PUR Guard
Lynx II DSA User's Manual – 7096089 171
This behavior is usually due to non-ideal characteristics of the preamp feedback resistor.
Events that fall on the tail of an event that exhibits this behavior will become corrupted or
distorted when minimal guard time is selected. In this case, the spectral peaks will be
distorted with excessively high or low side tailing at high count rates. Events that arrive
too close and are corrupted by the tail of the previous pulse can be rejected by increasing
the Guard Time. For problematic detectors, this will reduce spectral distortion at high
count rates, but at the expense of reduced throughput.
Figure 77: Preamplifier Secondary Time Constant
Scope:
Vert: 20 mV/div
Horiz: 10 µs/div
PUR Guard Setup
The default PUR Guard Time is 1.1x. This Guard Time is minimum and does not extend
the pileup rejection interval beyond the peaking time. For events that exhibit secondary
time constants or other anomalies, measure the pulse width from the leading edge to where
it returns to the baseline and becomes stable. This is shown as time T
W
in the second
figure of the previous section. The required guard time is determined by dividing T
W
by
the Peaking Time (1.1T
R
+ T
Flat Top
).
For example:
• The filter rise time is set to 5.6 µs and the flat top is set to 0.8 µs.
• The Peaking Time is: 1.1 x 5.6 µs + 0.8 µs = 7.0 µs. T
W
for a stable baseline is 15.0
µs.
• The desired guard time setting is: T
W
/Peaking Time = 15.0/7.0 = 2.1
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