PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
xix
3.5 Optimizing Parameters
Optimization of the Pixie-4’s run parameters for best resolution depends on the individual
systems and usually requires some degree of experimentation. The Pixie Viewer includes
several diagnostic tools and settings options to assist the user, as described below.
3.5.1 Noise
For a quick analysis of the electronic noise in the system, you can view a Fourier transform of
the incoming signal by selecting O
SCILLOSCOPE
→
FFT
. The graph shows the FFT of the
untriggered input sigal of the O
SCILLOSCOPE
. By adjusting the
dT
control in the O
SCILLOSCOPE
and clicking the
Refresh
button, you can investigate different frequency ranges. For best
results, remove any source from the detector and only regard traces without actual events. If
you find sharp lines in the 10 kHz to 1 MHz region you may need to find the cause for this and
remove it. If you click on the
Apply Filter
button, you can see the effect of the energy filter
simulated on the noise spectrum.
3.5.2 Energy Filter Parameters
The main parameter to optimize energy resolution is the energy filter rise time. Generally,
longer rise times result in better resolution, but reduce the throughput. Optimization should
begin with scanning the rise time through the available range. Try 2
s, 4
s, 8
s, 11.2
s, take
a run of 60s or so for each and note changes in energy resolution. Then fine tune the rise time.
The flat top usually needs only small adjustments. For a typical coaxial Ge-detector we suggest
to use a flat top of 1.2
s. For a small detector (20% efficiency) a flat top of 0.8
s is a good
choice. For larger detectors flat tops of 1.2
s and 1.6
s will be more appropriate. In general
the flat top needs to be wide enough to accommodate the longest typical signal rise time from
the detector. It then needs to be wider by one filter clock cycle than that minimum, but at least
3 filter clock cycles. Note that a filter clock cycle ranges from 0.026 to 0.853
s, depending on
the filter range, so that it is not possible to have a very short flat top together with a very long
filter rise time.
The Pixie Viewer provides a tool which automatically scans all possible combinations of
energy filter rise time and flat top and finds the combination that gives the best energy
resolution. This tool can be accessed by clicking the
Optimize
button on the
Settings
tab. Please
refer to the Online Help documentation for more details. A second option is to create a file
series where the energy filter parameters are modified for each file in the series. See section
3.6 for more details.
3.5.3 Threshold and Trigger Filter Parameters
In general, the trigger threshold should be set as low as possible for best resolution. If too low,
the input count rate will go up dramatically and “noise peaks” will appear at the low energy
end of the spectrum. If the threshold is too high, especially at high count rates, low energy
events below the threshold can pass the pile-up inspector and pile up with larger events. This
increases the measured energy and thus leads to exponential tails on the (ideally Gaussian)
peaks in the spectrum. Ideally, the threshold should be set such that the noise peaks just
disappear.