Triggerless DAQ system for a light based detector

The utilization of various types of photon detectors, generally referred to as light based detectors, is widespread in many fields of high energy physics, nuclear physics, astrophysics and medical imaging to measure scintillation photons or Cherenkov light. Both the traditional photomultiplier tubes (PMTs), still the most commonly used, and the silicon photomultipliers (SiPMs) in rapid development in the last years show good performance. The sensitivity down to single photoelectron (PE), the wide dynamic range and the fast timing characteristics can be exploited thanks to a careful optimization of the readout electronics, trigger and DAQ. In applications where a high efficiency is required at low energy threshold (as for rare-event search experiments), in a typical trigger-based DAQ architecture, lowering the number of coincident signals for the event selection is mandatory with the consequent heavy increase of data rate that could not be sustained by the DAQ chain. We have developed a triggerless data collection scheme for a light based detector that relies on a new generation of waveform digitizer board, developed by CAEN and able to manage an independent data acquisition on each channel. This paper describes the implementation and the performance of the triggerless DAQ system.