Pulse pile-up protection

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Role of the pulse processor
Analog pulse shaping
Time variant shaping
Digital pulse shaping
Fixed process time
Adaptive pulse shaping
Resolution & count rate
Pulse pile-up protection
Comparing different pulse processors
Summary

The pulse processor

Pulse pile-up inspection

Pulse pile-up inspection channels are used to ensure that only one photon is measured at a time.

Inspection circuits (Fig. 13b) sense the arrival of an incoming event. Each circuit has a time constant that determines the smallest voltage step and therefore the lowest energy event that can be inspected. If the time constant is too short, noise levels will be high and low energy events will not be efficiently detected, but if too long it will be unable to distinguish between closely arriving events and the pile-up protection efficiency of the circuit will be compromized. Pulse processors should have more than one of these circuits, each with a different time constant to ensure efficient pulse pile-up protection over the full range of detectable energies. This means the processor will be suitable for use with a thin polymer window detector for light element detection, as well as for measuring higher energy lines.

The result of efficient pulse inspection is to provide a spectrum with no pile-up artefacts such as sum peaks. These peaks, caused when two X-rays arriving close together are counted as one, may cause incorrect peak identification, and inaccurate results (Fig. 19). If a photon is missed by all the inspection channels, it can contribute to a tail or ‘pile-up continuum’ on the high side of every peak.

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