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

Razmik Mirzoyan, Alexander Hahn, David Fink, Antonios Dettlaff, Daniel Mazin, David Paneque, Olaf Reimann, Thomas Schweizer, Derek Strom, Masahiro Teshima, Yazhou Zhao

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.

Direct comparison of SiPMs and PMTs in operation with a bright background and prospects of using SPADs as truly digital 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.
Paper Structure (9 sections, 3 figures)

This paper contains 9 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Performance Metrics of an IACT
  3. The telescope and camera layout
  4. SiPM Module Prototypes
  5. Sensor Characteristics and Spectral Response
  6. Light of Night Sky and SiPM Intrinsic Noise
  7. Signal-to-Noise Ratio and Results
  8. SPAD arrays as the "Non Plus Ultra" solution and advantages over the use of SiPM
  9. Conclusion

Figures (3)

  • Figure 1: 3 SiPM modules based on SiPMs from Hamamatsu, EXCELITAS and SensL installed in the free corners of the MAGIC-I telescope imaging camera. In the top right one can see also a 4th module, which was used for test purposes. All these modules are operated along with the PMT modules in the MAGIC camera and treated in the same way. Picture taken from hahn_direct_2024.
  • Figure 2: PDE of Hamamatsu SiPM shown in red dashed-line, QE of MAGIC PMT shown in blue, LoNS spectrum (black, in arbitrary units (A.U.)) and Cherenkov spectra of air showers observed under $15\hbox{$^\circ$}\xspace$ (green, in A.U.) and $80\hbox{$^\circ$}\xspace$ (light green, in A.U.) zenith distance. The mirror reflectivity of the MAGIC-1 mirrors is shown in orange. Cherenkov and LoNS spectra were scaled for better visibility. Picture taken from hahn_direct_2024.
  • Figure 3: Dependence of the signal-to-noise ratio on the observational angle for the tested 3 SiPM modules and 2 PMT types. Except for the “old” SiPM from EXCELITAS from the year 2011, all the other sensors show comparable signal-to-noise ratios. Picture taken from hahn_direct_2024.