What is MPPC (SiPM)?*
MPPC is short for Multi-Pixel Photon Counter, and this detector is also known as silicon photomultiplier (SiPM). It is a solid state photodetector that uses multiple avalanche photodiode (APD) pixels operating in Geiger mode. The basic element (one pixel) of an MPPC is a combination of the Geiger mode APD and quenching resistor, and a large number of these pixels are electrically connected and arranged in two dimensions.
Each pixel in the MPPC outputs a pulse at the same amplitude when it detects a photon. Pulses generated by multiple pixels are output while superimposed onto each other. For example, if three photons are incident on different pixels and detected at the same time, then the MPPC outputs a signal whose amplitude equals the height of the three superimposed pulses. Each pixel outputs only one pulse and this does not vary with the number of incident photons. So the number of output pulses is always one regardless of whether one photon or two or more photons enter a pixel at the same time. This means that MPPC output linearity gets worse as more photons are incident on the MPPC such as when two or more photons enter one pixel. This makes it essential to select an MPPC having enough pixels to match the number of incident photons. The following two methods are used to estimate the number of photons detected by the MPPC.
How to use
The MPPC characteristics greatly vary depending on the operating voltage and ambient temperature. In general, raising the operating voltage increases the electric field inside the MPPC and so improves the gain, photon detection efficiency, and time resolution. On the other hand, this also increases unwanted components such as dark count, afterpulses, and crosstalk which lower the S/N. The operating voltage must be carefully set in order to obtain the desired characteristics.
The MPPC can be used by various methods according to the application. Here we introduce a typical method for observing light pulses. Using a wide-band amplifier and oscilloscope makes this measurement easy. Below shows one example of a connection to a wide-band amplifier. The 1 kΩ resistor and 0.1 μF capacitor on the power supply portion serve as a low-pass filter that eliminates high-frequency noise of the power supply. The 1 kΩ resistor is also a protective resistor against excessive current.
The MPPC itself is a low-light-level detector, however, in cases where a large amount of light enters the MPPC, for example, when it is coupled to a scintillator to detect radiation, a large current flows into the MPPC. This may cause a significant voltage drop across the protective resistor, so the protective resistor value must be carefully selected according to the application. The amplifier should be connected as close to the MPPC as possible.
The following table provides an overview of the characteristics of the four different types of detectors: Photodiodes; Avalanche Photodiodes; Multi-Pixel Photon Counter and Photomultiplier Tubes
|PD APD MPPC PMT|
|Gain 1 102 to 106 to 107|
|Quantum efficiency Highest High Medium Low|
|Operation voltage 5 V 100 to 500 V 30 to 60 V 800 to 1000 V|
|Large area No No Scalable Yes|
|Multi channel with
narrow gap Yes Yes Yes No
|Readout circuit Complex Complex Simple Simple|
|Noise Low Middle Middle Low|
|Uniformity Excellent Good Excellent Good|
|Response time Fast Fast Very Fast Fast|
|Energy resolution High Medium High High|
|Temperature sensitivity Low High Medium Low|
|Ambient light immunity Yes Yes Yes No|
|Magnetic resist Yes Yes Yes No|
|Compact ＆ Weight Yes Yes Yes No|
[*] This article was originally published by Hamamatsu Photonics K.K who retain all rights.