Direct in-chamber radon-220 (thoron) emanation measurements for rare-event physics experiments

TL;DR

The paper introduces a direct in-chamber method to measure ${}^{220}\mathrm{Rn}$ emanation by placing the sample inside the detector, overcoming rapid decay losses that plague transfer-based approaches. In-chamber measurements show a 3× sensitivity improvement over flowthrough in air, rising to ~5× with helium as the carrier gas, enabling faster and more sensitive screening of low-background materials. An absolute calibration approach using a ${}^{222}\mathrm{Rn}$ response yields ${A_{220}} = 69 \pm 19~\mathrm{mBq}$, in agreement with the independently measured flowthrough value ${A_{220}} = 76 \pm 20~\mathrm{mBq}$, demonstrating both relative and absolute measurement viability. The method offers practical benefits for rapid pre-screening of surface treatments and radon mitigation studies, and could be adapted to ${}^{222}\mathrm{Rn}$ to further enhance high-sensitivity radon measurements in next-generation rare-event experiments.

Abstract

Measuring radon emanation from detector materials is a key method for controlling radon, a significant background in rare-event physics experiments. Methods for measuring radon emanation are well-established but have predominantly focused on the 222Rn isotope, the dominant radon isotope for these backgrounds. However, measurements of 220Rn (thoron), the second most abundant radon isotope, remain relatively unexplored. 220Rn emanation measurements are challenging because the 220Rn must be transferred from the emanation chamber to the active detector within its short 55 s half-life. In this study, a direct in-chamber approach for measuring 220Rn emanation is presented in which the sample is placed directly within the active detector chamber, thereby minimising losses during transfer. The method was demonstrated with a DURRIDGE RAD8 electrostatic radon detector, which measured 220Rn emanation from low-activity thoriated rods with an activity of 76 +/- 20 mBq. Compared with a conventional flowthrough 220Rn emanation setup, the in-chamber method increased sensitivity by a factor of 3. Using helium as the carrier gas provided a further sensitivity increase, giving an overall sensitivity gain of ~5. These results indicate that in-chamber 220Rn emanation measurements provide an effective tool for low-background experiments and have the potential to accelerate radon studies by exploiting the shorter half-life of 220Rn.

Figures (10)

• Figure 1: Schematic of an electrostatic chamber, showing the mechanism of $^{220}\mathrm{Rn}$ detection.
• Figure 2: Decay chain of ${}^{220}\mathrm{Rn}$ and its decay modes. Image adapted from internachiThoronDecayChain.
• Figure 3: DURRIDGE standard thoron flowthrough configuration. Image from sadler_thoron_calibration_2024.
• Figure 4: Low-activity thoron source assembly inside the acrylic emanation chamber.
• Figure 5: Alpha energy spectra for the thoron source (red) and empty emanation chamber background (black), measured with the flowthrough method over a 3 hour run, with the RAD8 calibrated energy windows A, B, C and D indicated.