Dark matter Experiment with Argon and Pulse-shape discrimination
DEAP-1 uses liquid argon as both the target volume and detection medium. Argon produces 40 photons per keV of energy deposited by electrons. Thus even low-energy nuclear recoils of order 40 keV (equivalent to about 12 keV electron energy) are detectable.
The largest potential background in DEAP-1 is from the beta decay of Argon-39 which has an activity of approximately 1 Bq/kg in natural argon. Fortunately, the time structure if scintillation light is very different for electromagnetic events and nuclear-recoil events. The reason is most of the energy deposited by nuclear recoils leaves argon in dimers in a singlet state which decay in a few nanoseconds. However 2/3 of the argon in electromagnetic events is left in a triplet state with a 1.5 microsecond lifetime. Thus the time structure of events are very different. This allows a simple single phase (and scaleable detector) to be built.
