Author Topic: WDS Detector Dead Time calibration acquisition now in StartWin application  (Read 2830 times)

sem-geologist

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I’d encourage everyone to measure deadtimes for multiple X-ray lines per spectrometer and not to rely on just Si Ka and Ti Ka.  It’s a time-consuming but worthwhile exercise.  I see variation in deadtime from one X-ray line to another, but I can’t see any obvious pattern to it -- it is not a simple function of X-ray energy. 

Brian,
Had You done those measurements in differential mode or in integral mode?
I don't know how Jeol (and if) had cut corners in pulse processing... Are Your tests done one by one spectrometers or all at once?

I as well try to stay in low count range (for Cameca probes that is about 10kcps) where pulse-pileup probability is below 0.5% in integral mode. But I am looking forward to improve this and go up to reliable 50kcps with differential mode and modified equation of deadtime for diff mode (which needs to take into account counts of integral mode). At first I want to finish gathering the evidences and fact-checking, before presenting the full picture (This is going to take some time). I want to emphasize that deadtimes on WDS are partly from different sources than those on EDS. In coming separate post I will go through pulse pipeline and discuss the parts which majorly or only minorly can influence the dead time and how (and why) it differs from EDS counting. As for Cameca probes, currently I had done live oscilloscope based observations of low pressure (on SX100, last year) and high pressure (on SXFiveFE, revisiting this issue) GPC generated signals, and results (at first surprising a bit) are quite different, but well explainable taking all pieces together. There are many factors which could influence or for sure influence the miss events of some pulses.
« Last Edit: March 28, 2022, 02:17:45 AM by sem-geologist »

Brian Joy

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I’d encourage everyone to measure deadtimes for multiple X-ray lines per spectrometer and not to rely on just Si Ka and Ti Ka.  It’s a time-consuming but worthwhile exercise.  I see variation in deadtime from one X-ray line to another, but I can’t see any obvious pattern to it -- it is not a simple function of X-ray energy. 

Brian,
Had You done those measurements in differential mode or in integral mode?
I don't know how Jeol (and if) had cut corners in pulse processing... Are Your tests done one by one spectrometers or all at once?

I as well try to stay in low count range (for Cameca probes that is about 10kcps) where pulse-pileup probability is below 0.5% in integral mode. But I am looking forward to improve this and go up to reliable 50kcps with differential mode and modified equation of deadtime for diff mode (which needs to take into account counts of integral mode). At first I want to finish gathering the evidences and fact-checking, before presenting the full picture (This is going to take some time). I want to emphasize that deadtimes on WDS are partly from different sources than those on EDS. In coming separate post I will go through pulse pipeline and discuss the parts which majorly or only minorly can influence the dead time and how (and why) it differs from EDS counting. As for Cameca probes, currently I had done live oscilloscope based observations of low pressure (on SX100, last year) and high pressure (on SXFiveFE, revisiting this issue) GPC generated signals, and results (at first surprising a bit) are quite different, but well explainable taking all pieces together. There are many factors which could influence or for sure influence the miss events of some pulses.

I use integral mode and only use results up to about 30 kcps.  In this way, I avoid large shifts in the pulse amplitude distribution and stay within the roughly linear region on a plot of cps/nA versus cps.  For a given X-ray line, I use as many spectrometers simultaneously as possible.
« Last Edit: March 28, 2022, 12:32:38 PM by Brian Joy »
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

sem-geologist

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If someone using EDS is looking to this thread and think "oh those WDS-dudes, we on EDS have none of these dead time problems" - I would like to curb Your enthusiasm, especially if Your machine is running Bruker/Rontec EDS SDD detectors.

I had just discovered that dead time (live time) estimations can have very huge uncertainties in case the beam is blanked before running the analysis or mapping. Long counting (>30s) is not so affected, but short counting (<1s, few hundred of ms of pixel  dwell time) i.e. for mappings will be a lot affected. Live time of EDS is counted with pulser method (0eV pulse at set frequency, if it miss - it means the counting electronics is busy). Frequency of pulser on Bruker signal processing unit is set (absolutely automatically, with no programmable access) depending from input counts which are constantly monitored and value just before launching the EDS acquisition is used for automatic setup of pulser frequency. In case it is 0cps (beam is blanked) it decides it is very low count rate and set that to (on SX100 tailored Bruker Nano XFlash) 1000Hz. In case input count rates would be near 1Mcps it would set 50000Hz (and other frequencies in between scaling to the input count rate). It does not depend from selected throughput. For mapping, where pixel dwell time is few hundred ms it will produce severely shortened live time and net counts from such short dwell time maps will be not comparable with long EDS measurements of standard material.

The fix is to add 2s waiting time before the start of acquisition (beam-on), so that SPU would get adequate estimation of input count rate and would select adequate zero peak frequency. There is caveat. if mapping starts in the epoxy (left-top corner) and beam would wait those 2 s there, SPU would get quite underrated count rate and would select underestimated 0eV peak frequency.
« Last Edit: April 12, 2022, 09:44:36 AM by John Donovan »

Probeman

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... dead time (live time) estimations can have very huge uncertainties in case the beam is blanked before running the analysis or mapping...

This is exactly correct.

We have noticed this also on the Thermo EDS (NSS or Pathfinder) systems as well:

https://probesoftware.com/smf/index.php?topic=939.msg8735#msg8735

One solution is to select a fixed dead time constant for the system (do not use the Auto dead time mode) based on a typical spot on the sample, or as suggested, allow the system to "equilibrate" by adding a delay of several seconds after the cup is removed but before the EDS counting starts.
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