Demonstration of Efficient Radon Removal by Silver-Zeolite in a Dark Matter Detector

We present the performance of an efficient radon trap using silver-zeolite Ag-ETS-10, measured with a spherical proportional counter filled with an argon/methane mixture. Our study compares the radon reduction capabilities of silver-zeolite and the widely used activated charcoal, both at room temperature. We demonstrate that silver-zeolite significantly outperforms activated charcoal by three orders of magnitude in radon capture. Given that radon is a major background contaminant in rare event searches, our findings highlight silver-zeolite as a highly promising adsorbent, offering compelling operational advantages for both current and future dark matter and neutrino physics experiments. Furthermore, this not only offers great promise for developing future radon reduction systems in underground laboratories, but also paves the way for innovative, multidisciplinary advancements with far-reaching implications in science, engineering and environmental health.

💡 Research Summary

This paper presents a groundbreaking experimental demonstration of a highly efficient radon removal technique using silver-exchanged zeolite (Ag-ETS-10), specifically tailored for rare-event search experiments like dark matter detection. Radon-222 is a pervasive radioactive noble gas that emanates from materials and creates significant background noise in ultra-sensitive detectors, masking potential dark matter signals. Traditional mitigation methods, such as activated charcoal traps, often require cryogenic temperatures for optimal performance, adding complexity and cost to large-scale experiments.

The research team developed and tested a radon trap system within a closed-loop gas circulation setup, mimicking realistic conditions for the NEWS-G (New Experiments with Spheres-Gas) dark matter experiment. The core detector was a 30-cm diameter spherical proportional counter (SPC) filled with an Argon/Methane mixture. The trap itself was a stainless-steel tube containing 10 grams of the adsorbent material—either Ag-ETS-10 or, for comparison, conventional granular activated charcoal (Silicarbon). Three separate experimental campaigns were conducted: two with silver-zeolite and one with activated charcoal, all operating at room temperature.

The experimental procedure was meticulously designed in four phases: (I) a background measurement, (II) radon injection into the SPC, (III) monitoring of natural radon decay without the trap, and (IV) opening the trap to circulate and purify the gas. Data analysis employed sophisticated techniques to handle high event rates, including corrections for pileup and dead-time effects using Monte Carlo simulations and Markov Chain Monte Carlo (MCMC) fitting. Two event selection cuts were applied: a “quality cut” to remove muons and electronic noise, and a more stringent “214Po cut” that isolated events from the high-energy alpha decay of Polonium-214, a radon daughter, thereby achieving a near-background-free signal.

The results were striking and conclusive. Visually, the event rates in phase IV plummeted to background levels when the silver-zeolite trap was engaged, whereas a much slower, exponential decrease was observed with activated charcoal. To quantify the performance, the researchers calculated the Radon Reduction Ratio (R-value), defined as the expected event rate (without a trap) divided by the observed rate (with the trap). At a 90% lower confidence limit, the R-values for silver-zeolite campaigns ranged from 3.8 × 10^3 to 6.2 × 10^3. In stark contrast, the R-value for the activated charcoal campaign was only between 5.4 and 11.4. This represents an improvement of approximately three orders of magnitude (a factor of 1000) for silver-zeolite over activated charcoal at room temperature.

The study concludes that silver-zeolite Ag-ETS-10 is a superior adsorbent for radon mitigation in particle physics experiments. Its high efficiency at room temperature offers compelling practical advantages: it eliminates the need for complex cryogenic systems, simplifies detector design, and reduces operational costs. This advancement not only promises to enhance the sensitivity of current and future dark matter and neutrino experiments by drastically reducing a dominant background source but also paves the way for innovative applications in underground laboratory infrastructure and environmental radon remediation, showcasing significant multidisciplinary impact.