Multimodule Systems

The maximum count rate of a single TCSPC channel is limited not only by the counting loss due to the dead time of the TCSPC channel, but also by pile-up effects and the counting capability of the detector. 

Compared with classic systems, the dead time of advanced TCSPC systems has been considerably reduced. It is however, still on the order of 100 to 150 ns. The fraction of photons lost in the dead time - the counting loss - becomes noticeable at detector count rates higher than 10% of the reciprocal dead time (see Sect. 7.9.2, page 338). The counting loss can be compensated for by a dead-time-compensated acquisition time. Therefore, often a relatively high loss can be tolerated. The practical limit is the maximum useful count rate, which is defined as the recorded rate at which 50% of the photons are lost. For currently available TCSPC modules, the maximum useful count rate ranges from 3 to 5 MHz, corresponding to a detector count rate from 6 to 10 MHz. 

Pile-up is caused by the detection of a second photon within one signal period [104, 105, 238, 389, 549]. Because a second photon is more likely to be detected in the later part of a signal period, pile-up causes a distortion of the signal shape (see Sect. 7.9.1, page 332). The pile-up distortion is smaller than commonly believed (see Fig. 7.78, page 336), and reasonable results can be obtained up to a detector count rate of 10 to 20% of the signal repetition rate. Nevertheless, the pile-up sets a limit to the applicable count rate. 

The third limitation, the counting capability of the detector, depends on the detector type used, the voltage divider design, and the requirements for IRF stability, long-term gain stability, and detector lifetime. For conventional PMTs the practical limit for TCSPC is of the order of 5 to 10 MHz. For MCP-PMTs the maximum count rate is 200 kHz to 2 MHz, depending on the MCP gain used. 

Because of pile-up and detector effects, a breakthrough in the counting capability of TCSPC cannot be achieved by simply making the signal processing electronics of the TCSPC device faster. 

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B. Scale-Up Multimodule Systems for Pilot-Scale Purification... 

Another application of multimodule systems is in correlation spectroscopy of single molecules. If the photons detected in different detectors are recorded in different modules, the minimum correlation time is no longer limited by the dead time. By synchronising the macrotime clocks of the modules, a continuous correlation from the picosecond to the millisecond scale can be achieved, see Sect. 5.11.3, page 189. 

A potential application of multimodule systems is high-speed two-photon multibeam scaiming systems [53, 77]. FLIM systems with 4, 8 or even 16 beams and the same number of parallel TCSPC channels appear feasible. The problem is to direct the fluorescence signals from the individual beams to separate PMTs or separate charmels of a multianode PMT. If this problem is solved, two-photon lifetime images can be recorded with unprecedented speed and resolution. 

The efficiency versus the count rate of a single TCSPC channel and a four-module TCSPC system is shown in Fig. 5.94. The efficiency of the single-channel system remains better than 0.9 and the figure of merit better than 1.05 for count rates up to 1 MHz detector count rate. This is better than for any other lifetime imaging technique. For a detector count rate of 10 MHz, the values are 0.5 and 1.4, respectively. Higher count rates not only result in a substantial loss in efficiency but also increase lifetime errors by pile-up-effect (see Sect. 7.9.1, page 332). For detector count rates above 10 MHz the solution is multimodule systems see Sect. 5.7.5, page 146. 

Of course, multidetector TCSPC is unable to correlate the photons between the individual start detectors. More flexibility is achieved by using multimodule TCSPC systems. Multimodule systems can be used to obtain antibunching and FCS results simultaneously or even to correlate photons on a continuous time scale from the picosecond to the millisecond range (see Sect. 5.11.3, page 189). 

An even more efficient, yet more expensive way to reduee pile-up distortion is the multimodule technique. In a multimodule system the photons from several detectors are processed in parallel. Therefore, not only pile-up distortion but also eounting loss are reduced in proportion to the number of TCSPC ehannels. 

The only solution to the count rate problem is multimodule operation. Splitting the light into several detectors connected to independent TCSPC modules proportionally increases the counting capability. Of course, multimodule operation also increases the system cost and can cause space and power supply problems in the host computer. These problems have at least partially been solved since packages of relatively small and cost-efficient TCSPC modules are available. A fully parallel four-channel package (SPC-134, Becker Hickl, Berlin) is shown in Fig. 3.16. 

With a dead time of 100 ns per TCSPC channel, total useful count rates of the order of 20 MHz can be achieved. All four channels can be used for multidetector operation. The high count rate and the high number of channels make multimodule TCSPC systems exceptionally useful for diffuse optical tomography [34], and high count rate applications in laser scanning microscopy [39]. Details are described under Sect. 5.5, page 97 and Sect. 5.7, page 129. 

Figure 7.17, right, shows a system that splits a fluorescence signal into several wavelength intervals. The signals in these intervals are detected by individual detectors and recorded simultaneously either in one TCSPC channel with a router, or in a multimodule TCSPC system. 

The culture system described earlier is based on cylindrical tubes, which makes it difficult to calculate radiative transfer in the culture volume, which has to be solved numerically (Lee et al, 2014). As already described, the one-dimensional hypothesis where light attenuation occurs along only one main direction serves to obtain analytical relations to represent the light attenuation field (as with the two-flux model, Eq. 12). This enables accurate and easy determination of light attenuation conditions for any operating conditions and thus greater system control. Based on this statement, researchers designed a specific PBR. Like the multimodule external-loop airlift PBR, this system is of industrial size (130 L), but the unit is a flat panel with front illumination so as to respond to the one-dimensional hypothesis. It is also illuminated on both sides to increase specific illuminated area... 

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