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Silicon photomultipliers, often called "SiPM" in the literature, are solid-state single-photon-sensitive devices based on Single-photon avalanche diode (SPAD) implemented on common silicon substrate. The dimension of each single SPAD can vary from 10 to 100 micrometres, and their density can be up to 10000 per square millimeter. Every SPAD in SiPM operates in Geiger mode and is coupled with the others by a metal or polysilicon quenching resistor. Although the device works in digital/switching mode, most of SiPM are an analog device because all the microcells are read in parallel, making it possible to generate signals within a dynamic range from a single photon to 1000 photons for a device with just a square-millimeter area. More advanced readout schemes are utilized for the lidar applications. The supply voltage (Vb) depends on APD technology used and typically varies between 20 V and 100 V, thus being from 15 to 75 times lower than the voltage required for a traditional photomultiplier tube's (PMT) operation.
Typical specifications for a SiPM:
- Photo detection efficiency (PDE) ranges from 20 to 50%, depending on device and wavelength, being similar to a traditional PMT
- Gain (G) is also similar to a PMT, being about 106
- G vs. Vb dependence is linear and does not follow a power law like in the case of PMTs
- Timing jitter is optimized to have a photon arrival time resolution of about 100-300 ps
- Signal decay time is inversely proportional to square root of photoelectrons number within an excitation event
- The signal parameters are practically independent of external magnetic fields, in contrast to vacuum PMTs
- Afterpulsing probability (3-30%), defined as probability of spurious second pulses after single photon arrival
- Dark count density is frequency of pulses in absence of illumination (105-106 pulses/s/mm2)
- Small dimensions permit extremely compact, light and robust mechanical design
SiPM for medical imaging are attractive candidates for the replacement of the conventional PMT in PET and SPECT imaging, since they provide high gain with low voltage and fast response, they are very compact and compatible with magnetic resonance setups. Nevertheless, there are still several challenges, for example, SiPM requires optimization for larger matrices, signal amplification and digitization.