DEAP trigger and readout electronics

The DEAP experiment's aim is to detect Weakly Massive Particles through their interaction in liquid argon. It is a ball of liquid Argon surrounded by 255 Photo-multiplier tubes. As such it is a relatively small scale experiment; the challenge for the trigger and data acquisition electronics is to find the few possible interesting events occurring at a rate of up to 1 to 100 per year among a 3.6 kHz background due to the beta decay of 39 Ar isotopes. Electron recoil due to beta decay or gamma interaction can be very efficiently rejected by measuring the fraction of scintillation light emitted in the first 100 to 200 ns over the total scintillation light emitted over several Jls. The electron recoils are expected to be suppressed by a factor of 109 to 1010 with such pulse shape discrimination technique. The core of DEAP electronics is a set of 32 commercial CAEN VI720 digitizers that enable the identification of single photo-electron pulses. The readout speed of the digitiser is limited to about 1500 Hz due to the way the zero suppression is performed in CAEN proprietary firmware. A trigger system was hence implemented to discard 50% to 90% of the events. The trigger decision is made by analyzing 22 waveforms, which are the analog sums of the signals from 12 PMTs. Such a trigger system allows continuous monitoring of the data used to make the trigger decision and it provides a flexible way to alter the trigger condition should a background source happen to be larger than expected. Construction of the DEAP electronics system is nearing completion.