Pulse Height Spectroscopical Measurements
3.3 Gated Measurements
The gate input is useful if spectra should only be measured during a certain time or if
certain events should be rejected, e. g. if using a Compton shield as for low radiation
measurements. By software it can be set
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if gating is used or not
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what gate input level enables measurement (low or high)
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delay time. The gate level is checked an adjustable (0µs...25µs) delay time after
the rise of the preamplifier signal, then the decision is made.
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Discard or sort mode. In case of sort mode, the rejected events are counted in a
second spectrum. Sort mode is an option if doing stabilization with a LED pulser on
a NaI detector. See also measurements with stabilization.
The gate input signal has to be TTL-compatible (3.3V nominal, but input is 5V-tolerant).
An input voltage <0.8V is considered low, an input voltage >1.6V is considered high.
3.4 Measurements with Stabilization
Stabilization is an option if the gain of a detector system is not fully stable. It allows to
adjust fine gain during a measurement to keep a certain peak in the spectrum at its place.
Stabilization is mostly used with temperature sensitive detectors such as NaI and for long
term or series measurements.
For stabilization, a peak has to be selected from spectrum, which
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should be always present
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should be significant and not disturbed by other peaks
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is preferably in the upper part of the spectrum.
Typically peaks used would be the K40 peak (1460.5keV) which is always present in
background, Am241 peaks, (gamma or alpha) as Am241 as source is sometimes
integrated with NaI detectors, or a peak generated by LED light pulses, as present in
special NaI detectors.
For selecting a peak, a stabilization ROI has to be selected, and a centroid to which to
stabilize a peak.
If the selected peak originated from a LED light pulse, and the detector provides also a
gate signal for the LED signal, gating with sort mode should be selected, and then
stabilization on the gated spectrum. This causes that the stabilization light peak does not
disturb the measured spectrum, and the stabilization is not disturbed by real peaks in the
spectrum.
The quality of the stabilization can be influenced by the settings for stabilization area and
time.
The stabilization time is the minimum time for a stabilization cycle, where fine gain is
readjusted. Default is 10 seconds, and this is suitable for most cases.
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