Radon Decay Simulation and Background Analysis of the MAJORANA DEMONSTRATOR Experiment

The Majorana Demonstrator neutrinoless double-beta decay experiment is operating at the Sanford Underground Research Facility to demonstrate the required background and scalability of a Ge-based, ton-scale experiment. It consists of two cryostats of HPGe detectors enriched in the isotope ⁷⁶Ge within a graded shield containing low-background gamma and neutron shielding materials, a sealed purge volume, and an active muon-veto system. Our initial results show we have achieved low backgrounds and we are now developing a full background model based on Monte Carlo simulations and available data. A purge system delivers LN₂ boil-off to the shield volume to mitigate the entry of Rn from the surroundings. A validation between simulations and high-Rn commissioning data is based on an improved determination of the source of decays. Further, the purge flow has been modeled and combined with the new simulations to estimate the Rn background. The details of the simulations, the effectiveness of the purge system, and an analysis of the Rn backgrounds will be presented.