Direct comparison of SiPMs and PMTs in operation with a bright background and prospects of using SPADs as truly digital sensors

TL;DR

This study compares silicon photomultipliers (SiPMs) and traditional photomultiplier tubes (PMTs) in high-energy gamma-ray astronomy, evaluating their performance under bright background conditions and exploring the potential of SPADs as fully digital sensors.

Contribution

The paper provides a direct experimental comparison of SiPMs and PMTs in a real telescope environment and discusses the future prospects of SPADs as digital photon sensors.

Findings

SiPMs have higher photon detection efficiency but also higher noise from night sky background.

In long-term operation, SiPMs perform comparably to PMTs despite higher PDE.

SPAD arrays offer promising potential as fully digital photon sensors.

Abstract

The use of silicon photomultipliers (SiPMs) alongside conventional photomultiplier tubes (PMTs) is a remarkable technological development in modern ground-based very high energy gamma-ray astronomy. SiPMs exhibit comparable or even higher photon detection efficiencies (PDEs) than PMTs. The sensitivity of a PMT matches well the spectral shape of Cherenkov radiation from extended air showers. In contrast to a PMT, the sensitivity of a SiPM is shifted toward longer wavelengths, where the intensity of light of night sky (LoNS), considered as unwanted noise, increases significantly. It is obvious that a SiPM with a higher PDE will indeed measure more Cherenkov light than a PMT, but it will also detect significantly higher LoNS noise; the question is which factor will predominate in the signal-to-noise-ratio (SNR). To compare the performance of a PMT with that of a SiPM, we built SiPM-based modules and installed these and operated in parallel in the imaging camera of the 17 m diameter MAGIC telescope. Our long-term studies show that SiPM, despite their higher PDE, can deliver only a comparable to PMT performance. As already the name SiPM suggests, we use these semiconductor sensors analogously to classical PMTs: We amplify their small signals, digitize, and calibrate the converted amplitudes. Although SiPM is essentially a digital sensor, its common-anode design does not allow one to directly profit from it. Numerous arrays of single-photon avalanche diodes (SPADs) are being developed in various laboratories worldwide. Unlike SiPM, SPAD arrays digitize the incident photons from the outset and count their number. We will dwell on the potential further developments of SPADs.