Author Topic: New method for calibration of dead times (and picoammeter)  (Read 4025 times)

jlmaner87

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Re: New method for calibration of dead times (and picoammeter)
« Reply #90 on: September 08, 2022, 03:07:57 PM »
Here are some k-ratio measurement I performed on my Cameca SXFive-Tactis.

Background-corrected count rates (not corrected for dead time) are ~11 kcps on the 4 large crystals and ~ 3 kcps on the standard crystal (sp4) at 4 nA. Count rates are ~185 kcps and 113 kcps at 250 nA for large and standard crystals, respectively.

Traditional DT expression seems to work well up to 50 nA (100 kcps or 34 kcps for large and standard crystals, respectively). Logarithmic expression works well up to at least 150 nA (168 kcps on large crystals), if not higher, especially for sp4 standard size PET crystal.

Additional details are provided on the attached images.


Probeman

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Re: New method for calibration of dead times (and picoammeter)
« Reply #91 on: September 09, 2022, 11:49:14 AM »
James,
This is fantastic data, and congrats on an excellently calibrated instrument.  I love seeing those simultaneous k-ratios all agreeing with each other! 

Hey, did you by any chance acquire PHA scans at both ends of your beam current range? 

The more I think about it, the more that I think that at least some of the deviation from constant k-ratios that we are seeing is due to the tuning of the PHA settings. We really need to make sure that our PHA distributions are above the baselines at both the low count rate/beam current and at the highest count rate/beam current.

Here's an example. When I ran some of my Ti Ka k-ratios on TiO2 and Ti metal, I checked the PHA distributions at both ends of the acquisition, first at 10 nA:



and then at 200 nA:



This is really important to check especially as we get to these high count rates. 
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Probeman

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Re: New method for calibration of dead times (and picoammeter)
« Reply #92 on: September 09, 2022, 12:03:09 PM »
A new feature that is worth taking advantage of in Probe for EPMA is to plot/export the raw on-peak counts on the x axis rather than the beam current. This is a new plot item found in the Output Standard and Unknown XY Plots menu dialog as seen here:



Now when plotting/exporting the raw k-ratios for the secondary standard (the primary standard k-ratio will always be 1.000!), the program will plot/export the raw on-peak counts for the secondary standard.  But it's the count rate on the primary standard that we really care about since that will generally be a higher concentration/count rate.  And therefore be more sensitive to the dead time correction. And of course since the primary standard intensity is in the denominator of the k-ratio, when it loses counts faster (at higher count rates), the k-ratio values will trend up!

So we need to export twice. First to select all the primary standards and export the raw on peak intensities for the primary standards, then select all the secondary standards and export all the k-ratios for the secondary standards.
 
We then combine the raw on peak counts from the primary standards with the k-ratios from the secondary standards and then we can obtain a plot like the following:

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Probeman

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Re: New method for calibration of dead times (and picoammeter)
« Reply #93 on: September 19, 2022, 12:39:54 PM »
I want to look at these PHA settings more closely because I think that some of what we are seeing, when performing these constant k-ratio measurements, is due to PHA peak shifting at high beam currents (count rates).

These effects will be different on Cameca and JEOL instruments obviously, so please feel free to share your own PHA scans at low and high count rates so we can try and learn more. This is of course complicated by the fact that on Cameca instruments, we tend to leave the bias fixed at a specific voltage (~1320v for low pressure flow detectors and ~1850v for high pressure flow detectors) and then simply adjust the PHA gain setting to position the PHA peak (normally around 2 to 2.5 v in the Cameca 0 to 5 v PHA range), but for the constant k-ratio method we want to instead position the peak to slightly *above* the center of the PHA range (at low beam currents) to avoid peak shifting from pulse height depression (at higher beam currents), so centered roughly around 3 volts or so.
 
Here for example is Mn Ka on Spc2, LPET, a low pressure flow detector at 30 nA:
 


Note that the peak is roughly centered around 3 volts. Now using the same bias voltage of 1320v here is the same peak at 200 nA:



Please note that the gain is *exactly* the same for both the 30nA and the 200 nA scans!   This is really good news because it means that we don't need to adjust the PHA settings as we go to higher count rates.

But the PHA peak at 200 nA has certainly broadened and shifted down slightly to 2.5 volts or so (which is why we set it a little to the right of the center of the PHA range to begin with!), probably due to pulse height depression.  Note that even though it has broadened out, because we are in integral mode, we don't have to worry about cutting off the higher side of the PHA peak.  The important thing is to keep the peak (including the escape peak!), above the baseline cutoff.

How about a high pressure flow detector? This is a PHA scan on Spc3 LLIF which is a high pressure flow detector, first at 30 nA:



and again at 200 nA using the same (1850v)  bias voltage:



Again, the gain setting is the same, and very little change in the PHA peak (though it is again, slightly shifted down and broadened). Now admittedly we are getting a somewhat less count rate on the LLIF crystal than the LPET, so I do want to try this again on Spc3 LPET, but still very promising.

Again the take away point: check your PHA distributions at both the lowest and highest count rates to be sure you are not cutting off any emission counts when performing the constant k-ratio method.

On JEOL instruments that is an entirely different story because usually the gain is fixed and the bias voltage is adjusted. Question is: can we keep the JEOL PHA distributions above the baseline as we get to higher count rates using a single pair of bias and gain values?  Anette's initial data suggests, no we can't:



I should mention that this PHA shift effect (more pronounced on the higher concentration Si metal primary standard), would tend to produce a constant k-ratio trend as we see in this post, because the primary standard is in the denominator (and as the primary intensity decreases, the k-ratio tends to increase):

https://probesoftware.com/smf/index.php?topic=1489.msg11230#msg11230

Can we see some more JEOL PHA data at low and high count rates for both P-10 and Xenon detectors?
« Last Edit: September 19, 2022, 02:50:52 PM by Probeman »
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Probeman

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Re: New method for calibration of dead times (and picoammeter)
« Reply #94 on: September 26, 2022, 09:41:51 AM »
Here are the constant k-ratios from Anette's most recent run, first for the TAP spectrometer:



When going from the logarithmic to exponential expression we clearly need to reduce the dead time constant from 1.26 usec to 1.18 usec.  Interesting that the predicted count rates are slightly different for these two models at these two slightly different parametric constants in the middle of the count rate range.

Now for the TAPL crystal (beware it ain't pretty):



The logarithmic expression does a pretty good job (at least up until around 450 kcps) but the exponential expression loses it completely as the product exceeds 1/e (so no dead time correction can be applied at higher count rates).
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