Performance of SiPMs and pre-amplifier for the wide field of view Cherenkov telescope array of LHAASO

Abstract

The Wide Field of View Cherenkov Telescope Array (WFCTA), a main component of the Large High Altitude Air Shower Observatory (LHAASO), is designed for cosmic rays spectrum measurement from 30 TeV to several EeV. Silicon photomultipliers (SiPMs) have been adopted for the telescope cameras as they do not suffer any aging even with strong light exposure and thus they can be operated with moonlight. The physics of WFCTA requires a dynamic range of SiPMs to be within 10 p.e. (photoelectrons) to 32, 000 p.e. at any energy range. And the dynamic range of SiPMs is proportional to the total number of APDs and depends on the distribution of light hitting on the active area. This light distribution is affected by the use of light concentrator and also by the use of spherical mirrors of the telescope. The effect of these two factors on the Cherenkov light distribution has to be taken into account to evaluate the dynamic range and in this work they have been simulated with a ray-tracing software. The non-uniform light distribution caused by the light concentrator and the spherical mirror will be below 2% for 32, 000 p.e. at the condition if the SiPM has 360, 000 cells. The characteristics of SiPMs from three different producers – Hamamatsu, FBK, and SensL – are measured in laboratory, and the dynamic range of them has also been measured using a uniform light flux. The results of dynamic range correspond with the calculation by the function which describes the relationship between the number of fired APDs and the total number of APDs in the SiPM. The performance of the SiPMs and PMTs under long time duration light pulses up to 3 μs are compared, in order to investigate the stability of the gain of the SiPMs for the fluorescence mode.