If the calculated guard time falls in between available selections, set the PUR Guard
for the next higher setting. The pileup rejection interval will now be extended beyond
the peaking time. Subsequent events that occur within the pileup reject interval of 15
µ
s will be rejected. After this instance, the anomaly associated with the tail of the
previous pulse is over and subsequent events can be accepted. As noted earlier,
extending the PUR interval by adding Guard Time will degrade throughput. Highest
throughput is obtained with the PUR Guard set for minimum; x=1.1.
The PUR Guard adjust function is located on the Gain Device Adjust screen. The
value can be decremented/incremented using the adjust slide bar. The adjust range is
1.1x to 2.5x (the default setting is 1.1x).
PUR Guard Adjustment Using a Live Spectrum
As mentioned earlier, detector/preamplifier induced effects on the trailing edge of the
shaped signal will cause spectral distortion; low or high side tailing.
At moderate to high count rates, observe the shape of the spectral peaks. They should
appear symmetrical. Low or high side tailing may indicate the presence of
preamplifier-induced effects corrupting the trailing edge of the shaped signal. This
could also be due to a misadjusted pole/zero. Verify the Pole/Zero is correctly opti-
mized (refer to “Detector Matching” on page 38.
If the Pole/Zero is not the problem, set the PUR Guard to 2.5x and acquire a new spec-
trum. If the symmetry of spectral peaks improves, this affirms that trailing edge pileup
effects associated with the shaped signal are responsible. Reduce the PUR Guard time
to the next lower setting of 2.3x and re-acquire a spectrum. If the symmetry and
FWHM of the spectral peaks remain good, reduce the PUR Guard time again to the
next lower setting. Repeat this procedure until spectral distortion begins to reappears,
then set the PUR Guard time to the next higher setting.
Live Time Correction Using the LFC Module
The Model 9660 Digital Signal processor can be used with the Model 599 Loss Free
Counting Module (LFC). The 599 LFC provides the ability of correcting for counting
losses as they occur in real time, rather than extending the measurement duration as in
conventional live time correction. The LFC module dynamically monitors the count-
ing losses and generates a fractional weighting value N. The LFC and MCA update the
intended memory address using an ADD-N memory transfer instead of the traditional
ADD-1.
The intended MCA must support multiple ADD-N or multiple ADD-1 operations.
Please reference the 599 LFC Operator Manual for specific details and operating in-
structions.
Chapter 7 - PUR/LTC Operation
54
Model 9660-9660A ICB Digital Signal Processor