Development and test of a SiPM cluster for a SiPM version of the Cherenkov Telescope Array LST camera

The Large Size Telescopes (LSTs) are the largest telescopes of the Cherenkov Telescope Array (CTA). Their cameras are equipped with 1855 photomultiplier tube (PMT) pixels with GHz readout, to image the flashes of Cherenkov light emitted from atmospheric air-showers initiated by cosmic gamma rays. Silicon photomultipliers (SiPMs) are becoming valid alternatives for PMTs, and in fact many of the smaller telescopes in the array will feature a SiPM camera. To evaluate the performance of an LST SiPM-based camera we are building one of the LST camera elements (a cluster of 7 pixels with the readout electronics), replacing the PMTs with SiPM pixels. Most of the hardware of the baseline design is maintained, to keep the evaluation process simple and to use the existing calibration facilities. We discuss the design and construction of the demonstrator unit and some preliminary characterizations.