Pulse pileup

On-demand beam pulsing has been shown to be effective for eliminating pulse pileup in the X-ray detection system, minimizing the energy dissipated in delicate specimens, yet maximizing the data throughput of the overall system. In essence. 

For cell isolation purposes an important cytometer sort feature is pulse-pileup also referred to as peak-pileup or PPU. This recognizes split peak intensities arising from two or more cells in the same droplet that are strung together in chains or that coincidentally partially eclipse the laser beam. The two split peaks would each have the same forward- and side-scatter... 

Importantly, the dead-time of TACs and TDCs is comparatively long, typically 125-350 ns. When a photon arrives within this time interval after the detection of a photon, it will not be observed. Therefore, care must be taken that the count rate of the experiment is sufficiently low to prevent this pulse-pileup. TACs and TDCs usually operate in reversed start-stop geometry. Here, the TAC is started by the fluorescence signal and stopped by the laser trigger. 

A common cause of inaccuracy in SPC-based time domain detection is pulse-pileup, that is, the arrival of photons during the dead-time of the detection system. Because the higher probability of emission (and detection) in the earlier part of the decay, pulse-pileup is more probable in this part of the decay. Consequently, the decay will be distorted and the lifetime will be biased towards higher values. Moreover, pulse-pileup will also result in a reduction of the detection efficiency (see Fig. 3.7 and Eq. (3.4)). Therefore, care should be taken to avoid excitation rates too close to the efficacy count rate (i.e., the inverse of the dead-time) in order to minimize these effects. 

Samples and standards were irradiated for 8 h at a flux of 5 X 10 n/cm s. Following a 6-day decay, the samples were counted for 2 h each using an intrinsic germanium detector (FWHM at 1333 Co of 1.71 keV), and data were collected on an 8192-channel MCA. Data processing included corrections for pulse pileup and peak interferences. Samples were then allowed to decay for 30 days postirradiation and were recounted for 4 h using the same system. Table I presents a summary of the elements sought and quantified. 

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

With the long pulse durations used for the Si(Li) detector, there is a high probability that two or more x-ray photons will arrive at the detector within the duration of one slow amplifier output pulse. As a result the amplifier output pulse is distorted. The resulting pulse shape is the linear sum of the pulse shapes contributed separately by each photon. This distortion process is called pulse pileup since the... 

As was mentioned earlier, there can be no more than one photon detected per 50-100 source pulses. Electronics for SPG only allows detection of the first arriving photon. Once this photon is detected, the dead time in the electronics prevents detection of another photon resulting from the same excitation pulse. The apparent decay time becomes shorter as the number of arriving photons increases because the TAG is stopped by the first arriving photon. The pulse pileup problem is being solved to some extent by the development of dedicated electronic detection... 

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