Challenges of arbitrary waveform signal detection by Silicon Photomultipliers as readout for Cherenkov fibre based beam loss monitoring systems

Silicon Photomultipliers (SiPMs) are well recognised as very competitive photodetectors due to their exceptional photon number and time resolution, room-temperature low-voltage operation, insensitivity to magnetic fields, compactness, and robustness. Detection of weak light pulses of nanosecond time scale appears to be the best area for SiPM applications because in this case most of the SiPM drawbacks have a rather limited effect on its performance. In contrast to the more typical scintillation and Cherenkov detection applications, which demand information on the number of photons and/or the arrival time of the light pulse only, beam loss monitoring (BLM) systems utilising Cherenkov fibres with photodetector readout have to precisely reconstruct the temporal profile of the light pulse. This is a rather challenging task for any photon detector especially taking into account the high dynamic range of incident signals (100K – 1M) from a few photons to a few percents of destructive losses in a beam line and presumably an arbitrary temporal distribution of photons (localisation of losses). Nevertheless, a number of advantages and ongoing improvements of SiPM technology are considered to be a reasonable ground for this feasibility study of SiPM application in BLM systems. Transient SiPM responses to light pulses over a wide range of intensities have been measured and an analytical model has been applied to describe the results. Non-linearity of SiPMs due to the limited number of pixels and non-instant pixel recovery time is found to be a source of transient and history-dependent distortions of output signals.