PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
xx
The settings of the trigger filter have only minor effect on the resolution. However, changing
the trigger conditions might have some effect on certain undesirable peak shapes. A longer
trigger rise time allows the threshold to be lowered more, since the noise is averaged over
longer periods. This can help to remove tails on the peaks. A long trigger flat top will help to
trigger better on slow rising pulses and thus result in a sharper cut off at the threshold in the
spectrum.
3.5.4 Decay Time
The preamplifier decay time
is used to correct the energy of a pulse sitting on the falling
slope of a previous pulse. The calculations assume a simple exponential decay with one decay
constant. A precise value of
is especially important at high count rates where pulses overlap
more frequently. If
is off the optimum, peaks in the spectrum will broaden, and if
is very
wrong, the spectrum will be significantly blurred.
The first and usually sufficiently precise estimate of
can be obtained from the
Auto
Find
routine in the
Energy
tab of the P
ARAMETER
S
ETUP
panel. Measure the decay time several times
and settle on the average value.
Fine tuning of
can be achieved by exploring small variations around the fit value (±2-3%).
This is best done at high count rates, as the effect on the resolution is more pronounced. The
value of
found through this way is also valid for low count rates. Manually enter
, take a
short run, and note the value of
that gives the best resolution.
Pixie users can also use the fit routines in the O
SCILLOSCOPE
to manually find the decay time
through exponentially fitting the untriggered input signals. Another tool is the
Optimize
routine
in the
Energy
tab of the P
ARAMETER
S
ETUP
panel. Similar to the routine for finding the optimal
energy filter times, this routine can be used to automatically scan a range of decay times and
find the optimal one. Please refer to the Online Help documentation for more details. A further
option is to create a file series where
is modified for each file in the series. See section 3.6
for more details.
3.6 File Series
3.6.1 File Series to break up long data acquisition runs
When taking long data acquisitions, it may be beneficial to break up the run into smaller sub
runs. This helps to save data in case of power failure or system crashes, since only the most
recent sub run is lost. Also list mode files tend to get large and unwieldy for analysis in longer
runs.
The Pixie Viewer thus has a method to create a series of files at specified intervals. In the D
ATA
R
ECORD
O
PTIONS
panel, opened with the
Record
button in the
Run Control
tab of the
P
ARAMETER
S
ETUP
panel, there is a checkbox named
New files every
, followed by a control
field to enter a spill (or time) interval N. If checked and a run is started, every N spills (or, in
MCA runs, every N seconds) the data file is closed, spectra, settings and statistics are saved,
and then a new run is started. This is equivalent to manually clicking first the Stop Run button
and then the Start Run button. It is recommended to enable the automatic increment and auto-
store options as shown in Figure 3.9 as well.