Pulse Height Spectroscopical Measurements
3.2.13 Low Frequency Rejection
Low frequency rejection is a special triple differentiating spectroscopic filter for use in
environments with strong low frequency noise components, e. g. microphonics with a
HPGe or higher leakage currents with a CZT. Principally this filter does not only evaluate
the voltage step, but also the slope of the baseline before and after the voltage step.
Because of this, the low frequency sensitivity is such low that Pole zero adjustment is not
necessary. However, as this filter is triple differentiating, the amount of high frequency
noise is increased, even as this filter is optimized for lowest error. This filter gives best
results with twice or triple as high shaping times, so throughput may be a bit limited with
this. Resolution can be almost as good as with the standard filters in a quiet environment.
In a noisy environment, the low frequency filter can yield orders of magnitude better
results and be invaluable when forced to work in such environments. Use this option in the
presence of significant low frequency interference. Default is off, this feature is not
available with MCA527L.
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Figure 24: Preamplifier signal of a HPGe detector in a quiet and noisy environment
Left: Preamplifier signal of a HPGe measuring Am241 in a quiet
environment.
Right: Preamplifier signal of a HPGe measuring Am241 in an extremely noisy
environment, here a grinding machine nearby running. The signal
exhibits oscillations in the 300 Hz range, which may be twice as high as
the signal measured.
