Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy β and nuclear recoils in liquid argon with DEAP-1

The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination down to an electron-equivalent energy of 20 keV(ee). In the surface dataset using a triple-coincidence tag we found the fraction of,6 events that are misidentified as nuclear recoils to be < 1.4 x 10(-7) (90% C.L.) for energies between 43-86 keV(ee) 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 x 10(8) 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 < 2.7 x 10(-8) (90% C.L.) between 44-89 keV(ee) 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 10(-10) for an electron-equivalent energy threshold of 15 keVee for a detector with 8 PE/keV(ee) 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 10(-46) cm(2), assuming negligible contribution from nuclear recoil backgrounds. (C) 2016 Elsevier B.V. All rights reserved.