Pile-up rejection

Since the fast centrifuge system SISAK is equipped with liquid scintillation counting LSC [12,58], it is in principle capable of investigating short-lived a-decaying nuclides of the transactinides, p/y pulses and a pulses are distinguished by pulse-shape discrimination PSD and pile-up pulses are rejected by a pile-up rejection system PUR. This analog electronics proved to result in insufficient background suppression. Thus, two new approaches... 

Dead time correction is made by empirical measurement of observed count rates as a function of increasing concentrations of activity. From these data, the dead time loss is calculated and a correction is applied to the measured data to compensate for the dead-time loss. Other techniques, such as use of buffers, in which overlapping events are held off during the dead time, use of pulse pile-up rejection circuits, and use of high-speed electronics, have been applied to improve the dead time correction. 

Figure 4.23 The principle of the spectroscopy amplifier pile-up rejection system...

Amplifiers are available which also provides an alternative means of pile-up rejection by detecting the delayed peaking time of a pile-up pulse. Since pulses affected by ballistic deficit or charge trapping will also be delayed, these pulses will also be rejected. 

Pile-up rejection is useful for reducing random summing at moderate to high count rates. At low count rates, it should be switched out. 

Non-Poisson statistics due to pile-up rejection and loss-free counting... 

In Chapter 4, I discussed random summing in connection with the pile-up rejection circuitry in amplifiers. We came to the conclusion that even with pile-up rejection there must be some residual random coincidences. There is then, whether or not pile-up rejection is available, a need to be able to correct for random summing in high count rate spectra. In some circles, there seems to be an assumption that pile-up rejection is 100% effective... 

Because the summing is random, this correction is applicable to aU peaks in the spectrum - to apply it we need to know the resolution time, t. Without pile-up rejection, we can expect it to be of the same order of magnitude as the shaping time constant of the amplifier, i.e. a few jjts. It is best estimated by experiment. 

Not all dead time sources are accounted for. This might arise, for example, if the dead time signal from the pile-up rejection circuit is not connected up properly. 

As with random summing, the event results in loss of counts from the full-energy gamma-ray peaks and a loss of efficiency. However, unlike the random summing that I discussed in Section 4.8, the summed pulse will not be misshapen and cannot be rejected by pile-up rejection circuitry. 

In order to minimize random summing, make sure that pile-up rejection is enabled and that the count rate is low. 

As we have seen above, each component of the electronic system has its contribution to make to overall dead time TRP reset, amplifier busy signals, pile-up rejection gate, ADC busy and MCA store-to-memory. It would appear that there is no industry standard for timing. 

This test is similar to the procedure outlined earlier in Chapter 7, Section 7.6.8 to derive a random summing correction factor for conventional systems. It was pointed out that, regardless of the use of pile-up rejection circuitry, there will always be a small proportion of random... 

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