Reducing pileup

Certain types of photodetectors, such as MPPC (Multi-Pixel Photon Counter), are so sensitive that they allow the detection of a single photon. This is incredibly useful for High-Energy Physics experiments where there is a need to detect single photonabsorption events.


The MPPC single photon resolution is excellent because of its high internal gain and good cell-to-cell gain uniformity. However, for some applications, the electrical output signal speed from the detector might not fit the needed accuracy to replicate with enough accuracy the time dynamic of the optical signal.


In the ideal scenario, as shown in Figure 1, the MPPC electrical output of each optical signal is completely separated from the view of the counter. These pulses have completely discrete levels of pulse height with little randomness derived from gain fluctuation and white noise.

Figure 1: MPPC without pileup

Pulse accumulation may occur in instances where an MPPC exhibits an extended fall time due to elevated terminal capacitance depending on a particular application, such as in scenarios involving a high photon rate. Additionally, the potential for pulse pile-up is heightened in situations characterized by a high Dark Count Rate, increased likelihood of delayed crosstalk, and afterpulse events, as illustrated in Figure 2 below.

Figure 2: MPPC with pileup

Due to the pileup, we will count a lower number than the actual number of MPPC output pulses. This will show fewer pulses versus the discriminator threshold plot, hindering the accurate determination of the number of counts of each plateau.


Additionally, clusters of pulses may be grouped as a 'single large pulse,' particularly for pulses with less than 1 photoelectron (p.e.), where the population is higher. Consequently, the count of pulses at low thresholds may appear smaller than the actual value.


In a broader context, these effects contribute to an increased prompt crosstalk probability compared to actual values. The count of pulses exceeding the 0.5 p.e. threshold is diminished, while the count of pulses with heights surpassing the 1.5 p.e. threshold is inflated.


For applications involving background light, such as atmospheric Cherenkov experiments, effective pileup suppression is crucial.


As shown in Figure 3 below, Hamamatsu can provide MPPCs with suppressed pileup.

Figure 3: Example of suppressed pileup

Explore the performance of our MPPCs and reach out to Hamamatsu for a more in-depth discussion to identify the ideal device for your specific application requirements.

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