Classic Pile-Up Effect

The classic pile-up effeet is shown in Fig. 7.77. A single TCSPC channel is unable to record a second photon in a single signal period. Consequently, the second photon is lost. (Actually the term pile-up is not quite correct. It comes from nuclear partiele detection and means the detection of several particles within the luminescence lifetime of a scintillator.) 

Detection and loss of a second photon is more likely to occur in the later part of the signal, therefore the reeorded waveform is distorted. Pile-up distortion becomes notieeable if the count rate exeeeds a few percent of the pulse repetition rate. 

The pile-up distortion of the signal shape is predictable if the detector count rate and the signal repetition rate are known [103, 104, 105, 238, 389, 549]. Suppose the number of counts in a time channel, i, is iV, and the total number of excitation cycles is E. A photon in channel i cannot be detected if a photon in a previous channel, j i, is detected. The effective number of excitation cycles for channel i is then 

The relation can be used for a first-order correction of the pile-up distortion. For P exceeding 10% the influence of the pile-up oniV,. becomes noticeable, and a detection probability of 

The influenee of the (uncorrected) pile-up on a single-exponential lifetime ean be estimated as shown below. The probability, that a photon appears at a time corresponding to ehannel j in one signal period is 

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The repetition rate of Ti Sapphire lasers is fixed by the resonator length. Rates from 78 to 92 MHz are common. The high repetition rate helps to minimise classic pile-up effects in TCSPC measurements. However, it can cause problems if fluorescence lifetimes longer than 3 or 4 ns have to be measured, since the fluorescence does not decay completely within the pulse period. Data analysis can account for incomplete decay to a certain degree. However, if the lifetime becomes equal to or longer than the pulse period, the accuracy of the obtained lifetime degrades. The pulse repetition rate must therefore be reduced by a pulse picker. 

If the signal period is longer than the sum of the start-stop time and the TAC/ADC dead time (Fig. 7.82, left) the dead time ends before the next laser pulse. If a second photon is detected during this signal period it is lost. Consequently, the situation is exactly described by the classic pile-up effect. 

The situation for reversed start-stop and high repetition rate signals is shown in Fig. 7.85. The TAG is started when a photon is detected and stopped with the next laser pulse. Within the time between the start and the stop, the TAG is unable to record a second photon. The resulting loss is the classic pile-up effect. 

Fig. 7.77 Effect of classic pile-up on the recorded waveforms, a correct curves. b,c curves distorted by pile-up...

Fig. 7.79 Effect of classic pile-up on signals recorded with pulse-by-pulse multiplexing...

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