Radioreduction and Radiopurity of Acrylic in the DEAP-3600 Dark Matter Experiment
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The DEAP-3600 experiment is a single-phase liquid argon dark matter detector employing pulse discrimination to mitigate background-like events and isolate WIMP-like events. The vessel that holds 3600~kg of target, and the flowguide that guides convective flow, are made of ultrapure acrylic plastic. Diffusion of radioactive radon and radon progeny within air into the acrylic post-fabrication result in a surface layer that is orders of magnitude more radioactive than the bulk material. The robotic sanding of the vessel and the manual sanding of the flowguides, to reduce surface contamination to that of the bulk, are discussed: the Resurfacer removed 360 ± 30 μm of acrylic from the vessel surface as a rate of 10.5 ± 2.3 g/hr, and a minimum of 37 μm was manually removed from the argon-exposed flowguide surfaces. The empirical assay of acrylic bulk contamination is also discussed: a content of (2 ± 2) × 10^-19 grams of Pb-210 per gram of acrylic was measured, with a procedural Pb-210 recovery efficiency of 47 ± 13%.