Performance study of charcoal-based radon reduction systems for ultraclean rare event detectors

The continuous emanation of radon due to trace amounts of uranium and thorium in detector materials introduces radon to the active detection volume of low-background rare event search detectors. $^{222}$Rn produces as a particularly problematic background in the physics region of interest by the "naked" beta decay of its $^{214}$Pb daughter nucleus. While charcoal-based adsorption traps are expected to be effective for radon reduction in auxiliary circulation loops that service the warm components of current generation 2 (G2) detectors at slow flow rates ($0.5-2\;SLPM$), radon reduction in the entire circulation loop at high flow rates ($\mathcal{O}$(100's)$ SLPM$) is necessary to reach high sensitivity in future generation experiments. In this article we explore radon dynamics with a charcoal-based radon reduction system in the main circulation loop of time projection chamber detectors. We find that even for perfect radon traps, circulation speeds of $2,000\;SLPM$ are needed to reduce radon concentration in a 10 ton detector by 90%. This is faster by a factor of four than the highest circulation speeds currently achieved in dark matter detectors. We further find that the effectiveness of vacuum swing adsorption systems, which have been employed very successfully at reducing atmospheric radon levels in clean-rooms, is limited by the intrinsic radon activity of the charcoal adsorbent in ultra-low radon environments. Adsorbents with about ten times lower intrinsic radon activity than in currently available activated charcoals would be necessary to build effective vacuum swing adsorption systems operated at room temperature} for rare event search experiments. If such VSA systems are cooled to about $190\,K$, this factor drops from 10 to about 2.5. This may be in reach by the time future generation experiments can be realized.