Low dark count rate for increased accuracy

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 photon absorption events. Contrary to other standard photodetectors, MPPCs have special characteristics, some of which are influenced by temperature, such as Dark Count Rate (DCR). 


During the MPPC operation, pulses are produced not only by photon-generated carriers but also by thermally generated ones. The pulses created by the latter effect are called dark pulses. These can appear randomly and are uncorrelated with photon-initiated output pulses. They are observed along with the signal pulses and cause a decrease in measurement precision.


In the context of an application, the influence of DCR on measurement precision is contingent upon the configuration of measurement parameters. For instance, when the time window for a specific measurement is set at 10 ns, the probability of a single dark count occurring during that timeframe is notably low, approximately 0.01 event for a DCR of 1,000 kcps (kilo counts of photons per second). This scenario enhances the feasibility of detecting single photons with a high level of statistical confidence using the MPPC. 


Nevertheless, if the above timing constraints on a single-photon event cannot be met, DCR can be reduced through thermoelectric cooling of the MPPC at a rate close to halving the DCR for every approx. 10° C reduction in temperature as shown in Figure 1.

Figure 1: Thermoelectric cooling of the MPPC to reduce DCR

In applications where multiple photons are involved in a light event, DCR can be drastically reduced by increasing the discriminator level. As a general rule, for every single-photoelectron (1 p.e.) output pulse height increase in the discriminator’s threshold level, DCR readout diminishes by about an order of magnitude as shown in Figure 2.

Figure 2: Increasing the discriminator’s threshold level reduces DCR

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