Radon depletion in xenon boil-off gas

An important background in detectors using liquid xenon for rare event searches arises from the decays of radon and its daughters. We report for the first time a reduction of $^{222}$Rn in the gas phase above a liquid xenon reservoir. We show a reduction factor of $\gtrsim 4$ for the $^{222}$Rn concentration in boil-off xenon gas compared to the radon enriched liquid phase. A semiconductor-based $\alpha$-detector and miniaturized proportional counters are used to detect the radon. As the radon depletion in the boil-off gas is understood as a single-stage distillation process, this result establishes the suitability of cryogenic distillation to separate radon from xenon down to the $10^{-15}\,$mol/mol level.