This is a post on recent "constant" k-ratio measurements and some strange observations regarding the results.
Now that I think I've finally learned to properly adjust my PHA gain settings to work at count rates from zero to ~600 kcps, I've been noticing some consistently odd non-linearities in the constant k-ratio results. These are quite small variations but they seem to be reproducibly present on my Cameca instrument, though maybe not on Anette's JEOL instrument. But maybe that is just a question of the larger dead time constant on the Cameca instrument?
Beginning this story with Anette's instrument here are some TiO2/Ti metal k-ratios on her PETL spectrometer:
Take a look at the Y-axis range and you can see that we are seeing consistent accuracy in the TiO2/Ti k-ratios from 15 kcps to 165 kcps count rates of Ti ka in TiO2 and count rates from 28 kcps to 392 kcps in Ti metal (10 nA to 140 nA) within roughly a thousand or so PPM. Pretty darn good!
This very nicely demonstrates the sensitivity of the constant k-ratio method because the Y-axis can be expanded indefinitely as the slope of the k-ratios approaches zero (as they should in an well calibrated instrument!). Her JEOL data was taken at 15 kV. Now here are some Ti Ka k-ratio data from my Cameca at 20 kV:
First note that the count rates are almost the same (at 20 keV) as Anette's JEOL instrument at 15 keV. Next note that the k-ratio variation in the Cameca Y-axis range is larger than Anette's instrument though still within a percent or so. But that's still a pretty significant variation in the k-ratios as a function of count rate. So the question is, why is it so "squiggly" on the Cameca instrument? Though I should add that if we look really closely at Anette's JEOL data, there is an almost imperceptible "squiggle" to her data as well... though seemingly smaller by about a factor of 10. So what is causing these "squiggles" in the constant k-ratios?
And also note the fact that at 140 nA, the k-ratios are starting to "head north" is simply because at that beam current the count rate on the Ti metal is approaching 600 kcps! And on the Cameca with a 2.6 usec dead time constant, the logarithmic dead time correction is around 200% and really just can't keep up any more!
But more interestingly (and also incomprehensibly) to me is that these "squiggles" appear on all the spectrometers, even those with lower count rates as seen here:
So that might indicate to me that maybe these squiggles are due to a picoammeter non-linearity, but if you've been following along with these discussions you will remember that when using the constant k-ratio method, we measure both the primary and secondary standard at the *same* beam current. Therefore any picoammeter linearity should normalize out. And in fact the picoammeter non-linearity on my instrument is much worse than these k-ratio data show, as previously plotted here:
https://probesoftware.com/smf/index.php?topic=1466.msg11324#msg11324So I don't think it's the picoammeter. Now, it is worth pointing out that using a traditional dead time calibration one would never see such tiny variations in the data. To demonstrate this, here are the same Cameca k-ratio data as above, but this time plotted using the traditional linear dead time expression:
These k-ratio variations are even less evident in a traditional plot of a single material intensity vs. beam current as seen here:
The PHA data is here, first adjusted at 200 nA to ensure that the Ti Ka escape peak is above the baseline:
and here at 30 nA:
Remember, in integral mode, the counts to the right of the 5v X-axis are counted in the integration as shown previously:
https://probesoftware.com/smf/index.php?topic=1475.msg11356#msg11356 And are
not cut off as we might expect, given the display in the PeakSight software. And by the way, Anette has sent me some preliminary "gain test" data from her JEOL, and even though she had to deal with a shifting baseline, she also sees a constant intensity as a function of gain. She will post that data here soon I hope.
In the mean time does anyone have any theories on what could be causing these "squiggles" in the constant k-ratio data on my Cameca? And why are they so much more pronounced than on Anette's JEOL instrument?