microDXP Technical Reference Manual
Version 3.15
October 7, 2019
61
will never exceed some maximum width, defined by MAXWIDTH. Thus, if the width of
the fast filter output pulses is measured at threshold and found to exceed MAXWIDTH,
then fast channel pileup must have occurred. This is shown graphically in Figure 4-9, where
pulse 3 passes the MAXWIDTH test while the piled-up pair of pulses 4 and 5 fail the
MAXWIDTH test. Thus, only pulse 1 passes both pileup inspection tests and, indeed, it is
the only pulse to have a well-defined flattop region at time PEAKSAMP in the slow filter
output.
MAXWIDTH should be set according to the preamplifier rise time t
R
and fast filter length
and gap: FASTLEN and FASTGAP.
MAXWIDTH = F 2* F (t
R
/ sampling interval)
Equation 4-10
4.8 Input Count Rate (ICR) and Output Count Rate (OCR)
During data acquisition, x-rays will be absorbed in the detector at some rate. This is the
input count rate, which we will refer to as ICR. Of the detected x-rays, some fraction will
also satisfy pileup tests and have their values of V
x
captured and placed into the spectrum.
This number is the output count rate, which we refer to as the OCR. The DXP normally
returns statistics in addition to the collected spectrum: the REALTIME for which data was
collected; the triggering channel LIVETIME, or sum of time during which the triggering
channel was below threshold (and thus ready to detect a subsequent x-ray); the
FASTPEAKS, or number of times the triggering channel threshold was surmounted, and
the EVTSINRUN, or number of good events captured into the spectrum. From these
values, the ICR can be computed according to Equation 4-11, and the OCR according to
Equation 4-12.
ICR = FASTPEAKS / LIVETIME
Equation 4-11
OCR = EVTSINRUN / REALTIME
Equation 4-12
Note: The fast channel LIVETIME should only be used to determine the input count rate
according to Equation 4-11. Specifically, it is NOT related to the energy filter live time and
should not be interpreted as the inverse of the processor deadtime. The energy filter live
time can be calculated according to Equation 4-13.
Energy filter live time = REALTIME * OCR / ICR
Equation 4-13
4.9 Throughput
Figure 4-10 shows how the output count rate (OCR) varies with input count rate (ICR) for
a few different Peaking Times. Functionally, the OCR is seen to initially rise with
increasing ICR and then saturate at higher ICR levels. The theoretical form, from Poisson
statistics, for a channel that suffers from paralyzable (extending) dead time is given by:
