New method for calibration of dead times (and picoammeter)

[jlmaner87]

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]

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. 

[Probeman]

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:

[Probeman]

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?

[Probeman]

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).

[Probeman]

This was a test of our (new) WDS board on our SX100 that appears to have been installed, along with PeakSight 6.1, in 2015.

Please note that this is a very unusual dead time test because instead of using the default "integer" (enforced) dead time values of 3 usec, we instead wanted to see if the NEW WDS board improved the "intrinsic" dead times of the electronics compared to the OLD PHA board from the test performed in 2010. This "intrinsic" dead time test is performed by setting the "integer" (enforced) dead time values of the Cameca spectrometers to the lowest value possible which is 1 usec on the SX100 and SXFive instruments. And then seeing what the resulting dead times actually are.

Here are the "intrinsic" dead time values we obtained in 2010 (using the traditional linear dead time calibration method - Carpenter's Excel spreadsheet) using Ti Ka (running PET or LPET on all spectrometers):

Spc 1: 1.71 usec
Spc 2: 2.00 usec
Spc 3: 2.19 usec
Spc 4: 1.61 usec
Spc 5: 1.85 usec

The above test from 2010 was run up to 180 nA and even the LPET crystals produced no more than 230 kcps running at 15 keV.  Unfortunately in the new test I did yesterday, I ran at 20 keV (which increased the count rate significantly) and also I did not utilize all PET/LPET crystals (I should have checked the setup from 2010 first!). Still worth showing the data I think...

For this test I edited the Camca "integer" (enforced) dead time values in the SCALERS.DAT file in the Probe for EPMA ProgramData folder. These values are found on line 35 of the SCALERS.DAT file. They can be edited using any text editor such as NotePad.

Please also note that if either of the following keywords in the Probewin.ini file are set to non-zero values, Probe for EPMA will *not* set the spectrometers to the PHA values in the SCALERS.DAT file on startup:

UseCurrentConditionsOnStartUp=0   ; non-zero = read current instrument condition on software start
UseCurrentConditionsAlways=0   ; non-zero = read current instrument conditions on each acquisition

Here is the full setup that I ran yesterday:

On and Off Peak Positions:
ELEM:    ti ka   ti ka   ti ka   ti ka   ti ka
ONPEAK 31402.0 31504.0 68259.0 31456.0 68269.0
OFFSET 28.0293 -73.971 32.4297 -25.971 22.4297
HIPEAK 32898.5 32925.9 69214.5 33026.1 69097.5
LOPEAK 30052.3 29843.1 67323.8 30103.9 67509.0
HI-OFF 1496.50 1421.90 955.500 1570.10 828.445
LO-OFF -1349.7 -1660.9 -935.20 -1352.1 -760.00

PHA Parameters:
ELEM:    ti ka   ti ka   ti ka   ti ka   ti ka
DEAD:     2.85    2.80    2.80    3.00    3.00
BASE:      .29     .29     .29     .29     .29
WINDOW    4.50    4.50    4.50    4.50    4.50
MODE:     INTE    INTE    INTE    INTE    INTE
GAIN:     942.    864.   1369.    818.    864.
BIAS:    1320.   1320.   1850.   1320.   1850.

Last (Current) On and Off Peak Count Times:
ELEM:    ti ka   ti ka   ti ka   ti ka   ti ka
BGD:       OFF     OFF     OFF     OFF     OFF
BGDS:      EXP     EXP     LIN     EXP     LIN
SPEC:        1       2       3       4       5
CRYST:     PET    LPET    LLIF     PET     LIF
ORDER:       1       1       1       1       1
ONTIM:   60.00   60.00   60.00   60.00   60.00
HITIM:   10.00   10.00   10.00   10.00   10.00
LOTIM:   10.00   10.00   10.00   10.00   10.00

I then automated a constant k-ratio acquisition for Ti metal and TiO2 at 10, 20, 30 , 60, 80, 120, 160 and 200 nA.  Let's look at the PHA scans first.  Here are the 10 nA PHA scans:



Notice that the LPET shows the highest count rate as expected. Also note that I've adjusted the PHA gains so that the PHA peaks are all around 3 volts in the 0 to 5 volt range of the Cameca PHAs. This is done because at the higher count rates, the PHA peaks will shift to the left due to pulse height height depression.

And here are the PHA scans at 200 nA (at the same bias and gain settings as the 10 nA PHA scans):



First of all I note that the high count rate intensities for all the crystals (other than the LIF) all seem to be about the same intensity.  I think this is due to an artifact of the PHA scan method which utilizes 8 bit MCA channels, so what happens in PFE is it just stops once one of the MCA bins gets full, so the high count rate spectrometers all get "normalized" to roughly the same count rate.

But more importantly note that all the peaks have shifted to the left, but not by so much that they are getting cut off by the baseline (hence the reason for setting the PHA peaks to the right of center at 10 nA to begin with).

So this means that our constant k-ratios should all be good to acquire from 10 nA to 200 nA.  I will show those in the next post.

Edit by Probeman: as SEM Geologist correctly points out below the escape peak in the spec 2 LPET PHA scan at 200 nA *is* getting cut off by the baseline level!

[sem-geologist]

But more importantly note that all the peaks have shifted to the left, but not by so much that they are getting cut off by the baseline (hence the reason for setting the PHA peaks to the right of center at 10 nA to begin with).

I am going to be a bit picky: I see something different - the Ar esc peak got out of PHA, that is about ~5% of counts (in particularly for 2nd spectrometer with Large PET).

[Probeman]

But more importantly note that all the peaks have shifted to the left, but not by so much that they are getting cut off by the baseline (hence the reason for setting the PHA peaks to the right of center at 10 nA to begin with).

I am going to be a bit picky: I see something different - the Ar esc peak got out of PHA, that is about ~5% of counts (in particularly for 2nd spectrometer with Large PET).

Yeah, OK I see that. That's probably why that spectrometer's k-ratio got really crazy at the highest beam currents.

[Probeman]

But more importantly note that all the peaks have shifted to the left, but not by so much that they are getting cut off by the baseline (hence the reason for setting the PHA peaks to the right of center at 10 nA to begin with).

I am going to be a bit picky: I see something different - the Ar esc peak got out of PHA, that is about ~5% of counts (in particularly for 2nd spectrometer with Large PET).

Yeah, OK I see that. That's probably why that spectrometer's k-ratio got really crazy at the highest beam currents.

I have to say that am very pleased that SEM Geologist pointed out the loss (due to pulse height depression) of the Ar esc peak in the PHA scans at 200 nA compared to 10 nA on the LPET spectrometer 2!  It's not as large an effect as it might be (he estimated around 5%), but it certainly exacerbates the loss of intensities as measured on the Ti metal primary standard. See the cyan line in the 10 nA and 200 nA PHA scans in this post and compare them:

https://probesoftware.com/smf/index.php?topic=1466.msg11309#msg11309

I myself should point out that both the 10 nA and the 200 nA plots in the link above were acquired using the normal 3 usec enforced dead time on the SX100, so once again here are the 200 nA PHA scans at 200 nA, where one can see the Ar escape peak has shifted below the baseline (see cyan line for the LPET spectrometer):



This really makes a great point about how important it is to check our PHA peaks when operating at these very high count rates!

But before I started the constant k-ratio acquisitions, I thought to myself: maybe I should re-acquire the PHA scans using the 1 usec (integer) enforced dead times... and so I did:



When I first glanced at these acquisitions as they were acquired one by one, I merely thought to myself, well they look pretty much the same as the 3 usec enforced dead times.  And in terms of there shapes they are almost exactly the same, but it wasn't until I plotted them all together that I noticed the Y-axis had changed significantly!

Note that the PET crystal spectrometers are almost 3 times the count rate as when I used the 3 usec enforced deadtimes.  That makes sense of course.  Because we expect that the intrinsic dead times will be lower using a lower enforced dead time!  The question is, will these intrinsic dead times be significantly lower using the new SX100 WDS electronics board, compared to the values we obtained in 2010 when we used the old WDS electronics board.

I'll discuss that in the next post, but in the meantime here is the k-ratio plot from the Spc 2 LPET spectrometer where we see some interesting effects given its "terrifying" count rate of 632 kcps on Ti metal at 200 nA (extrapolating from 10 nA):



The decrease in the k-ratio for the 80 and 120 nA acquisitions is due to the fact that these k-ratio intensities were corrected using the original 2.80 usec software dead time correction calibrated using a 3 usec enforced dead time. Therefore we will need to decrease these software dead time constants as described in the next few posts since we utilized an enforced dead time of 1 usec, but how much will we need to decrease them to obtain a *constant* k-ratio!   

[sem-geologist]

Very interesting,
I rather would argue that this steep shot of k-ratios up are due to Ar esc AND the part of normal counts being cut out, and that shallow decrease is from too large dead time constant used for count recalculation.
Your PHA graphs and k-ratio observations makes me question: Are the lower Voltage pulses rejected by PHA or is it rejected earlier in the pipeline by Pulse-hold chips low skew rate (the rate how fast Voltage can drop at output of pulse hold chip back to 0V (or below that value) after holding the voltage for A/D conversion) - the lag of that chip will make Comparator-PulseHold chip tandem to miss the pulse even in integral mode - the PHA would not get the pulse for rejection/acceptance by baseline of PHA (if that is taking place at all on Cameca hardware in integral mode). I think We are tricked to believe that whole distribution (except Ar esc peak) is preserved with PHA shift, as left slope of PHA distribution is not sharp vertical line, but we see smooth inclined curve. If PHA would be cutting out the distribution by its baseline - we should see sharp precise cutoff at that value. However mentioned "cutout" of "shifted to low V" pulses by lag of pulse-hold low skew rate would make such cut-off line inclined and curved as would depend from pulse height before this "cutout/missed" low V pulse.

Things to check (I would like to check on my own): skew rate of that chip is fixed, but by decreasing the gas and analog gain (thus average pulse height coming into comparator---pulse-hold chip tandem) there should be less missed pulses - that would look absolutely counter intuitive measure for PHA shift.

[Probeman]

I rather would argue that this steep shot of k-ratios up are due to Ar esc AND the part of normal counts being cut out, and that shallow decrease is from too large dead time constant used for count recalculation.

Yes, I agree, though not sure about losing any normal counts as the spec 2 intensities looks to get very close to zero just above the baseline in the 200 nA PHA scan.

You are correct though, when the primary intensity loses counts either from a too small dead time constant or an escape peak shifting out of range, the k-ratio will increase. While a too large dead time constant will decrease the k-ratio assuming the primary standard has the larger concentration...

By the way, the 160 and 200 nA intensities on spec 2 cannot be corrected using the logarithmic dead time correction, but at 120 nA the dead time correction for spec 2 is almost 1800%!   

On-Peak (off-peak corrected) or EDS (bgd corrected) or MAN On-Peak X-ray Counts (cps/1nA) (and Faraday/Absorbed Currents):
ELEM:    ti ka   ti ka   ti ka   ti ka   ti ka   BEAM1   BEAM2
BGD:       OFF     OFF     OFF     OFF     OFF
SPEC:        1       2       3       4       5
CRYST:     PET    LPET    LLIF     PET     LIF
ORDER:       1       1       1       1       1
   61G 1671.9931348.41  772.79 1373.29  169.11 124.210 124.180
   62G 1674.0632404.42  774.92 1375.77  169.16 124.195 124.195
   63G 1675.7632614.40  774.65 1373.35  169.33 124.210 124.210
   64G 1673.5032133.87  775.21 1373.92  169.06 124.210 124.210
   65G 1674.0432615.82  776.04 1373.10  169.09 124.226 124.180
   66G 1679.0833348.09  778.36 1374.96  169.22 124.210 124.195

AVER:  1674.7432410.83  775.33 1374.07  169.16 124.210 124.195
SDEV:     2.45  658.39    1.83    1.08     .10    .010    .014
1SIG:      .36     .49     .28     .34     .15
SIGR:     6.78 1355.15    6.47    3.18     .68
SERR:     1.00  268.79     .75     .44     .04
%RSD:      .15    2.03     .24     .08     .06
DEAD:     2.85    2.80    2.80    3.00    3.00
DTC%:     72.8  1742.7    30.3    61.7     6.5

Dang, now that's a big dead time correction!

[Probeman]

So ignoring spec 2 LPET for the time being let's plot up the constant k-ratios with the enforced dead time set to 1 usec and still using the optimized dead time constants from when the enforced dead times were all 3 usec:



After adjusting the dead time constants for obtaining a zero slope k-ratio fit we obtain the following plot:



OK, so let's now compare the dead times for the 1 usec enforced dead time calibration from 2010 (using the old WDS board electronics) with this recent 1 usec enforced dead time calibration (using the new WDS board electronics):
                   
                             1         2          3          4          5
1 usec (2010)    1.71    2.00     2.19     1.61     1.85      <--- OLD WDS board
1 usec (2022)    1.58    1.60     1.80     1.65     2.00      <--- NEW WDS board

OK, well that's a pretty mixed bag!  Specs 1, 2 and 3 went down in dead time, while 4 and 5 went up in dead time with both measurements at 1 usec enforced dead time.

But since all the values are 2 usec or less, I think my next task is to re-run the constant k-ratio calibration, this time using a 2 usec enforced dead time, and see what values we obtain...

[sem-geologist]

OK, so let's now compare the dead times for the 1 usec enforced dead time calibration from 2010 (using the old WDS board electronics) with this recent 1 usec enforced dead time calibration (using the new WDS board electronics):
                 
                             1         2          3          4          5
1 usec (2010)    1.71    2.00     2.19     1.61     1.85      <--- OLD WDS board
1 usec (2022)    1.58    1.60     1.80     1.65     2.00      <--- NEW WDS board

OK, well that's a pretty mixed bag!  Specs 1, 2 and 3 went down in dead time, while 4 and 5 went up in dead time with both measurements at 1 usec enforced dead time.

But since all the values are 2 usec or less, I think my next task is to re-run the constant k-ratio calibration, this time using a 2 usec enforced dead time, and see what values we obtain...

This mixed bag have its very clear explanation.

Old electronics has two analog signal multiplexers and 2 ADC, where spect 1,2,3 used one, and spect 3 and 4 used the second multiplexer and ADC. Both ADC sent its results in parallel; thus Your old dead times are very logic: spect 1 to 3 raises up, and then spect 4 and spect 5. Spect 4 had lower dead time as it needed then to share the ADC only with 1 other detector, and spect 1 compete with 2 spectrometers. Probably there was some prioritization if pulses comes to multiplexer from few spectrometers at the same time, and spectrometer with low number is then prioritized. (thus dt of 1st spect < 2nd spect < 3rd spect; and 4th < 5th spect).

With new board all spectrometers have its own ADC. However ADC results are sent with shared 8bit bus, and thus multiplexed in that digital bus. Because of multiplexing there should be prioritization if few ADC wants to send the result to FPGA at the same time, and thus again highest priority has lowest spectrometer number, and lowest priority has the spectrometer with the largest number. But wait, someone would say that 3rd and 4th spectrometers does not fit this scheme! - There is something strange with 3rd and 4th, because on our SXFive (with 5 spectrometers) these are switched in places in few places on the pipeline which is confusing. It is highly possible that FPGA see 3rd and 4th spectrometer in different order and thus prioritize the 4th before 3rd. So in this mixed bag the 5th spectrometer looks in disadvantage on new board. I am a bit surprised with these results, I was expecting all dead times better than before. Maybe multiplexing of that bus is not so much faster than I had anticipated...

Instead of going to 2us integer dead time test, I would propose you to redo same test but only with a single spectrometer (setting other spectrometer gain to 0 so that it would produce no counts) and see if dead times would not improve significantly. Especially for 5th spectrometer.

P.S. a quick test on our SXFive setting 975nA and Ti Ka on TiO2 and LPET on 2nd and 5th spect shows no influence in count rate when switching on/off one or the other - thus this prioritizing thing probably is not here. BTW max seen raw counts on both spectrometers at these insane current are 322.9kcps! maybe it needs more spectrometers to saturate the bus...

[Probeman]

Instead of going to 2us integer dead time test, I would propose you to redo same test but only with a single spectrometer (setting other spectrometer gain to 0 so that it would produce no counts) and see if dead times would not improve significantly. Especially for 5th spectrometer.

Sounds like an interesting test.

I suggest that you run some multiple (all 5) and single spectrometer constant k-ratio tests at 1 usec, and I'll run multiple and single spectrometer k-ratio tests at 2 usec and we can compare.

[Probeman]

I was not able to run the constant k-ratio test at 2 usec enforced dead time last weekend because they shut down all the chillers on Sunday because the process water was being turned off on Monday morning, but I will try to get to that hopefully this weekend.

In the mean time I took another look at the 1 usec enforced dead time measurements I did the weekend before (see above posts), and because I finally also remembered to run the primary standard before each secondary standard, the standard intensity drift correction in PFE did not confuse things (once it was turned off in the Analysis Options dialog). 
 
So here is the picoammeter linearity test produced from the same constant k-ratio data set shown above, but using only a single primary standard, which produces a nice check for the picoammeter calibration (since we are extrapolating to each different beam current). As I suspected from previous datasets we definitely have a small picoammeter calibration issue on our SX100:



This means that when running say major elements at 30 nA, and then minor/trace elements at 60 nA, there will be a ~3% accuracy error between the two sets of element intensities. A bit more if we consider the matrix corrections.  Hopefully this will get fixed once our instrument engineer gets our high accuracy current source built (he's been swamped with instrument problems throughout the facility because of power outages from all the construction next door!).

Have you run a constant k-ratio check on your instrument?  What are you waiting for?      If you have Probe for EPMA it's super easy with the latest version. See the PDF for a nice description of the acquisition process available from the Help menu (it only takes a couple hours of automation!). For those without PFE, see this post and the next few:

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