Pulse-pile-up

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. 

This derivation ignores the possibility of more than two pulses piling up and is therefore applicable only when the counting losses are not too high. 

An ideal spectroscopy amplifier should have a constant amplification for pulses of all amplitudes without distorting any of them. Unfortunately, some pulse distortion is always present because of electronic noise, gain drift due to temperature, pulse pile-up, and limitations on the linearity of the amplifier. 

Since the time of arrival of pulses is random, it is inevitable that a pulse may arrive at a time when the previous one did not fully decay. Then the incoming pulse piles up on the tail of the earlier one and appears to have a height different from its true one. Pulse pile-up depends on the counting rate. 

The output from a voltage sensitive or resistor feedback preamplifier is a tail pulse with a rather long decay time. Hence, some pulse pile-up is unavoidable, except at very low count rates. Pile-up will cause the average level of this signal to increase with pulse rate, which may approach the limit of linear operation of the preamplifier. 

Positron Annihilation Lifetime Spectroscopy (PALS). A fast-fast PALS system using cylindrical (40 mm diameter x 15 mm thick) Bap2 scintillators (14) arranged at 90 to each other to avoid pulse pile up problems. A count rate of 150-300 cps was achieved with a a source and a instrument resolution of 220-240 ps FWHM for a... 

Considerable eomputer power is dedieated to eorreeting for pulse pile-up and for extraetion of data from poorly resolved speetra. These elaborated correction processes tend to be based on empirieal relationships that may change with time, so eontinuous vigilance is required in order to obtain ehemical data of adequate precision. 

Zr(n,n ) Zr and Zr(n,2n) Fig. VII-4 gives the apparent oxygen concentration for a zirconium sample as a function of the delay between irradiation and start of the measurement. It is clear that a 6 s delay allows an interference free oxygen determination in the conditions used (185). Also for lead, pulse pile-up may lead to systematic errors. By the Pb(n,2n) " pb... 

If the half-life of the radionuclide that gives rise to pulse pile-up is long compared to that of this effect can be detected by measuring the sample e.g. 90 s after the irradiation, i.e. after the decay of The counting will differ significantly from the natural background if pulse pile-up occurs. 

Under optimal conditions with respect to the occurrence of pulse pile-up 1 mg of oxygen corresponds to 160 counts, S = 160 counts/mg, 12 s after... 

To meet the demand arising from high resolution spectroscopy, a considerable improvement has been introduced on the built-in base-line restorers and pile-up rejectors in order to avoid spectral distortions that may arise from baseline fluctuations at high counting rates and from pulse-on-pulse pile-up. 

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