GBS Elektronik MCA-527 - Manual (Page 39)

Pulse Height Spectroscopical Measurements
3.2.11 Baseline Restoring
A disadvantage of the conventional approach of pole zero compensation by adding a DC
component is that now the result becomes sensitive to DC and low frequency
disturbances such as often found HPGe preamplifier offset drift. Furthermore it is desired
that an energy of 0keV is found in channel 0 and the spectrum does not have offset. This
is corrected by the base line restorer. The baseline restorer applies the spectroscopical
filter to the signal immediately before the event if possible, and this result for the baseline
is subtracted from the evaluated value for the event. The disadvantage of this is that the
baseline measurement has itself an error, which adds to the total error and leads to some
peak broadening. As the baseline should not change to fast, it is possible to do averaging.
There are settings from 1 to 32 possible; 1 means no averaging and the baseline value
always new measured, 32 is that the last measured value contributes only to 1/32 to the
actual baseline.
Default is 16, which is best for good resolution and not too fast changing baseline. With
increasing count rate and low frequency noise in the signal, smaller values down to 4 may
be more optimum. For MCA527L the setting is always 4.
There is also a second very slow baseline restorer which concerns the analog part of the
device and just readjusts the analog offset such that the base line is at the 10% or 90%
input range of the ADC.
39
Figure 22: Dependence of resolution on base line restorer setting, as example a planar
HPGe detector operating with different count rates.
1 10 100
400
600
800
1000
1200
1400
1600
1800
2000
59keV @ 30kcps
14keV @30kcps
662keV @ 30kcps
59keV @ 20kcps
14keV @ 20kcps
662keV @ 20kcps
59keV @ 12kcps
14keV @ 12kcps
Shaping Time [µs]
F
W
H
M
@
66
2k
eV
[k
eV
]