In the surface dataset using a triple-coincidence tag we found the fraction of β events that are misidentified as nuclear recoils to be 232&_mathId=si27.gif&_user=111111111&_pii=S0927650516301232&_rdoc=1&_issn=09276505&md5=c58c520db4087595e1bbdf6b34f56698" title="Click to view the MathML source"><1.4×10−7 (90% C.L.) for energies between 43–86 keVee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement to be done with only a double-coincidence tag.
The combined data set contains 1.23 × 108 events. One of those, in the underground data set, is in the nuclear-recoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the level of electronic recoil contamination is 232&_mathId=si28.gif&_user=111111111&_pii=S0927650516301232&_rdoc=1&_issn=09276505&md5=d09ca474920f54e3075292af39f2d236" title="Click to view the MathML source"><2.7×10−8 (90% C.L.) between 44–89 keVee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale.
We developed a general mathematical framework to describe pulse-shape-discrimination parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approximately 232&_mathId=si29.gif&_user=111111111&_pii=S0927650516301232&_rdoc=1&_issn=09276505&md5=8b21620fcebeefcd998ad1b201b54af2" title="Click to view the MathML source">10−10 for an electron-equivalent energy threshold of 15 keVee for a detector with 8 PE/keVee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of 232&_mathId=si30.gif&_user=111111111&_pii=S0927650516301232&_rdoc=1&_issn=09276505&md5=f1d2302d1a7756c5ea83d2ac1f9ca032" title="Click to view the MathML source">10−46 cm2, assuming negligible contribution from nuclear recoil backgrounds.