Development of the Readout System for CTA Using the DRS4 Waveform Digitizing Chip

Abstract

We have developed a prototype of the photomultiplier tube (PMT) readout system for the next generation VHE gamma-ray observatory, the Cherenkov Telescope Array (CTA). Several thousand PMTs along with their readout systems are arranged on the focal plane of each telescope, with one readout system per 7-PMT cluster. The signal from a PMT detecting Cherenkov light from an air shower is amplified, and the waveform is then digitized at a sampling rate of the order of GHz using an analog memory ASIC developed at Paul Scherrer Institute (PSI), called the Domino Ring Sampler (DRS4). The sampler has 1,024 capacitors per channel and the ability to cascade channels for increased depth. After a trigger is generated in the system, the charges stored in the capacitors are digitized by an external slow sampling ADC and then transmitted via Gigabit Ethernet. An onboard FPGA controls the DRS4, trigger threshold, and Ethernet transfer. In addition, the control and monitoring of the Cockcroft–Walton circuit that provides high voltage for the 7-PMT cluster are done by the same FPGA. Using a prototype named Dragon we successfully obtained a pulse shape of a PMT signal at a sampling rate of 2 GS/s and a single photoelectron spectrum.