PUR Guard
Lynx DSA User's Manual - 9240227J
195
For example:
•
The filter rise time is set to 5.6
µ
s and the flat top is set to 0.8
µ
s.
•
The Peaking Time is: 1.1 x 5.6
µ
s + 0.8
µ
s = 7.0
µ
s. T
W
for a stable baseline
is 15.0
µ
s.
•
The desired guard time setting is: T
W
/Peaking Time = 15.0/7.0 = 2.1
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; Guard Time = 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
optimized (refer to
Methods of Detector Matching
on page 196).
If the Pole/Zero is not the problem, set the PUR Guard to 2.5x and acquire a new
spectrum. 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.
