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Assuming randomly occurring pulses, the effect of dead time on the output count rate is
governed by Poisson statistics for paralyzable systems with pileup rejection7. This means the
output count rate OCR (valid pulses) is a function of filter dead time Td and input count rate ICR
given by
OCR = ICR * exp(-ICR* 2 * Td),
(4)
which reaches a maximum OCRmax = ICRmax/e at ICRmax = 1/(2*Td). Simply speaking, the factor
2 for Td comes from the fact that not only is an event E2 invalid when it falls into the dead time
of a previous event E1, but E1 is rejected as piled up as well. This filter dead time is recorded in
the SFDT counter in each processing channel.
Fig. 6.7. Pulse dead time. A pulse arriving at T0 will incur slow filter dead time (for energy
measurement) until T1. At T1, the pileup status is latched – for a single pulse, it is logic low and the
event is accepted. A second pulse arriving at T0' will extend the dead time and cause the pileup
status to be logic high. Unless pileup rejection is disabled, both events are rejected.
2. Fast trigger dead time (FTDT)
Fig. 6.9. Fast Trigger Dead Time (FTDT). A second pulse is not detected if the trigger filter output
is still above threshold.
An second type of dead time only affects the trigger filter. Triggers are issued when the trigger
filter output goes above the trigger threshold set by the user. However, depending on the length
7
G. Knoll, Radiation and Measurement, J Wiley & Sons, Inc, 2000, chapters 4 and 17.
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PIXIE-500 Express User’s Manual V3.20
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