Radon Background Control for the SuperCDMS SNOLAB Dark Matter Experiment*

The Super Cryogenic Dark Matter Search (SuperCDMS) experiment at SNOLAB will use solid-state germanium and silicon cylindrical detectors to measure ionization and phonons produced by the scattering of dark matter particles. The dominant expected background at low energy for SuperCDMS SNOLAB is from radon daughters that have plated out onto detector surfaces. Therefore, understanding and mitigating plate-out rates during detector fabrication, assembly, and installation is critical. A study of radon-daughter plate-out during detector polishing and assays of plate-out onto detector hardware provide limits on backgrounds. I will describe the construction and commissioning of the SuperCDMS SNOLAB radon mitigation system, which is built upon the design of the SD Mines prototype radon mitigation system that has achieved a 4000× reduction of radon to a cleanroom activity of ∼20 mBq/m³, and show the resulting expected background from radon daughters for the experiment. Recent results from an etch to reduce otherwise dominant sidewall backgrounds without damaging sensors on the detector faces will also be described.