Measurement of the Radiopurity of Acrylic for the DEAP-3600 Dark Matter Experiment



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Recording Details

Speaker(s): 
PIRSA Number: 
11070078

Abstract

The DEAP-3600 single-phase liquid argon detector at SNOLAB will increase the sensitivity to spin-independent WIMP-nucleon scatters by two orders of magnitude, allowing for the possibility of dark matter particle detection. The spherical detector will contain 3600 kg of liquid argon in an 85 cm radius acrylic vessel surrounded by 255 photomultiplier tubes (PMTs). After a collision between a WIMP and an Ar-40 nucleus, the scintillation light from the recoiling nucleus will be collected by PMTs. The separation of background events from WIMP events is critical. Detector materials contain levels of uranium and thorium, and these decay chains contain alpha, beta, and gamma decays. Alpha particles near the surface of the acrylic vessel are perhaps the most difficult background. A fraction of the alpha energy, or the recoiling nucleus from the alpha decay, could misreconstruct in the fiducial volume and result in a false candidate dark matter event. The maximum concentrations in the DEAP-3600 acrylic are 0.3 ppt, 1.3 ppt, and 1.1 x 10^-8 ppt for U-238, Th-232, and Pb-210, respectively. The concentrations of U-238, Th-232, and Pb-210 in the bulk acrylic will be measured by vaporizing acrylic, collecting the residue, and counting the contamination in a high-purity germanium well detector.