Characterization of pulse-shape discrimination for background reduction in the DEAP-1 detector

DEAP (Dark Matter Experiment with Argon and Pulse Shape Discrimination) is an experiment that aims to directly detect dark matter particles via nuclear recoils in liquid argon. The experiment uses the scintillation property of liquid argon as a means to discriminate the gamma and beta backgrounds from the expected signal. DEAP-1 is a 7 kg single phase liquid argon detector. It was constructed to demonstrate the scalability for a larger (3600 kg) detector. The detector was originally operated at Queen's University, where the background rejection level achieved was 6.3x10-8 for the recoil detection efficiency of 97.1%. The detector was relocated to SNOLAB, where the background in the energy region of interest was reduced by a factor of 7.7 (from 4.61+/-0.17 mHz to 0.60+/-0.05 mHz). The background rejection level of 9.64x10-9 (10.4 part per billion) was achieved from the combined data set (Queen's University and SNOLAB) for a recoil detection efficiency of 35.5 +/- 1.3%. With the current background rate, the background rejection level required for the 3600 kg detector (1.8x10-9) is projected to be achieved in 382 days at the neutron efficiency of 9.1+/-0.6%.