Pile-up effect

The pulser peak method gives better estimates of the effective counting time than the methods implemented in the ADC converter which estimate the time when the gate of the ADC is closed electronically, since in the former the influence of the pile-up effect on the peak areas is taken into account. However, an automatic analysing procedure evaluating the pulser peak area introduces systematic effects which are caused by the distortion of the shape of the pulser peak. These systematic effects are reflected in the dependence of the count rate from a source located at a fixed position on the total count rate in the spectrum. The effects arise only partially from the difference between the calculation of the pulser peak area and the areas of other peaks in the spectrum. The other sources of systematic effects originate in the difference between the pulser peak shape and the shapes of other peaks in the spectrum and in the relatively low background near the pulser peak. 

The. .disable count signal is activated if several photons are detected in different detectors within the response time of the router. It suppresses the storing of a detected photon in the memory of the TCSPC module. Thus the multidetector technique elegantly uses the. .disable count signal to reduce pile-up effects at low pulse repetition rates. If several photons appear within the same signal period, they are more likely to be detected in different detectors than in the same one. Therefore. the multidetector technique is able to detect a large fraction of the mul-... 

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. 

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. 

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. 

Loss of photons inside the TCSPC module results from the faets that a single TCSPC channel can record only one photon per signal period, and that the module is blind during the dead time, i.e. the time during which a detected photon is processed. The detection and consequent loss of a second photon in one signal period is usually called pile-up effect. The term counting loss covers both pile-up-related and dead-time-related loss. 

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. 

The count rate of TCSPC has been increased by a factor of 100 in the last 10 years. Further increase is limited by the pile-up effect. A decrease in dead time below the currently achieved 100 ns appears feasible and may result in reduced counting loss. However, a substantial increase in count rate for a single TCSPC channel can only be obtained by reducing the dead time noticeably below the typical signal duration. For the typical fluorescence lifetime, a dead time of less than 1 ns will be required. Even if dead times this short can be achieved within the electronics, there is currently no detector capable of delivering several individual single-photon pulses within this time. It is therefore more likely that higher count... 

It should be noted in this connection that a piling up effect is observed not only with very intensive chemiluminescence reactions, but also when measuring radioactive scimples with count rates exceeding 10 to 10 cpm. We have studied this effect in experiments with a tritium sample with an activity of 4 x 10 dpm. 

Amplifier throughput is inversely proportional to the shaping time — the narrower the pulses, the more through the system per second. So, at any particular input rate there can be a trade-off between the throughput capability and resolution. If the optimum time constant is halved, then we expect twice as many counts to be processed before pile-up effects become a problem, at the cost of slight... 

Although the time structure of the primary positron beams may be of advantage to some experiments (see e g. Howell et al. [3.18]), the saturation and pile-up effects inherently connected with high-intensity bunched beams often cause problems in other experiments. Therefore, a number of present electro-producing positron facilities have been equipped with storage and pulse-stretching devices (see, e.g., Ebel et al. [3.17], Akahane and Chiba [3.19], Ito et al. [3.20], and Hulett et al. [3.21]). 

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