{-2} $, corresponding to radon-leakage flows of $ (2.9 \pm 0.3) \times 10^{-11} $ and $ (2.6 \pm 0.4) \times 10^{-11} $ $ \thinspace $m$^{3}$ $ \thinspace $ $\mathrm{s}^{-1}$, respectively. An extrapolation to a 60-tonne LXe TPC such as XLZD suggests that the radon leakage could amount to $ 1.2 \times 10^{-2} $ $ \thinspace $mBq, which is negligible compared to the expected natural radon emanation inside the detector, typically 3$ \thinspace $mBq. These results demonstrate that flange-based mechanical sealing provides an effective solution for realizing radon-isolated inner detectors in large-scale LXe experiments.

Figures (7)

• Figure 1: Schematic diagram (left) and photograph (right) of the hermetic detector developed in this study.
• Figure 2: Baseline of helium leak rate as a function of the torque applied to the bolts. Red and blue points correspond to gasket widths of 3 mm and 2 mm, respectively.
• Figure 3: Schematic overview of the gas circulation system. The brown and blue lines correspond to the inner and outer circulation loops, respectively. The mixing buffer is connected to both loops and allows for controlled exchange between them.
• Figure 4: Pressure of the system during the radon run. Upper: Red and blue lines represent the pressure for inner volume ($P_{\mathrm{inner}}$) and outer volume ($P_{\mathrm{outer}}$), respectively. Lower: Pressure difference between inner and outer volume. Data within the shaded region were excluded from the analysis. See the text for details.
• Figure 5: Energy spectra of alpha particles observed by the electrostatic radon detectors during (a) the calibration run and (b) the radon run. Blue and red histograms correspond to the outer and inner volumes, respectively. The shaded regions indicate the energy window used in the analysis, corresponding to the range [7.0, 7.8] MeV.