Setting the TCSPC System Parameters

In practice the effect of changes in the pulse shaping network is often hard to predict. A longer DEL 2 may result in an unfavourable zero transition shape but a higher bipolar pulse amplitude (Fig. 7.59, right). However, the bandwidth of the discriminator input stage is usually unknown, and how the discriminator actually sees the pulse shape is not predietable. The only way to find it out is to try different delays. 

Detector modules with internal diseriminators, sueh as the Hamamatsu H7421 PMT modules or the Perkin Elmer SPCM-AQR single photon APD modules, deliver stable output pulses without amplitude jitter. The timing performance is defined by the internal diseriminator of the deteetor module, not by the CFD of the TCSPC device. Thus changing the CFD eonfiguration does not improve the time resolution of these detectors. 

The CFD threshold determines the minimum amplitude of the input pulses that trigger the CFD. The threshold of the CFD in the deteetor channel has a considerable influence on the efficieney of a TCSPC system. As described under Sect. 6.2, page 222, the single-photon pulses of a PMT have a strong amplitude jitter. The general shape of the amplitude distribution is shown in Fig. 7.60, left. 

The pulse amplitude distribution consists of three major eomponents. There are the regular photon pulses, i.e. the pulses originating from eleetron emission at the photocathode, which form a wide peak at relatively high amplitudes. Thermal emission, photoelectron emission, and reflection of primary electrons at the dyn-odes forms a secondary peak at lower amplitudes. At very low amplitudes electronic noise, either from the preamplifier or from the environment, causes a third peak of extremely high eount rate. 

The TCSPC system should record the regular photon pulses, but not the dynode pulses and the electronie noise background. Therefore, the optimum CFD threshold is in the valley between the regular photon pulse distribution and the peak caused by dynode emission. 

---