Online ${}^{222}$Rn removal by cryogenic distillation in the XENON100 experiment

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column is integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant ${}^{222}$Rn background originating from radon emanation. After inserting an auxiliary ${}^{222}$Rn emanation source in the gas loop, we determined a radon reduction factor of R > 27 (95% C.L.) for the distillation column by monitoring the ${}^{222}$Rn activity concentration inside the XENON100 detector.