What is MPPC ?

The MPPC (multi-pixel photon counter) is one of the devices called SiPM (silicon photomultiplier). It is a new type of photon-counting device using multiple APD (avalanche photodiode) pixels operating in Geiger mode. Although the MPPC is essentially an opto-semiconductor device, it has an excellent photon-counting capability and can be used in various applications for detecting extremely weak light at the photon counting level. The MPPC operates on a low voltage and features a high multiplication ratio (gain), high photon detection efficiency, fast response, excellent time resolution, and wide spectral response range, so it delivers the high-performance level needed for photon counting. The MPPC is also immune to magnetic fields, highly resistant to mechanical shocks, and will not suffer from “burn-in” by incident light saturation, which are advantages unique to solid-state devices. The MPPC therefore has a potential for replacing conventional detectors used in photon counting up to now. The MPPC is a high performance, easy-to-operate detector that is proving itself useful in a wide range of applications and fields including medical diagnosis, academic research, and measurements.

  
 

Structure

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.
 

 
 

Basic operation

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.
 
· Observing the pulse
· Measuring the output charge
 
(1) Observing pulses
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.
 

 
(2) Integrating the output charge
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.
 

 
 

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. Figure shows one example of a connection to a wide-band amplifier. The 10 Ω 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 10 Ω 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.

 
 

Characteristics

 

Applications

Distance measurement (LIDAR)

MPPC can be used for distance measurement applications using the TOF method such as in-vehicle LIDARs, hand-held rangefinders, and human body sensors.

 

PET (positron emission tomography)

MPPCs that are arranged 360° around an object detect pair annihilation gamma rays, and the location of a target, such as cancer, can be determined on the basis of the detected intersections.

 

 

Flow cytometry

To detect the type, quantity, and nucleic acid (DNA, RNA), etc. of cells, laser light is incident on a fast running solution containing the cells. This enables the MPPC to capture the minute fluorescence that is emitted.

 

Fluorescence measurement

The MPPC module can detect minute fluorescence emission of reagents.

 

Particle counter

When a laser is made to pass through a chamber that contains a gas or liquid, the quantity and size distributions of the particles in the chamber can be determined through the detection of the light that is scattered by the particles.

Scanning laser ophthalmoscope (SLO)

In ophthalmoscopy, for safety reasons the light that is irradiated into the eyeball must have a low intensity. MPPC and APD modules can be used to detect faint reflected light from the eye with superior resolution and contrast.

 
 

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