GBS Elektronik MCA-527 - Manual (Page 43)

Pulse Height Spectroscopical Measurements
3.2.14 Number of Channels
The number of channels the spectrum is distributed to can be chosen between 128 and
16384 (16k). The useful setting depends mainly on the detector connected. If setting is too
low, details of peaks may not be seen, if setting is too high, the statistics for a single
channel is bad so the spectrum looks very noisy, and of course storage of the spectra
takes more space.
A good setting for most cases is achieved if the FWHM of interesting peaks is between 3
and 8 channels.
The most useful setting when taking full range gamma spectra with a HPGe is 8k. 16K
may be only beneficial when taking spectra with a good planar HPGe in a range of 3MeV,
but the interest is in low energy peaks; or for electrical performance testing.
For CZT or LaBr,1024 channels are reasonable, more than 2k is not necessary. For a NaI,
typically 512 channels are enough.
Default is 4k with MCA527 and 1k with MCA527L. MCA527L offers only up to 2k
channels.
3.2.15 Threshold
This setting is a legacy of MCA166, where the default setting is 2% and everything below
is cut away. The effect of this setting is very similar to LLD, just that here the value is in
percent of the full spectrum and not in channels.
The MCA527 allows to measure much more in the low energy range. Depending on
amplification and trigger filter, the low energy cutoff may be at 0.2%, therefore allowing a
very high dynamic range.
Default setting is 0, but it may be a good idea to set this to 0.3%...0.5% to cut out the
noise peak.
3.2.16 LLD / ULD
The LLD / ULD settings (lower level discriminator, upper level discriminator) determine,
which part of the spectrum is actually counted. This setting is most useful in MCS mode
for taking time dependent rate for a special part of the spectrum. In PHA mode, typically
the full spectrum is counted.
Default setting for LLD is 0, for ULD it is the resolution - 1; e. g. For a 4k resolution this is
4095.
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