7. PUR/LTC Operation
The Model 9660 ICB Digital Signal Processor includes a pileup rejector and live time
corrector. The pile up rejector inspects for pulse pileup and allows only non-piled up
events to be processed and stored into the spectrum. The result is a reduced number of
counts in the pileup region and reduced spectral interference for improved quantitative
measurement and analysis.
To compensate for dead times associated with rejected pulses and amplifier processing
times, the Model 9660 generates a dead time (DT) signal which extends the collection
time by the appropriate amount.
Pileup Rejection With a Live Source
The pileup rejector monitors the signal processing activities of the fast discriminator
(fast channel) and digital filtered signal (slow channel) and allows only signals result-
ing from a single detector event to be processed and stored in the spectrum. The fast
discriminator detects the arrival of input events and is capable of discriminating be-
tween multiple events separated by less than 500 ns. If the fast discriminator detects
two or more events within the processing time of the slow channel, the event is con-
taminated by pileup and is discarded.
The fast discriminator threshold is automatically adjusted just above the system noise
level for accurate operation. However, for the discriminating researcher or special cir-
cumstances the threshold can be optimized manually. For instructions on adjusting the
threshold manually, see “Manual Fast Discriminator Threshold” on page 72.
The following steps will demonstrate the operation of the Pileup Rejector and its abil-
ity to reduce spectral interference.
1.
Connect the Model 9660 and set it up as described in “Spectroscopy System
Setup” on page 34.
2.
For the following demonstration of the Pileup Rejector, a
57
Co source will be
used. As a result the system gain will need to be increased; set the gain as
follows:
Coarse Gain: x40
Fine Gain: x1.6000
SF Gain: 0.0000e
-2
User’s Manual - ICN 9231014G
47
Pileup Rejection With a Live Source
