MCA527
For best pulse pair resolution, the preamplifier signal should be steep, but it should not
exhibit any ringing. In case of ringing or overshoot, the trigger filter will trigger multiple
times on a single signal and therefore consider this erratically as pile up. A preamplifier
ringing or overshoot problem can be suspected if dead time is much higher than expected.
In such a case try a single differentiating trigger filter such as -1, 0, 1 or -1, 1. If this does
not help, switch pile up rejection off.
The MCA527L only supports the 1, 0, -2, 0, 1 trigger filter.
3.2.5 Trigger Level
The trigger level is normally automatically adjusted to 7 times the evaluated RMS noise
level by default. This works fine for almost all tasks. However, a detector may exhibit non
Gaussian noise or there may be other high frequency disturbances around and therefore
a reason to readjust trigger level. Some software allows this.
Symptoms of too low trigger level are:
●
There is a unusual high count rate with some detector and no sources present
●
Left of the normal spectrum cutoff there is a significant peak right at 0keV in the
energy scale
If this noise count rate and the noise peak are annoyingly high, the trigger level may be
increased. It may be increased just such high, that the noise peak disappears. Further
increasing of the trigger level just increases the low energy cutoff unnecessary and
impairs the ability to reject pile up with low energy events.
30
Figure 10: Low end spectrum cutoff depending on trigger filter. Spectrum range in all
cases is 8MeV, whereas the 1, -2, 1 cuts off the spectrum already at a quite
high energy of 48keV, the -1, 1 and the 1, 0, -2, 0, 1 go down to 23keV, and
the -1, 0, 1 allows still energies down to 13keV to be seen.
0
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-11Co60.spe
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Energy [keV]
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