Chapter 5 Web-Based Operations
194
Lynx DSA User's Manual - 9240227J
For the example shown above, the second event begins before the first returns to the
baseline. This is not normally a problem and the second event should be accepted for
maximum throughput. However, if the tail of the first event exhibited detector-
induced anomalies, the second event would be corrupted and should not be accepted.
To prevent acceptance of this corrupted event, the PUR Guard should be increased as
shown.
Some detectors with RC preamps may exhibit secondary time constants which is
evident by a short lived undershoot or ring on the trailing edge of the shaped signal
(see the following figure).
This behavior is usually due to non-ideal characteristics of the preamp feedback
resistor. Events that fall on the tail of an event which exhibits this behavior will
become corrupted or distorted when minimal guard time is selected. In this case, the
spectral peaks will be distorted with excessive 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 105: Preamplifier Secondary Time Constant
Scope:
Vert: 20 mV/div
Horiz: 10
µ
s/div
PUR Guard Setup
The default PUR Guard Time is 1.1x. This Guard Time is minimum and does not
extend the pile up 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
).
