FACT - How stable are the silicon photon detectors?

TL;DR

This paper discusses the development and operation of a feedback system for silicon photon detectors (G-APDs) in the FACT telescope, demonstrating high precision, stability, and potential advantages over traditional photo-multiplier tubes in Cherenkov astronomy.

Contribution

The paper presents a novel feedback system for G-APDs that enables high-precision operation and long-term stability without external calibration, advancing their application in Cherenkov telescopes.

Findings

High-precision single-photon spectrum extraction

Stable long-term operation of G-APDs

Elimination of external calibration devices

Abstract

The First G-APD Cherenkov telescope (FACT) is the first telescope using silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise a higher photon detection efficiency, more robustness and higher precision than photo-multiplier tubes. Since the properties of G-APDs depend on auxiliary parameters like temperature, a feedback system adapting the applied voltage accordingly is mandatory. In this presentation, the feedback system, developed and in operation for FACT, is presented. Using the extraction of a single photon-equivalent (pe) spectrum as a reference, it can be proven that the sensors can be operated with very high precision. The extraction of the single-pe, its spectrum up to 10\,pe, its properties and their precision, as well as their long-term behavior during operation are discussed. As a by product a single pulse template is obtained. It is shown that with the presented method, an additional external calibration device can be omitted. The presented method is essential for the application of G-APDs in future projects in Cherenkov astronomy and is supposed to result in a more stable and precise operation than possible with photo-multiplier tubes.