Photosensor modules

The typical IRF of an H5773P-0 is shown in Fig. 6.40. The output pulses were amplified by a 20 dB, 1.6 GHz preamplifier, and the response to 50 ps pulses from a 650 nm diode laser was recorded by an SPC-730 TCSPC module. 

The response function has a prepeak about 1 ns before and a secondary peak 2 ns after the main peak. The prepeak is caused by low amplitude pulses, probably by photoemission at the first dynode. It can be suppressed by properly adjusting the discriminator threshold. The secondary peak is independent of the discriminator threshold. 

The variation of the IRF for an H5773-20 over the active area is shown in Fig. 6.41. The FWHM of the IRF varies from about 120 ps to about 140 ps, and the first moment shifts over 90 ps (see insert). This means that the IRF width for small spots can be substantially shorter than for the full active area. However, the improved resolution can only be exploited if the location of the illuminated spot is kept stable within less than 0.1 mm. If timing stability is important it is probably better to spread the light over the full cathode area. 

There is also some variation in the efficiency, see total count numbers in the insert ofFig. 6.41. 

The afterpulsing is shown in Fig. 6.42. The devices show relatively strong afterpulsing, especially the -02 (extended red) and -20 (high sensitivity extended red) tubes. 

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A number of typical detectors are described under Sect. 6.4, page 242. The main selection criteria are the transit-time spread and the spectral sensitivity. Together with the laser pulse shape, the transit-time spread determines the instrument response function (IRF). As a rule of thumb, lifetimes down to the FWHM of the IRF can be measured without noticeable loss in accuracy. For shorter lifetimes the accuracy degrades. However, single-exponential lifetimes down to 10% of the IRF width are well detectable. Medium speed detectors, such as the R5600 and R7400 miniature PMTs, yield an IRF width of 150 to 200 ps. The same speed is achieved by the photosensor modules bases on these PMTs (see Fig. 6.40, page 250). 

Fig. 5.12 Fluorescence decay curves of indocyanine green (ICG). A in ethanol, B in water, C instrument response function. Time scale 400 ps / div, 1-cm cuvette, diode laser 650 nm, 50 MHz. Left Detected by a cooled H5773-20 photosensor module. Right Detected by an R3809U MCP PMT...

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. 

In some detectors a bump in the TCSPC instrument response function appears a few ns before the main peak. The size of the bump depends on the discriminator threshold. Normally the bump can be suppressed or reduced in size by increasing the discriminator threshold. The effect is probably caused by photoelectron emission from the first dynode. The corresponding pulses reach the anode prior to the photons from the cathode, and have a lower amplitude. Figure 6.20 shows an example for an H5773P-01 photosensor module. 

Fig. 6.20 Prepulses in a H5773P-01 photosensor module can cause a bump about 1 ns before the main peak of the IRF. The prepulses have a lower amplitude than the regular pulses and can be suppressed by increasing the CFD threshold. Time scale 200 ps/div...

Another possibility to reeord a pulse height distribution in a TCSPC system is to run a CFD threshold sean. Starting from the lowest possible value, the CFD threshold is gradually inereased and the number of photons reeorded in a given time interval is reeorded versus the CFD threshold. An example for an H5773-20 photosensor module is shown in Fig. 6.24. 

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. 6.39 H5773 and H5783 photosensor modules and radiant sensitivity of the different cathode versions. From [213]...

Fig. 6A1 Variation of the IRF over the active area of an H5773-20 photosensor module. Response to focused diode laser, 650 nm, 50 ps pulse width. Time scale 100 ps/div...

Fig. 6.42 Afterpulsing of H5773 photosensor modules. Autocorrelation of photon pulses measured with continuous light at 10 kHz count rate. Left H5773P-00 (hialkali), gain control voltage 0.9 V and 0.81 V. Right H5773-20, gain control voltage 0.9 V and 0.77 V. Please note the different scale of the correlation coefficient...

Figure 7.31 shows recordings of a fluorescence signal (top) and of the dark counts (bottom) for a cooled H5773-20 photosensor module (left) and an R3809U-50 MCP-PMT (right). 

Figure 7.33 and Fig. 7.34 show the eount-rate dependent timing drift of the IRF for an XP2020 linear-focused PMT and an H5773-20 photosensor module. The eurves were reeorded with a BHL-600 diode laser of 40 ps pulse width and 650 nm wavelength, and an SPC-140 TCSPC module (both Beeker Hiekl, Berlin). The... 

Fig. 7.34 IRF of an H5773-20 photosensor module for count rates of 30 kHz, 300 kHz, and 4 MHz in linear scale left) and logarithmic scale right). The shift between 30 kHz and 4 MHz count rate is <2 ps and not discernible in the IRF curves...

Fig. 7.44 Output signal of an H5773 photosensor module used for reference signal generation. 200 ps diode laser pulse, gain control voltage 0.45 V, 0.6 V, and 0.77 V...

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

Typical pulse shapes for LEDs driven by pulses of 3.6 ns FWHM from a Hewlett Packard HPlllOA pulse generator are shown in Fig. 7.88, left. Pulses from a 5 mW, 650 nm laser diode driven by 1 ns pulses from a HP8131A pulse generator are shown right. The detector was a Hamamatsu H5783P photosensor module. 

Hamamatsu Photonics K.K., H5773/H5783/H6779/H6780/H5784 Photosensor modules (2001)... 

MSFIA-CL manifold for the determination of trace levels of orthophosphate in waters. Detector chemiluminescence detector (flow-through solid-phase optical sensor-i- photosensor module) HC holding coil PMT photomultiplier RC reaction coil V solenoid valve. 

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