Article révisé par les pairs
Résumé : The First G-APD Cherenkov Telescope (FACT) is the first operational test of the performance of silicon photomultipliers (SiPM) in Cherenkov Astronomy. These novel photo detectors promised to be an inexpensive and robust alternative for vacuum photomultiplier tubes, but had, up to now, never been applied in an Imaging Air shower Cherenkov Telescope (IACT). For more than three years FACT has operated on La Palma, Canary Islands (Spain), with the primary aim of long-term monitoring of astrophysical sources. Stable performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken with regards to their temperature and over voltage dependence through implementation of a feedback method for keeping their properties stable. Several independent long term measurements were conducted to analyze and verify SiPM gain stability. Dark count spectra, which also make for an excellent self calibration mechanism, were used to study and correct for temperature dependencies. Rate scans make it possible to derive a method, for quickly finding appropriate trigger thresholds by measuring pixel currents, and thus allow for a consistent data acquisition rate. Dedicated measurements using an LED flasher are used to study the correct application of SiPM bias voltages. In this paper, the results of the long term studies will be presented and the applicability of SiPMs in IACTs for long term monitoring will be shown.

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