Pulse shape discrimination studies in liquid argon for the DEAP-1 detector

Abstract

A detector with a target mass of 7 kg of liquid argon was designed, constructed and operated at Queen’s University. This detector is a scaled model for the DEAP project toward a tonne-scale argon detector to search for the WIMP candidate of the so far undetected, dark matter of the universe. The primary intent of the scaled detector was to measure the achievable level to reject background events by use of pulse shape discrimination, being based upon the scintillation timing properties of liquid argon. After refining the apparatus and components, the detector was in operation from the 20th of August until the 16th of October 2007 before being moved to its current location in SNOLAB. During this time, a population of 31 million well-tagged gamma events were collected, of which 15.8 million were in the energy range of interest for calibration. This population was sufficient to demonstrate the discrimination of background events by pulse shape discrimination at the level of 6.3 × 10-8. An analytical model was constructed, based on the scintillation processes and detector response, and has been sufficiently investigated to make predictions of further achievable discrimination.