New method for calibration of dead times (and picoammeter)

[John Donovan]

With regards to Probeman's post above we've updated our Constant K-Ratio method document which is attached below (remember to log in to see attachments), to emphasize the importance of constructing the k-ratio with both the primary and secondary standards being measured at the same beam current for each beam current measurement.

[Probeman]

We are pleased to announce the publication of our new paper on improving dead time corrections in WDS EPMA:

John J Donovan and others, A New Method for Dead Time Calibration and a New Expression for Correction of WDS Intensities for Microanalysis, Microscopy and Microanalysis, 2023

https://academic.oup.com/mam/advance-article-abstract/doi/10.1093/micmic/ozad050/7165464

How to get the "best" dead time constant which produces the "best" zero slope "k-ratio v. current" line in your constant k-ratio method? I don't have the PROBESOFTWARE, so can I still perform dead time calibration by applying the constant k-ratio method?

Yes, you can perform the dead time calibration using the constant k-ratio method for general use as described here:

https://smf.probesoftware.com/index.php?topic=1466.msg11102#msg11102

But if you do not have the Probe for EPMA software, you will not be able to take advantage of the new non-linear expressions described in our paper for count rates above 30 to 50 kcps. In other words, you could perform the dead time calibration using the constant k-ratio method using the traditional linear expression, but you probably won't see a huge difference in the dead time value you obtain below 30 to 50 kcps.  Above those count rates, the tradition linear expression fails as shown in the linked post below.

That said, I think the constant k-ratio method is much easier to use and more precise for dead time calibrations whatever dead time expression is being used...

See here also:

https://smf.probesoftware.com/index.php?topic=1466.msg11173#msg11173

I like to know how the ideal dead time constant is produced. In the example of "TiO2/Ti"  shown in your post, when you found the DT constant (1.32 us) was too high, you dropped it  to 1.28 us and got a more "flat" line. So my question is why it is 1.28, but not 1.27 or 1.29. How is the exact number produced, using some algorithm or by repeated iterate and trial? If the later is the case, how do you determine the observed "k-ratio v. current" line， which depends on the exact DT constant, is the best?

It is easy. One simply adjusts the dead time constant (depending on which dead time expression is being utilized (because the dead time constant is actually a *parametric* constant as described in the paper) in order to obtain a trend with a slope close to zero.

Therefore, the goal is to obtain a dead time constant (with a suitable dead time expression) that yields a flat (zero slope) response from low count rates to the highest possible count rates.  One can do this visually by inspection because a zero slope is easy to evaluate.

Obviously at some point (above several hundred kcps) the dead time correction becomes very large and quite sensitive to small changes in the dead time constant.  But we found the logarithmic dead time expression can yield quantitative results from zero to several hundred kcps count rates once the dead time value was properly adjusted.

Don't forget, as described in the paper one can also utilize the same constant k-ratio dataset to check their picoammeter linearity and also the agreement of (simultaneous) k-ratios from one spectrometer to another (and even from one instrument to another given the same materials), which can be used to check one's effective take-off angle for each (WDS and EDS) spectrometer.

It should be pointed out that one can switch simply to the logarithmic WDS dead time correction method and keep using the dead time constant that was originally obtained using a linear extrapolation (limited to 30 to 40 kcps throughput):



Why do I say this?

Because even though you might be slightly over estimating your dead time at very high throughput (> 50 kcps) using the logarithmic expression, your accuracy will still be significantly better than using the old (traditional) linear dead time correction method:



And also, below 50 kcps the linear and logarithmic expressions give essentially identical results.