STJ Quick Start Guide v 2.01
April 1, 2021
Page 18
centroid of the inflection point in the green (bias current) plot. Note that the value in the Zero Adjust
changes with the cursor selection, and is highlighted yellow if the value is different from the saved STJ
Zero.
•
Move the blue cursor to the inflection point in the green plot, then press the “Set STJ Zero”
button.
•
Repeat for all channels!
•
Press the Start Scan button again to acquire new data with the voltage offsets removed. Note
that the inflection point should now be centered along the x-axis, at 0V.
•
In the past we have had to repeat this entire process a few times to get the offsets set
properly. This may have been necessary because of faulty ground connections that led to
interactions between the STJ pixels.
SET STJ BIAS
The STJ bias should be chosen in the appropriate voltage range, and at a voltage that is not associated
with a Fiske-mode peak. The noise level plot is useful for identifying the Fiske-mode peaks.
•
Move the yellow cursor to the desired voltage, then press the “Set STJ Bias” button.
•
Repeat for all channels!
At this point, all of the STJ detector elements should be properly biased.
SCOPE PANEL
The Scope panel displays raw ADC and filter data for debugging purposes. Data acquisition occurs
simultaneously for all channels in the currently selected modules. We strongly recommend reviewing this
section before trying to acquire MCA or list-mode Mapping data.
•
Set the Trace Type to “ADC”
•
Set the Trigger Type to “Threshold”
•
Turn on the x-ray source, or alternatively, enable the pulser in the Auxiliary tab
•
Press the Get Trace button to simultaneously acquire a plot of the ADC for all 32 channels
•
You may need to adjust the Trigger Threshold (in the Acquisition tab) to properly trigger the
acquisition. Note that the hardware times out after a second or so if no trigger is generated,
and just acquires un-triggered data.
MEASURE THE RC DECAY TIME
You should see positive RC decay pulses in the ADC data (either from the on-board pulser or from the
detector). To get the best energy resolution, the Decay Time setting in the Detector tab must be set
properly. Use the cursors in the ADC plot area to measure the decay time
τ
. Note that energy resolution
can be degraded significantly if this setting is way off, but small errors won’t really affect the data quality:
Don’t worry about getting it exactly right. For a more thorough explanation of the measurement, please
review section 3.8.3.4 of the
microDXP Development Kit Manual
, noting that the signal rise time is
typically much longer for STJ detectors than for conventional x-ray detectors, e.g. silicon drift.
•
If you’re using the pulser, the Decay Time is 5
µ
s (with PULSERATE=0)
