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. More details and a comparison of different photodetectors are below.
The structures of SPAD and MPPC are shown below.
SPAD is configured with one pixel, in which a Geiger mode APD and a Quenching resistor are combined as one set. MPPC is configured with a plurality of pixels, in which said SPADs are arranged in plural numbers and electrically connected in parallel.
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.
When light enters an MPPC at a particular timing, its output pulse height varies depending on the number of photons detected. Figure shows output pulses from the MPPC obtained when it was illuminated with the pulsed light at photon counting levels and then amplified with a linear amplifier and observed on an oscilloscope. As can be seen from the figure, the pulses are separated from each other according to the number of detected photons such as one, two, three photons and so on. Measuring the height of each pulse allows estimating the number of detected photons.
The distribution of the number of photons detected during a particular period can be estimated by measuring the MPPC output charge using a charge amplifier or similar device. Figure shows a distribution obtained by discriminating the accumulated charge amount. Each peak from the left corresponds to the pedestal, one photon, two photons, three photons and so on. Since the MPPC gain is high enough to produce a large amount of output charge, the distribution can show discrete peaks according to the number of detected photons.
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. Figure 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.
|Gain||1||102||to 106||to 107|
|Operation voltage||5 V||100 to 500 V||30 to 60 V||800 to 1000 V|
|Multi channel with
|Ambient light immunity||Yes||Yes||Yes||No|
|Compact ＆ Weight||Yes||Yes||Yes||No|
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