Optical feasibility of an upgrade of the CTA LST camera to SiPM

The Large Size Telescope (LST) is the largest telescope of the Cherenkov Telescope Array project, with a diameter of 23 m and a focal plane instrumentation of 4 square metres. In the current design, it comprises cameras equipped with arrays of 1855 photomultiplier tubes (PMTs). Each PMT has a light concentrator in front to reduce the stray light as well as reduce the dead space between PMTs. These cameras are built to detect the nanoseconds flash of Cherenkov light emitted from atmospheric air-showers generated by a cosmic gamma ray entering the atmosphere. Thanks to a rapid development, silicon photomultipliers (SiPMs) are becoming valid alternatives for PMTs in several fields, due to their lower operating voltage, larger photon efficiency, reduced ageing, insensitivity to magnetic fields, and possibly lower costs. These properties make SiPMs suitable for gamma-ray astronomy and future development for imaging atmospheric Cherenkov telescopes. Here we discuss a minimal-effort scenario for an upgrade of an LST PMT-based camera to a SiPM-based camera, in which most of the hardware is maintained. Thanks to a ray-tracing software, we show that the minimal valid solution consists only in replacing each PMT by several SiPMs. In particular, the current PMT-tailored lightguides in front of each pixel do not have to be exchanged considering the angular distribution of light at the SiPM surface and its angular response. We briefly discuss the effect on the sensitivity of the instrument equipped with SiPM.