Study of radon reduction in gases for rare event search experiments

Abstract

The noble elements, argon and xenon, are frequently employed as the target and event detector for weakly interacting particles such as neutrinos and Dark Matter. For such rare processes, background radiation must be carefully minimized. Radon provides one of the most significant contaminants since it is an inevitable product of trace amounts of natural uranium. To design a purification system for reducing such contamination, the adsorption characteristics of radon in nitrogen, argon, and xenon carrier gases on various types of charcoals with different adsorbing properties and intrinsic radioactive purities have been studied in the temperature range of 190-295 K at flow rates of 0.5 and 2 standard liters per minute. Essential performance parameters for the various charcoals include the average breakthrough times (τ), dynamic adsorption coefficients (k a ) and the number of theoretical stages (n). It is shown that the k a -values for radon in nitrogen, argon, and xenon increase as the temperature of the charcoal traps decreases, and that they are significantly larger in nitrogen and argon than in xenon gas due to adsorption saturation effects. It is found that, unlike in xenon, the dynamic adsorption coefficients for radon in nitrogen and argon strictly obey the Arrhenius law. The experimental results strongly indicate that nitric acid etched Saratech is the best candidate among all used charcoal brands. It allows reducing total radon concentration in the LZ liquid Xe detector to meet the ultimate goal in the search for Dark Matter.

The LUX-ZEPLIN (LZ) radioactivity and cleanliness control programs

“…A second mode of operation for the system uses 57 g of activated carbon (a synthetic charcoal sourced from Carbo-Act International [26]) as a radon collection trap. In larger emanation volumes, the radon is initially absorbed into the cooled trap while the carrier gas passes through.…”

“…Sensitivity projections for LZ presented in [1] include the effect of the online charcoal-based radon-removal system, operating continuously to scrub gaseous Xe [3,26]. Projections also assume the expected suppression of radon diffusion in certain materials at low temperatures.…”

Projected WIMP sensitivity of the LUX-ZEPLIN dark matter experiment

“…For these estimates a conservative approach is adopted, only taking credit for a reduction at LXe temperatures if there is direct knowledge that such a reduction will occur. The LZ gas handling apparatus [23] includes a radon reduction system that can take a small stream of gas from problem areas, such as the cable conduits, and perform online radon purification [60]. The current best estimate for emanation from LZ components results in a 222 Rn specific activity of 1.53 μBq=kg of LXe.…”

A Study on the radon removal performance of low background activated carbon

“…For activated carbon used in low background experiments, it is necessary to reduce the internal radioactive impurities because radon emanated from the carbon itself which ultimately limits its radon adsorption capacity. Carboact actived carbon [11] has the lowest background (∼ 0.3 mBq/kg [12,13]) but the price is extremely expensive(15000 USD/kg) [12]. Saratech actived carbon [14] is a good alternative for many physicists because its background is comparable to Carboact (∼ 1 mBq/kg [12]) while the price is much cheaper(35 USD/kg) [12].…”

“…Carboact actived carbon [11] has the lowest background (∼ 0.3 mBq/kg [12,13]) but the price is extremely expensive(15000 USD/kg) [12]. Saratech actived carbon [14] is a good alternative for many physicists because its background is comparable to Carboact (∼ 1 mBq/kg [12]) while the price is much cheaper(35 USD/kg) [12]. In order to study the application prospect of Saratech activated carbon in low background experiments, a radon emanation system has been developed to measure the background, and an adsorption performance research system has been developed to study its radon adsorption capacity at various temperatures.…”