Study on the gain and photon detection efficiency drops of silicon photomultipliers under bright background conditions

TL;DR

This study investigates how bright background light affects the gain and photon detection efficiency of silicon photomultipliers (SiPMs), crucial for gamma-ray telescopes, by analyzing factors like saturation, temperature, and voltage drops.

Contribution

It provides a detailed measurement and characterization of SiPM performance degradation under high background conditions, highlighting the importance of compensating for multiple factors.

Findings

SiPM gain and efficiency are affected by avalanche saturation, temperature increases, and voltage drops.

The combined effects of these factors can be modeled to improve calibration accuracy.

Experimental data confirms the relationship between background conditions and SiPM performance degradation.

Abstract

The use of silicon photomultipliers (SiPMs) in imaging atmospheric Cherenkov telescopes is expected to extend the observation times of very-high-energy gamma-ray sources, particularly within the highest energy domain of 50-300 TeV, where the Cherenkov signal from celestial gamma rays is adequate even under bright moonlight background conditions. Unlike conventional photomultiplier tubes, SiPMs do not exhibit quantum efficiency or gain degradation, which can be observed after long exposures to bright illumination. However, under bright conditions, the photon detection efficiency of a SiPM can be undergo temporary degradation because a fraction of its avalanche photodiode cells can saturate owing to photons from the night-sky background (NSB). In addition, the large current generated by the high NSB rate can increase the temperature of the silicon substrate, resulting in shifts in the SiPM breakdown voltages and consequent gain changes. Moreover, this large current changes the effective bias voltage because it causes a voltage drop across the protection resistor of 100-1000 {\Omega}. Hence, these three factors, namely the avalanche photodiode (APD) saturation, Si temperature, and voltage drop must be carefully compensated for and/or considered in the energy calibration of Cherenkov telescopes with SiPM cameras. In this study, we measured the signal output charge of a SiPM and its variation as a function of different NSB-like background conditions up to 1 GHz/pixel. The results verify that the product of the SiPM gain and photon detection efficiency is well characterized by these three factors.