Dark counts

The dark current in current detection always exceeds the dark-current equivalent of dark counts in photon counting because of the leakage. Also, no component of the dark current can be eliminated by discrimination as in photon counting. 

The longest value of ( r ) that can be reliably measured is determined by the longest sampling interval in the correlator times the number of channels, the dark count in the photomultiplier tube, the long term stability in the laser, and whether full correlation or clipping is employed. At present 100 s is a practical maximum for measured values of (r). In order to determine a relaxation time of 100 s, it is desirable to measure the correlation function for at least 1000 relaxation times. This means that run times of 105 s are required. This places severe requirements on the long term stability of all parts of the system. Routine measurements of (r) are probably better restricted to 10 s. 

The 5 ns pulses of about 10 electrons released at the anode by a photon absorbed by the photocathode of a PM tube can be used to count photons. In such instruments the intensity of light is displayed as a count per second which varies between about 15 (dark count) and 105. A photon-counting detector system is of course much more complex than the simple PM/ampli-fier used in conventional spectrofluorimeters. Figure 7.27(a) is a block diagram of such a photon counter (b) gives a simple illustration of the important process of pulse selection through a discriminator. The output of... 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

It is possible to approach shot-noise-limited performance in many optical experiments. When light levels are low, photomultipliers serve as noise-free quantum amplifiers with a gain of 10 . For absorption measurements, detectors with the highest quantum efficiency and uniformity of response, such as end-on semitransparent photocathode styles, are better than the high gain, opaque photocathode, low dark count types that are used for luminescence measurements. If one needs to measure absorption with a precision of AA 10, then 10 photons need to be accumulated at each data point. At these light levels, the dark count usually may not contribute greatly to the S/N. However, in absorption... 

Measurements at low light levels are routinely performed with photon-counting techniques. The development of ultrasensitive optical detectors has made great progress in the last couple of years. Integrated photon-counting modules with cooled avalanche photodiodes (APD) have been available for some years [31]. These detectors can have quantum efficiencies of 50% with less than 10 dark counts per second. The light sensitive area of such a device has a diameter of about 200 (im and can serve directly as a pinhole in a confocal detection channel. 

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 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...

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. 

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. 6.24 CFD threshold scan for an H5773-20 photosensor module. Gain control voltage 0.9 V, preamplifier 20 dB. Upper curve recorded at 100,000 counts per second, lower curve recorded with dark counts...

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

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