Dark count rate

The dark count rate of the detector sets a limit to the sensitivity of a photon counting system. The dark count rate of a PMT depends on the cathode type, the cathode area, and the temperature. The dark count rate is highest for cathodes with high sensitivity at long wavelengths. Typical dark count rates for the commonly used photoeathodes are 

Cathode Type Spectral Range nm Dark Count Rate s , at 22 °C 

The dark count rate of a PMT can increase dramatically after the photocathode has been exposed to daylight. For traditional cathodes the effect is reversible, but full recovery can take several hours. An example for a Flamamatsu F15773P-01 (multialkali cathode) photosensor module is shown in Fig. 6.19. To show the full size and duration of the recovery effect, the experiment was performed at an ambient temperature of 5° C. 

If the cathode of an operating PMT is exposed to daylight or another strong source of light, the dark count rate can be permanently increased by several orders of magnitude. The tube can be damaged beyond recovery. 

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The dark count rate decreases by a factor of 3 to 10 for a 10 °C decrease in temperature. Cooling is therefore the most efficient way to keep the dark count rate low. Figure 6.18 shows the dark count rate versus temperature for different cathode versions of the Flamamatsu R3809U MCP PMT [211]. 

Fig. 6.18 Dark count rate of the Hamamatsu R3809U versus temperature for different cathode versions. From [211], S20 = Multialkali, S25 = extended red multialkaU...

Fig. 6.19 Decrease of the dark count rate (in counts per second) of a H5773P-01 PMT module after the cathode was exposed to daylight. The detector was cooled down to 5°C. The peaks are caused by scintillation effects. Total time scale 11 hours...

Fig. 6.43 Dark count rate versus ambient temperature for different H5773P-01 modules...

In the last years it became possible to manufacture silicon SPADs in standard epitaxial processes as they are used for high-speed CMOS devices [117, 245, 246, 354, 424, 459]. The diodes are characterised by a small thickness of the depletion region. The thin depletion region results in a relatively low breakdown voltage, high time resolution, and low dark count rate. The drawback of the thin depletion region is a reduced quantum efficiency in the near infrared. Laser pulses recorded with an id 100-20 SPAD detector of id Quantique [245] and an SPC-144 TCSPC module of Becker Hickl are shown in Fig. 6.57. 

The choice of the detectors is dictated by the required time resolution, speetral range and sensitivity, tolerable dark count rate, deteetor area, available count rates, ruggedness, and possible budgetary constraints. The paragraphs below attempt to... 

The most common cathode types for PMTs are the bialkali, the multialkali, the extended multialkali, and the GaAs and GaAsP cathodes. Typical curves of the cathode radiant sensitivity are given in Fig. 6.16, page 230. The selection of the cathode is often a tradeoff between red and NIR sensitivity and dark count rate. 

A PMT can develop instability or permanently increased dark count rate after being heavily overloaded for a period of several seconds or longer. The reason is probably heating of the anode and the last dynodes which releases gas from these structures. 

The voltage divider can dissipate a considerable amount of heat. The heat should be kept away from the tube in order to keep the dark count rate low. In addition, good electrical shielding of the tube and the voltage divider is essential to obtain a good time resolution, see Sect. 7.5.4, page 311. 

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