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Pile-Up Rejection When two gamma rays arrive at the detector within the width of the spectroscopy amplifier output pulse, their respective amplifier pulses pile up to form an output pulse of distorted amplitude [Fig. 22(a)]. For detectors whose charge collection time is very short compared to the peaking time TP of the amplifier output pulse, a pile-up rejector can be used to prevent analysis of these distorted pulses. The pile-up rejector is implemented by adding a "fast" pulse shaping amplifier with a very short shaping time constant [Fig. 22(b)] in parallel with the "slow" spectroscopy amplifier. In the fast amplifier, the signal-to-noise ratio is compromised in favor of improved pulse-pair resolving time. A fast discriminator is set above the much higher noise level at the fast amplifier output to convert the analog pulses into digital logic pulses [Fig. 22(c)]. The trailing edge of the fast discriminator output triggers an inspection interval TINS [Fig. 22(d)] that covers the width TW of the slow amplifier pulse. If a second fast discriminator pulse from a pile-up pulse arrives during the inspection interval, an inhibit pulse is generated [Fig. 22(e)]. The inhibit pulse is used in the associated ADC or multichannel analyzer to prevent analysis of the piled-up event. As demonstrated in Figure 23, the pile-up rejector can deliver a substantial reduction in the pile-up background at high counting rates with germanium and Si(Li) detectors.
Figure 22. Basic Waveforms in the Pile-Up Rejector.
Figure 23. Demonstration of the Effectiveness of the Pile-Up Rejector in Suppressing the Pile-Up Spectrum with a Germanium Detector and a 60Co Spectrum at 50,000 Counts/s. |
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