Probe Software Users Forum
Hardware => Cameca => Topic started by: lucaSX50 on August 13, 2013, 12:12:45 PM
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Edit by John: it should be pointed out that the procedure for setting the deadtime constants in Probe for EPMA (editing the SCALERS.DAT file) is the same for the Cameca SX50/SX51//SX100 and SXFive.
Dear All,
We are trying to optimize the dead times for our detectors to work with high currents (300 nA). It turns out that the calculations suggest dead times like 2.3 ms. If I set a time like that in the cameca software, the value defaults to 2 (this is the dead time we are using for all our detectors). If I set the dead time from the SXLocal using: sacq sp4 dtim 2.3 the value is accepted (I receive no error message). However there's not a display of this value anywhere. The graphic software still shows 2 as dead time. Same thing if I use disp from the SXLocal.
Is this just a display limitation or it's actually the hardware that does not support those values?
Is it actually possible to set dead times with fractions of milliseconds in the SX50?
Also ... is the dead time correction something built into the cameca software/hardware but simply not accessible for modifications (i.e. like turning it on/off or selecting when to apply it. Similar to what is possible with Probe for EPMA)?
I am new at this (as you can imagine) so ANY suggestion/explanation/advice will be greatly appreciated.
Thanks.
Luca
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Hi Luca,
It is quite confusing.
There is a philosophical difference in the way the Cameca and JEOL deadtime electronics works. In the JEOL the measured deadtime is just that. What one measures. It includes a contribution from the detector gas amplification and the pulse shaping electronics. However, it is NOT a constant. As Paul Carpenter has demonstrated and I have confirmed, the deadtime is affected by a number of factors including the bias voltage and the incident x-ray energy.
Because these effects can contribute to a "variable" deadtime constant, Cameca microprobes incorporate a circuit for masking the variation in the "intrinsic" deadtime to a moderate but known (and constant) deadtime value which is set in the hardware.
The intrinsic deadtime of the Cameca electronics and detectors is somewhat larger than the JEOL and is about 1 to 2 microsecs and one can set it to zero and effectively measure only the "intrinsic" deadtime.
On the SX100 the software will not allow one to set the masking deadtime to zero and so one can not directly measure the intrinsic deadtime. However, I set the the "masking" or "forced" deadtime to 1 microsec and it gave 1.03, 1.38, 1.44, 0.91, and 1.13 microsecs for the 5 spectrometers, using Ca Ka when measured using Paul Carpenter's Excel sheet for calibrating your deadtime. See attached documents. One for measurement and one for calculations.
So on the Sx100 instrument the intrinsic deadtime is probably around 1 microsec but when set the "masking" or "forced" deadtime (DTIM parameter) is set to 3 microseconds and then measured, the ACTUAL measured pulse widths should be used for the software correction.
The beauty of this method is that although the underlying intrinsic deadtime may change due to changes in bias voltage and x-ray energy, the measured pulse width (and hence actual deadtime) never varies.
So on the Cameca there are three deadtimes:
1. The intrinsic or native deadtime which can vary with detector bias and x-ray energy.
2. The "forced" or "masking" deadtime used to mask the variation and is set to an integer value by the hardware (DTIM on the SX50). In Probe for EPMA, one sets the Cameca "forced" or "masking" deadtime in the SCALERS.DAT file on line 35.
3. The actual measured deadtime of the integer "forced" or "masking" DTIM value which used for the deadtime correction in software for each spectrometer and crystal combination on lines 72-77 of the SCALERS.DAT config file.
So in summary:
One should determine the intrinsic deadtime with the SX50 DTIM set to 0 us (or the Sx100 set to 1 us). Then one needs to set the imposed deadtime to some larger value (to have it function as a constant mask as originally intended by Cameca) and then, and only then...
measure the actual deadtime at that imposed integer deadtime using the classical method of increasing beam current, for use in the software correction.
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The deadtime spreadsheet in the previous post is used for automated acquisition of the data set though the Probe Software supplied Remote Automation software interface. It acquires 5 count cycles of 60 seconds over a range of beam currents.
That intensity data is then pasted into the deadtime calculation documents which are attached here.
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From the PFE User Reference manual:
Line 35 (Cameca integer deadtimes)
0 0 0 0 0 0 0 0 "Cameca integer deadtimes"
This line is used to explicitly specify the integer deadtime constants used to set the Cameca PHA hardware (non-extendible or "enforced" deadtime. Therefore this data is used only by the Cameca SX100/SXFive hardware interface and the values are not accessible from within the program. These integer deadtime values are distinct from the single precision deadtimes specified on line 13 above which are used to perform the actual deadtime correction in the analysis routines.
The typical procedure is to set the deadtimes on the Cameca PHA hardware all to zero and then to measure the "intrinsic" deadtime of the system using a range of beam currents from approximately 10 to 200 nA on a pure metal x-ray line such as Si Ka (PET and TAP) or Ti Ka (PET and LIF). A sufficient counting time should be used to obtain .2% precision or better. The best method is to find the "worst case" deadtime for each spectrometer since the deadtimes may vary somewhat as a function of detector bias and x-ray line energy.
Assume that the "intrinsic" deadtimes measured (when all spectrometers are set to "DTIM"=0) are 2.23, 3.14, 3.45, 3.78 and 2.15. The next step would be to set the DTIM deadtime parameter for each spectrometer to a value large enough to completely "mask" this intrinsic" deadtime, that is values of 3, 4, 4, 4 and 3. Now since these integer deadtime are not accurately set by the Cameca hardware, the operator must now re-run the deadtime calibration measurement using these new values and note the actual deadtimes. In this case depending on the instrument, the measured deadtimes will be somewhat larger, say, 3.75, 4.65, 4.12, 4.89 and 3.32.
In the example just described, the "DTIM" values of 3, 4, 4, 4 and 3 should be entered on line 35 for setting the Cameca hardware and the measured values that correspond to them, that is, 3.75, 4.65, 4.12, 4.89 and 3.32 should be entered on lines 72-77 below to use in the software correction.
The integer "DTIM" deadtime values must be between zero and ten. If a value of zero is read, then the program will load the single precision deadtime from line 13 and truncate to integer each value for setting the Cameca PHA hardware to an approximate value.
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Luca and John,
Here is the reply I sent to the SX users list earlier today after Les Moore responded...
First, the hardware imposed deadtime is corrected using the same value for the software deadtime correction. This is, of course, fine in theory as long as, for example, the 2 us called for actually actually ends up 2us. However, we find this does not hold up, so when you enter 2us, you may actually end up with something more like 3 or 4 us actual deadtime when you measure it doing a classical deadtime test. On the SX50 with the SXRayN50 software/firmware, these two things are coupled, which is a real problem. On PeakSight for the SX100 platform, this has been de-coupled so you give it a hardware deadtime, then adjust the software correction to give good linearity. So, if you are using PfE for quantitative analysis on your SX50 (you have it there at VPI, right Luca?), then you should be able to adjust the software correction independently no matter what the hardware imposed deadtime is set to.
The second issue is with the picoammeter. No matter how well tuned it may have been, this will go out of spec at some point, and may need tuning frequently. If the current measurements are not linear, then your deadtime tests can give you a wrong apparent deadtime, or, in many cases, different apparent deadtimes depending on the current regime you measure. There are five gain loops in the picoammeter for each current range: <0.5nA, 0.5-5nA, 5-50 nA, 50-500nA, and 500-10000nA. In a perfect world, these are all linearized. In actuality, the gains and offsets tend to fall out of linearity, so you may see a completely different deadtime slope in the 5-50 range compared to the 50-500 range. Unfortunately, the 5-50 range resistors do not have trimmers (so you have to change resistors to affect the gain/slope), the others do, so they are all trimmed to match the 5-50 range. You will really see what is going on if you do your deadtime test, set the deadtime according to the full count/current range, then redo the test and plot intensity (cps/nA) vs. current. If your deadtime correction is perfect, you should be able to see a nice flat cps/nA plot for a calibration throughout the current range (of course, being sure to do this measurement on a material, like metals, that can take the high current). How big of a problem is this? If you calibrate and analyze at the same current, it's obviously not a problem at all. If you calibrate at, say 20nA, then do the analysis at 300nA to get high sensitivity, then you have a big problem if your current measurement ranges do not correspond well, especially if you intend to run major elements along with trace elements at high current. This is, of course, what we do in our facility, so we have to deal with this issue.
Mike J.
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Hi John,
ok, so I did the measurements using the spreadsheet. I used V Ka on PET LiF LiF PET. I set the Cameca SX100 deadtime to 1. Now I have some nice regression curves, with 4 different values (Mean DT All + Last and Regression DT All + Last).
What do I do with the values? How do I proceed from here?
Cheers
Ph
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ok, so I did the measurements using the spreadsheet. I used V Ka on PET LiF LiF PET. I set the Cameca SX100 deadtime to 1. Now I have some nice regression curves, with 4 different values (Mean DT All + Last and Regression DT All + Last).
What do I do with the values? How do I proceed from here?
Hi Philipp,
The procedure for editing the SCALERS.DAT file is described in this post:
http://probesoftware.com/smf/index.php?topic=394.msg2131#msg2131
and also in the Configuration Files section of the Probe for EPMA Help Reference Manual accessed from the Help menu and also in the posts above.
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Hi John,
Thanks for the reply. However, my question remains. I read all these documents, but I still have these questions. I know where and what scalers.dat is, but what to put into it? What value should I take and where exactly should I put it? I find this confusing. Do I choose that value for all? Do I choose the value for last? Fitted or measured? What would I set in PeakSight? What do I put as the Cameca hardware dead time?
It is not clear to me what to do with these values. Maybe too stupid...
Thanks!
Philipp
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Thanks for the reply. However, my question remains. I read all these documents, but I still have these questions. I know where and what scalers.dat is, but what to put into it? What value should I take and where exactly should I put it? I find this confusing. Do I choose that value for all? Do I choose the value for last? Fitted or measured? What would I set in PeakSight? What do I put as the Cameca hardware dead time?
The values that you should have in the SCALERS.DAT file on line 35 for the Cameca integer "enforced" deadtimes are whatever the hardware deadtime values were when you made your deadtime calibration measurements. The point being that the integer hardware values force the deadtime to a rough "enforced" value but then for the software correction in PFE you want to use an actual measured value.
Which deadtime value from the regressions should one use? They should be quite similar but if not that may indicate a problem with the detector and then it gets complicated. Did you read Paul's documentation on his Excel dead time spreadsheet?
I guess I'll quote from my reference manual in case the post I linked to above wasn't clear enough:
Line 35 (Cameca integer deadtimes)
0 0 0 0 0 0 0 0 "Cameca integer deadtimes"
This line is used to explicitly specify the integer deadtime constants used to set the Cameca PHA hardware (non-extendible or "enforced" deadtime. Therefore this data is used only by the Cameca SX100/SXFive hardware interface and the values are not accessible from within the program. These integer deadtime values are distinct from the single precision deadtimes specified on line 72-77 below which are used to perform the actual deadtime correction in the analysis routines.
The typical procedure is to set the deadtimes on the Cameca PHA hardware all to zero and then to measure the "intrinsic" deadtime of the system using a range of beam currents from approximately 10 to 200 nA on a pure metal x-ray line such as Si Ka (PET and TAP) or Ti Ka (PET and LIF). A sufficient counting time should be used to obtain .2% precision or better. The best method is to find the "worst case" deadtime for each spectrometer since the deadtimes may vary somewhat as a function of detector bias and x-ray line energy.
Assume that the "intrinsic" deadtimes measured (when all spectrometers are set to "DTIM"=0) are 2.23, 3.14, 3.45, 3.78 and 2.15. The next step would be to set the DTIM deadtime parameter for each spectrometer to a value large enough to completely "mask" this intrinsic" deadtime, that is values of 3, 4, 4, 4 and 3. Now since these integer deadtime are not accurately set by the Cameca hardware, the operator must now re-run the deadtime calibration measurement using these new values and note the actual deadtimes. In this case depending on the instrument, the measured deadtimes will be somewhat larger, say, 3.75, 4.65, 4.12, 4.89 and 3.32.
In the example just described, the "DTIM" values of 3, 4, 4, 4 and 3 should be entered on line 35 for setting the Cameca hardware and the measured values that correspond to them, that is, 3.75, 4.65, 4.12, 4.89 and 3.32 should be entered on line 13 above to use in the software correction.
The integer "DTIM" deadtime values must be between zero and ten. If a value of zero is read, then the program will load the single precision deadtime from line 13 and truncate to integer each value for setting the Cameca PHA hardware to an approximate value.
Extended Format SCALERS.DAT Lines 42-83
Lines 42-65 (default baseline, window, gain and bias and deadtime PHA settings for each crystal)
0. 0. 0. 0. 1. 1. .3 .45 "default PHA baseline voltages1"
0. 0. 0. 0. 1. 1. 1. 1. "default PHA baseline voltages2"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA baseline voltages3"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA baseline voltages4"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA baseline voltages5"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA baseline voltages6"
0. 0. 0. 0. 9. 9. 9.7 8. "default PHA window voltages1"
0. 0. 0. 0. 9. 9. 9. 9. "default PHA window voltages2"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA window voltages3"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA window voltages4"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA window voltages5"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA window voltages6"
0. 0. 0. 0. 32. 16. 32. 64. "default PHA gain1"
0. 0. 0. 0. 32. 32. 64. 64. "default PHA gain2"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA gain3"
0 0. 0. 0. 0. 0. 0. 0. "default PHA gain4"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA gain5"
0. 0. 0. 0. 0. 0. 0. 0. "default PHA gain6"
0. 0. 0. 0. 1674. 1764. 1750. 1650. "default detector bias1"
0. 0. 0. 0. 1750. 1800. 1800. 1700. "default detector bias2"
0. 0. 0. 0. 0. 0. 0. 0. "default detector bias3"
0. 0. 0. 0. 0. 0. 0. 0. "default detector bias4"
0. 0. 0. 0. 0. 0. 0. 0. "default detector bias5"
0. 0. 0. 0. 0. 0. 0. 0. "default detector bias6"
These lines are used to set the individual default PHA settings for the baseline, window, gain and bias on the different spectrometer and crystal basis. These crystal based parameter values replace the older default PHA settings in lines 24-27. The older values are still read and used as defaults for backward compatibility.
You only need to enter values for as many crystals as you have for each spectrometer (up to six crystals per spectrometer). The range of allowable values is the same as the PHA parameters for lines 24-27.
Lines 66-77 (default Inte/Diff mode and Deadtime PHA settings for each crystal)
0 0 0 0 0 0 0 0 "default PHA inte/diff modes1"
0 0 0 0 0 0 0 0 "default PHA inte/diff modes2"
0 0 0 0 0 0 0 0 "default PHA inte/diff modes3"
0 0 0 0 0 0 0 0 "default PHA inte/diff modes4"
0 0 0 0 0 0 0 0 "default PHA inte/diff modes5"
0 0 0 0 0 0 0 0 "default PHA inte/diff modes6"
0 0 0 0 0 0 0 0 "default detector deadtimes1"
0 0 0 0 0 0 0 0 "default detector deadtimes2"
0 0 0 0 0 0 0 0 "default detector deadtimes3"
0 0 0 0 0 0 0 0 "default detector deadtimes4"
0 0 0 0 0 0 0 0 "default detector deadtimes5"
0 0 0 0 0 0 0 0 "default detector deadtimes6"
These lines are used to set the individual default PHA settings for the inte/diff mode and deadtime on the different spectrometer and crystal basis. These crystal based deadtime parameter values replace the older default settings in line 13.
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Hi John,
thanks for the explanations. It is a bit clearer now. But there is something going wrong. Ok, as described: I used V Ka on PET LiF LiF PET. I set the Cameca SX100 deadtime to 1.
I got some dead time values out of the spreadsheet: 4.06 2.78 2.29 3.06 (all four values are pretty much the same for any one spectrometer, that seems right).
I then continued (as described in your text) and set the Cameca dead time to 5 3 3 4 and measured everything again. And now something is weird, because I get for
SP1 fit all 7.87 and fit last 6.78
SP2 fit all 6.97 and fit last 1.56
SP3 fit all 6.33 and fit last 1.64
SP4 fit all 6.07 and fit last 4.82
There is something weird going on here, right? What am I doing wrong?
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thanks for the explanations. It is a bit clearer now. But there is something going wrong. Ok, as described: I used V Ka on PET LiF LiF PET. I set the Cameca SX100 deadtime to 1.
I got some dead time values out of the spreadsheet: 4.06 2.78 2.29 3.06 (all four values are pretty much the same for any one spectrometer, that seems right).
I then continued (as described in your text) and set the Cameca dead time to 5 3 3 4 and measured everything again. And now something is weird, because I get for
SP1 fit all 7.87 and fit last 6.78
SP2 fit all 6.97 and fit last 1.56
SP1 fit all 6.33 and fit last 1.64
SP1 fit all 6.07 and fit last 4.82
There is something weird going on here, right? What am I doing wrong?
You are doing nothing wrong. Your detectors might be a little dirty is all.
Set the integer deadtimes to 5 3 3 4 and the software values to the deadtimes you measured.
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How can one explain the huge difference in SP2 and SP3 fit all and fit last? Isn't that monster big?
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How can one explain the huge difference in SP2 and SP3 fit all and fit last? Isn't that monster big?
I would ask Paul Carpenter. After all, his name is "Paul *deadtime* Carpenter" and it is his Excel macro!
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Hi Philipp,
In the meantime it is worth mentioning that one cannot set the hardware "enforced" deadtimes on the SX100/SXFive to zero (it was possible on the older SX50/51), so setting them all to 1 usec to determine your "base" deadtime is the correct thing to do- as you did.
Attached below are calculations on my Sx100 instrument with the hardware DT set to 1 for both Ti Ka and Si ka. Note that they results for Si Ka Lower energy), are quite a bit longer than Ti Ka. Hence Paul's comment that these deadtimes are *not* constant and depend on bias and x-ray energy.
But you will note that my Sx100 calculated deadtimes at 1,1,1,1,1 usec are much lower than yours, hence my comment about "dirty detectors".
Just FYI, if I got the values you got ( 4.06 2.78 2.29 3.06 usec) at 1,1,1,1 usec hardware DTs, then I would probably set the hardware DTs to 4,3,2,3 rather than 5,3,3,4 because the enforced deadtimes actual values are usually higher anyway, and you don't want to have a larger DT than you have to.
All this discussion relates to a suggestion I made to Curt Scheppman at Cameca that since they utilized the SIMS vacuum electronics for the new SXFive, they should likewise utilize the SIMS counting electronics which has much faster (shorter) intrinsic deadtimes than the existing SXFive counting electronics.
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All,
You have to provide a graph of the deadtime plot to get meaningful comments from us. The deadtime Excel sheet calculates a deadtime for each measurement point (x = cps, y=cps/nA), and regression deadtime fit using the Excel least squares function (which is done for two sections of the data set) and also displays an average of the discrete values. For a system that is linear and fast, you should get very similar values for the discrete data and the fitted data.
For systems that exhibit paralyzable behavior, the plot (which has a negative slope) has increasing negative slope as the count rate is increased, with a possible asymptotic relationship to some high count rate; that is, at some high count rate no further output of pulses is obtained with an increase in input count rate. You sure don't want to use this range in any measurements and you want to know about it, hence the reason for doing this exercise and using a large count rate range to detect that behavior.
Most plots have sigmoidal behavior, slightly concave up over one range and slightly concave down over the other range.
It is really important to have the target be fully conductive over the large probe current range being used. This is why the spreadsheet uses both probe and absorbed current, and calculates the ratio abs/probe which you should plot vs. probe current; this should be a plot with no real change in the ratio abs/probe and if so it indicates either charging or a problem with the measurement of probe current.
I recommend that these measurements be made with integral PHA mode because you will surely see some level of gain shift and pulse coincidence so you need to have an essentially wide open PHA setting to allow for this. I recommend doing PHA scans at progressive count rates to demonstrate you have the correct gain/bias and baseline settings. I expect that as the count rate increases, eventually the x-ray pulse moves to lower voltage and merges with the baseline noise because the system loses pulse energy resolution and cannot discriminate between the two.
So again, I ask that in addition to posting numbers, you really need to show us the plot so we can see the behavior. I have not really seen any Cameca deadtime plots so why don't you SX-50 and SX-100 folks show us what the plot looks like for these systems and the effect of including the enforced deadtime value.
You guys are using *ultra pure P-10* for your detectors, right? If you don't (and I observed this at a lab) you will almost certainly see problems that may well be due to contamination of the wire. That is completely avoidable by using the cleanest grade of P-10, and I have never seen problems on the 4 microprobes I have operated over the years. I had a discussion with Colin MacRae and they replace their Jeol sealed Xe counters every 2 years or so, but I have seen very good long term performance of them. Maybe people are leaving the detectors running at high voltage ~1850V for extended periods of time?
Cheers,
Paul
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Ok, I just saw what Donovan posted. Several comments from the Si Excel sheet you posted:
First, the plot of abs/probe shows ~20% variation so something is not right about the fundamental setting and/or measurement of probe current or sample conductivity.
For Si use only data from the TAP-equipped spectrometers because you probably are not going to get a high enough count rate for Si on a PET spectrometer unless you go to very high probe currents. So use the date from Tap2 and Tap4.
The two Si PET plots exhibit sigmoidal curves (apparently). I don't know the origin of this.
Secondly, you need to use the count rate range that is for example ~1000 cps up to ~200k cps. Donovan's sheet shows a plot using the LTap for which the lowest count rate is ~50kcps and it exhibits paralyzable behavior at the high end. The two linear fits show very different deadtime values and the paralyzable portion has a much steeper slope, so you don't want to use a data range like that. Again, if you were doing X-ray mapping using the LTAP at high probe current, you will likely observe different intensities on Si-rich phases that will not be the correct intensities unless a deadtime correction is being used.
I start out these runs by determining the low and high probe current needed for the range, dividing it up into the number of measurements and using that increment to span the count rate range. This is done using the Excel remote interfact to PFE, but your probe has to be able to set the probe current correctly over the required range. John has used this to collect the data, hence the values for the probe current being used.
John, thanks for posting that data.
Paul
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First, the plot of abs/probe shows ~20% variation so something is not right about the fundamental setting and/or measurement of probe current or sample conductivity.
Yes, sorry, I should have pointed that out. We've had bad sample current measurements for about 5 years now and our engineer is sure that the connector inside the stage isn't making good contact when the sample holder is inserted. For example, I'll be at 20 nA on the faraday and pull out the cup and the sample current goes to 200 nA which is impossible of course!
The engineer is supposed to drop the stage and deal with that but in the last 5 years we've only dropped the stage once for some other reason and he forgot to check it! But we don't use absorbed current for anything else so it hasn't been a priority... but it should be!
But I think the deadtime data attached above is pretty good because the specimens were pure Ti and pure Si on conductive mounts.
For Si use only data from the TAP-equipped spectrometers because you probably are not going to get a high enough count rate for Si on a PET spectrometer unless you go to very high probe currents. So use the date [sic] from Tap2 and Tap4.
Yes, ideally that would be best. One of the PETs was LPET, so that is 3x better intensity. I really will never buy another instrument that doesn't have all large area crystals (except for the 4 crystal spectros of course).
Secondly, you need to use the count rate range that is for example ~1000 cps up to ~200k cps. Donovan's sheet shows a plot using the LTap for which the lowest count rate is ~50kcps and it exhibits paralyzable behavior at the high end. The two linear fits show very different deadtime values and the paralyzable portion has a much steeper slope, so you don't want to use a data range like that. Again, if you were doing X-ray mapping using the LTAP at high probe current, you will likely observe different intensities on Si-rich phases that will not be the correct intensities unless a deadtime correction is being used.
Fortunately CalcImage does perform a full deadtime correction for quant! :)
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Again, if you were doing X-ray mapping using the LTAP at high probe current, you will likely observe different intensities on Si-rich phases that will not be the correct intensities unless a deadtime correction is being used.
This is a little off-topic but I should mention this since the issue of quant came up. In addition to all quant data in Probe for EPMA and CalcImage being deadtime corrected (along with all the other required corrections of course!), we also perform deadtime and beam drift corrections to all displayed raw intensities as well. That is for both point and x-ray map intensities.
That is, unless one has disabled these corrections from the Analytical | Analysis Options menu dialog as seen here:
(https://probesoftware.com/smf/oldpics/i60.tinypic.com/2a00x8w.jpg)
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Hi Paul, John,
thanks for all the info. When I'll get to the office on Monday, I'll post my excel sheets -- I don't have them here. Maybe together we can figure out how to determine that deadtime for our SX100R.
Sorry for not posting them earlier!
Edit: Now attached to this post!!!!
Best
Philipp
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thanks for all the info. When I'll get to the office on Monday, I'll post my excel sheets -- I don't have them here. Maybe together we can figure out how to determine that deadtime for our SX100R.
Hi Philipp,
I took a quick look at the first spreadsheet and it looks pretty good. You should update the comment on the first row as it still has my conditions noted there!
I assume these were acquired at 1 usec enforced deadtimes? If so, then I would set those Cameca integer values in the SCALERS.DAT file as 4, 3, 2, 3 usec and remeasure them again and then use the new deadtime values for the software deadtimes for those crystals in the lower section of the SCALERS.DAT files.
john
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Hi John,
thanks for checking this out. It would be great to also get the opinion from Paul.
So, I re-measured everything, now with the DTIM suggested by you. It looks better now, I attach the two xls files, the 1,1,1,1 one and the 4,3,2,3 one.
What do you both think? Possibly a good idea to also check on a Ti metal?
Which values would you finally set in PfE then? I understand that 4,3,2,3 should go into line 35. From the xls values, I take the worst one for line 13?
Happy Christmas!
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What do you both think? Possibly a good idea to also check on a Ti metal?
Absolutely. In fact ideally one should measure an emission line on each crystal, so you can populate the SCALERS.DAT lines starting at line 72 with values for each crystal.
In other words, because deadtime is somewhat dependent on photon energy (and detector bias) as Paul stated above, having a different deadtime value for each crystal (think of it as an energy range), means that one can nicely compensate for this variation by specifying a deadtime value for each crystal.
Which values would you finally set in PfE then? I understand that 4,3,2,3 should go into line 35. From the xls values, I take the worst one for line 13?
The values from your spreadsheet calculations go into lines 72-76. Line 13 should be ignored, it is obsolete. Yes, the 4,3,2,3 values go in line 35 for the enforced hardware values.
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The second issue is with the picoammeter. No matter how well tuned it may have been, this will go out of spec at some point, and may need tuning frequently. If the current measurements are not linear, then your deadtime tests can give you a wrong apparent deadtime, or, in many cases, different apparent deadtimes depending on the current regime you measure. There are five gain loops in the picoammeter for each current range: <0.5nA, 0.5-5nA, 5-50 nA, 50-500nA, and 500-10000nA. In a perfect world, these are all linearized. In actuality, the gains and offsets tend to fall out of linearity, so you may see a completely different deadtime slope in the 5-50 range compared to the 50-500 range. Unfortunately, the 5-50 range resistors do not have trimmers (so you have to change resistors to affect the gain/slope), the others do, so they are all trimmed to match the 5-50 range. Mike J.
Hi Mike,
I recently calibrated deadtime on our SX100.
Some results in attached file
1 ) At the moment I use Cameca integer deadtime = 3 µs for the 4 Spectrometers.
Can I reduce this value to 2 µs as the pulse stretcher circuit already seems to be active at this value for the 4 Spectrometers?
2 ) From the graph it's clear that the 5-50 nA and 50-500 nA range or not trimmed correctly at this moment.
- The deadtime values that I put in table or only valid for the 50-500 range? What to do if I want to calibrate standard (Pure Element )in the range 5-50 nA and do measurement sample (Minor Element) in the range 50-500 nA.
- Can I trim/tune the 50-500 nA range myself? How to do it?
Greetings,
Benedict Vos
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Hi all,
I was at EMAS 2019 in Trondheim, Norway for the 16th European Workshop on modern developments and applications in microbeam analysis. My poster presentation was about a "calibration device for accurate current measurement on a CAMECA SX100 EPMA".
The abstract and poster are given in annex.
With this calibration device, I'm sure that both the range 5 - 50 nA and 50 - 500 nA are very well trimmed now.
For more details feel free to contact me.
Greetings,
Ben Vos
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Hi all,
I was at EMAS 2019 in Trondheim, Norway for the 16th European Workshop on modern developments and applications in microbeam analysis. My poster presentation was about a "calibration device for accurate current measurement on a CAMECA SX100 EPMA".
The abstract and poster are given in annex.
With this calibration device, I'm sure that both the range 5 - 50 nA and 50 - 500 nA are very well trimmed now.
For more details feel free to contact me.
Greetings,
Ben Vos
Hi Ben,
Very interesting. Would you be willing to attach your poster as a pdf here?
john
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I was scratching my head for years (actually only 6 years, I am quite fresh in this field) about this dead time and PHA (those are tightly connected, and all missing counts at very high beam currents at intensive lines are missing due to that, I will explain that further). Unfortunately most of available educational material on these subjects are lacking and some are even completely misleading.
The "improvements" by vendors (i.e. enforced cutting off values bellow some energy in PHA, that is cutting out Ar esc-peak) hides the mechanism-behind away from such newcomers as me even more further. I should mention that our SXFIVE FE is equipped with 5 spectrometers where 4 have large xtals. Due to being a FEG machine it has only single condenser lens, which makes changing between high and low currents during single analysis not practical, the power of condenser needs to be changed enormous (compared to 2-lense system, i.e. sx100), and requires thermal equilibrium time to stabilize. So for this reason when we want to measure trace elements with high or very high (max available current, or "more power, Scotty") current, we also analyze major elements with same conditions. I had done similar experiments as here a few years ago: with total counts/real time vs cnts/beam current and wondered with few things: non linearity of that, and PHA peak deformation and shift. My "cure" for this problem was (and is up to now, but maybe it will change) making separate calibrations at high current - this minimizes the effect, but does not completely eliminates it, and generally is cumbersome.
I should tell that lately I am very into electronics (probably inherited genes, I have good memories how my dad was building synthesizers, and I love the smell of solder fumes (burnt rosin flux). Whole last month I got an excuse to apply my hobby obsession on our SX100, as it had broke down. Due to Covid crisis we had no clients for month, and I had plenty of time to be not bothered - just machine and me. After fixing major problems (lens supply) I got into hunting and troubleshooting small issues which had accumulated through years. Also Covid situation showed that remote control of machine is not easy, ergonomic or efficient, particularly that at home I have 3Mb/s internet - that simply sucks. And I know that plenty of our customers has similar connectivity problems, thus to overcome such problem I was inspecting options for custom hardware (+ software), which would send lossless video through network with lowest possible latency with lowest bandwidth. There is this unused EDS slot on electronics motherboard (we have no full version of Bruker Esprit, with full license that slot would be used by cable to EDS card for video and external scan generation). While my main aim is extracting the video signal, however led by curiosity I had probed (with oscilloscope) the WDS signal pins exposed there. It outputs counts as pulses (5V). wait... I am going to design the chip, which will use only video pins and leave those other signals just in peace? All kind of possibilities came into my mind, there are tons of limitations in Cameca Peaksight software and hardware acquisitions (limited mapping resolutions as an example, or limited resolution of WDS scans), and these pins with right hardware and software can at last let me go over it.
So the digital WDS pulse has 500 ns width. (i had no idea about WDS schematics at that point, nothing had broken there). And so I got many important questions in my head "what if". If I would want to count peaks what kind of counter chip I would need, 500ns stuffed at full side by side would do 2Mhz. Can two peaks exist side by side (which would exclude counter chip working on rising edges, thus would need more expensive complicated chip/system)? To find answer to this questions I had set beam to burning 3 micro-amperes to find out how dense these peaks can get! Ni Ma line on TAP for x-rays of stage just was enough. The x-ray meter on the monitor showed E6 level (white) - just excellent.
So with persistence set on oscilloscope for 2 seconds I had found out that digital pulses are aligned perfectly at 1 μs steps. The closest pulse (rising edge) to the triggered pulse was 4 μs counting from the rising edge of the triggered pulse. That is consistent with software set dead time to 3µs. So despite digital pulse being only 500ns, it represents 1μs step, and thus setting deadtime to non-integer values makes no sense. Changing the dead time changes the length of gap between closest pulses, or in other words enforces the rejection of anything at interval counting from 1+set_dead_time counting from rising edge of the last counted pulse. And then I asked myself the politically incorrect question: could there be "pile-up" peaks on proportional counter?
The short answer is.... (drumroll... dramatic music) yes. :o
So led by my curiosity, I hanged oscilloscope probe on the raw spectrometer signal (by raw I mean the signal coming out from spectrometer, not processed with spectrometer board). The set dead time in GUI does not affect anything there, and with high intensity beam the closest peak from triggered rising edge is 2μs apart. The peaks coming from pre-amplifier (and mild opamp, for spec- intermediate board transfer) have about 1μs width. So the smallest gap between two peaks is... 1μs. Interestingly looking at small time scale it is clear that peaks (and gaps) are aligned at 1μs steps. Now lets stop here, I understand why digital pulses are aligned like that, but analog pulses? Would that intend that electron beam is pulsing beam in 1MHz?
Anyway, this is another proof showing that setting dead time to non-integer values makes no sense. So if minimal observed gaps are 1μs thus physical dead time of counter is 1μs, why simply not set the dead time to 1μs and leave like that? We could do that if only there would be no pile-ups and higher order diff x-rays... because this physical dead time depends from amplitude of peak it follows. High order of more energetic peak (or pile-up peak) will produce following voltage drop which is proportional to amplitude of peak, and relaxation time is proportional too. It does not matter if PHA is set to diff or integral - those filtering is applied much latter in the processing pipeline, and does not eliminate physical influence and physical dead time of those higher energy peaks.
So lets talk about the pile-ups. I wouldn't believe it if I wouldn't see it. While running on low current with zoom out view in oscilloscope it is clear that dominating peaks have very similar amplitude. With increase of current to the moment there density of peaks increases, there starts to appear double of amplitude peaks, and with increase of current further triple amplitude and more appears. (i.e. dominating amplitude of ~300mV at 20nA, after going to very high current (2μA) some peaks with 1.5V amplitude appeared -that is quadruple pile-up!).
But that is not all, remember Ar escape peaks? Filtered out they said... filtered out single Ar escape events, but... with high intensity beam there is so many Ar escape peaks that they pile up on themselves, and pile up on the analysed line peak, and on piled-up peaks. And.... this is where You get right wing tail in PHA peak, and gap between peaks is smoothed out by Ar esc piling up.
So with oscilloscope probing the raw spectrometer output I had checked how well the auto PHA works for high current. It does not. It sets the bias too high, which leads to multitude of argon peaks, which gets hidden out ("good" job Cameca for dumbing PHA down). With oscilloscope probing, I could reduce bias to the level where Ar esc peaks just disappear.
According answer about PHA from one of Cameca engineers, at integral mode pulses up to 15 V are summed (5.5 to 15V is not probed for and thus not shown by PHA).
So why we get PHA peak shifts depending from current? I guess there is some smoothing algorithm before raw peaks are processed, with Ar esc peaks which gets very numerous the smoothed background would be much higher, and thus would decrease the relative amplitude of main peaks.
So my final words:
The 3μs is set by default as the best compromise universal dead time. It can safely be lowered to 1μs if working on low current, and no high order overlapping peaks are at the position. However if high order high energy overlap is expected (despite diff or integral), it is good idea to increase dead time. At high counting rate, due to pile-up'ed pulses physical dead time can be randomly larger than 3μs, and it would be safe to increase the dead time (actually increasing dead time is always most safe option, but not most efficient), albeit that won't fix missing counts, which is a product of counting pile-up peaks as single pulse (integral) or completely discarded (diff mode).
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Followup:
My recent thought is that maybe proper way would be use tight window with diff mode, discarding anything (pile-ups, ar+pile-up, ar+ar...), but for that those discarded peaks should too be added to dead time. so proper way to calculate the dead time would be calculate dead time for accepted peaks:
diff_counts * dead_time + (integral_counts - diff_counts) * (1 + dead_time)
Of course getting rid of Ar esc peaks from generation is important too, to prevent peak shift in PHA. However, how to do it with this "improved" PHA, and without an oscilloscope is huge challenge.
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Unfortunately (or actually fortunately) I need to correct some statements of mine above. I re-looked at my film material from that experiment of mine, and I see that I got a bit different first impression. My general advice stated above stays the same, however, I see after re-watching that there is no minimal gap between physical pulses in raw x-rays. So actually physically there is no dead time after most of peaks (that is the advantage of p-10 gas vs pure Ar), unless the height of peak is so huge, that following relaxation voltage drop is so intense preventing an avalanche. (2022 update: looks that I had too limited set of observations of these pulse behavior. After looking more I had got into numerous cases there other pulses are still arriving in that depression - thus in real at these petite X-ray intensities practically the proportional counter has no dead-time at all)
And my guess about argon escape peaks causing PHA shifts is not probably right. Actually it is more likely that shifting is due to closely following pulses where following pulse starts while charge of detector thread has still not recovered from previous pulse, and so while relative (to left background/side) height of peak is correct, the absolute height (to 0V reference) is smaller and after further amplifications and digitization in WDS card it lands at lower voltages in the PHA graph. With high counting rate (E5, E6 level), such closely packed pulses are very common. I think it is worth to try to find a day for concise video and photo documentation of how this works and maybe I could upload it here (I mentioned I have some recordings, but it is bad quality). I guess this would benefit the community. I am not sure about legal aspects - this basically reveals lots of internal working of SX detectors. But on the other hand this is the fundamental piece of methodology to understand.
The more I think about this, the more I am tempted to replace prop counters with SDD (tiny eds).
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Recently I accidentally had found this article:
"Radiation detector deadtime and pile up: A review of the status of science"
https://doi.org/10.1016/j.net.2018.06.014 (https://doi.org/10.1016/j.net.2018.06.014)
That can be used to go further with discussion on dead times, it demonstrates that old approaches for dead time is not well thought. It also supports my previous claim that Cameca SX100/Five detectors(+electronics) suffers from pile-up effects.
One way is to correctly calculate the deadtime, when its origination is well understood.
Another way is hardware modifications, to pimp-up the electronics that dead-time would be not bothered anymore.
I mentioned that it is tempting to replace proportional counter with tiny EDS (It was done on some custom tailored JEOL probe, pardon, can't remember all the details); I, however, had changed my view on that - I see that it is not actually SDD over gas proportional counter superiority (EDS windowing vs WDS PHA), but modern "state of art" counting electronics versus ancient "state of art" counting electronics which makes the difference. The spectrometer pre-amplifier electronics probably had not changed from age of SX50 (needs some confirmation). Used A203 charge preamplifier from AMPTEK was state-of-art few decades ago, but AMPTEK went with R&D through few generations of improvements. A203 signals (shaped pulses) has enormously long tail, and so pile-up with high counting rate is inevitable. That tail was wanted feature with slow low res ("fast" to old standards) ADC used in MCA. To achieve better performance, I think the least modification would be to use the shaped and amplified signal from spectrometer (without modifications) and convert it with high speed high resolution (at least 12-bit) ADC and feed that to DSP cores on some low/middle-end FPGA (no need to go with high-end). That should be able to process without any hiccup/dead-time with full re-pile-up in real time. I wonder what additional increase in performance could be achieved using modern "state-of-art" charge preamplifier (like A250), that would require to replace part of pre-amplifying electronics. While this would be more complicated - it still looks like peanuts if compared to mechanical complete replacement and fitting of SDD EDS in-place of proportional counter.
I wish I would have an access to a spare SX100 (or SX50) to experiment with...
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Looking through those spreadsheets I see that there is approach of fitting linear regression (fit all or fit last), but I want to point that due to two (not single) processes this is not linear. One is dead time - not measured counts; another - not the less important - peak-pile-up. With higher count rate the pile-up process dominates the "missing counts". Going up to 200 kcps and using this linear regression approach looks quite reasonable, albeit fun starts already at 100 kcps. Deadtime looks clearly tricky to Cameca peaksight as in peaksight 6.4 there is additional option for additional dead time correction with setting current threshold. That is clearly a wrong approach, as it depends from count rate, not directly from current, but it illustrates that the problem is not straightforward to understand.
At least with peaksight 6.4 it gets clear how Peaksight (and interpretor, and so I guess dll for communication of ProbeSoftware-to-machine) calculates the counts per second from counts with applied dead time. For set integer deadtime (in µs) additional 0.3 µs is added (it is shown in config window). So if we set it to 1 µs the count rate will be: cps = cts / (real_time - cts * (1µs + 0.3µs));
Why 0.3µs? I guess because without it the discrepancy between cps at different nA was to high to accept. As I already mentioned before, generated digital count pulses are emitted by WDS board aligned to exact 1µs time grid. Pulses from preamplifier comes with 1µs width - even if it would be possible forcibly to set dead time to 0, there is no way to register another peak at falling edge of pulse - for counting the rising edge of pulse is needed. So Why 0.3µs? - there is such chip which holds amplified signal for 700 ns so that slow-ass prehistoric ADC could read the amplitude. ...and that calculates well 0.7 + 0.3 + set_dead_time = time from rising edge of pulse after which electronics is ready to register next pulse.
This 0.3µs in my opinion is for compensating unrecognized by Cameca peak-pile-up process, which works miserably (is insufficient) with high count rate (>100kcps).
So to understand exactly how this dead-time is working I made a MC model. The model does two functions. First function generates randomized simplified model of signal emitted by preamplifier with length of 1 second with 1µs resolution (this simplification can be assumed good enough as electronics emits digital pulses aligned at 1µs grid, and pulses from pre-amplifier comes with ~ 1µs width). It requires to provide wished count rate produced by detector/preamplifier (or aimed raw counts). If it is modeled for low count rate there are only single pulses with vast time in between, but as density grows - the pile-ups starts to appear. The randomisation is semi-precise, there are cut few computational corners to boost modeling speed (it is written in python) and thus it does not create signal pulse by pulse, but in batch vectorised (numpy arrays) form, that is summing a number of generated randomized arrays. That produces sufficient patters which can be compared with those seen on oscilloscope, when probing the pre-amplifier-produced signals.
The second function processes such signal like counting electronics on the WDS card would do, and accounts all of pulse: it iterates through signal from beginning to the end, pooling counts by its height to different variables (so that in the end we could see how many pure pulses would be, how many double, triple and so on pile-ups). In the end it produces number of counts which cameca software/firmware would produce, and calculates count rate as Cameca software would calculate with given software dead time. I checked its results against SXFiveFE and it agrees very well with observations. What left to do is to fit non-linear regression for correct addressing of pile-ups. It demonstrates that up to real 500 kcps the linear approach kinda can work, with small overestimation in lower count rates. However to make it work close to 1 and up to 1.5 Mcps linear approach is not sufficient. i.e. Cr Ka on LPET on Cr2O3 will produce about 300 raw kcps, and after dead time correction up to ~500 kcps while in reality there should be three times more (quanti analysis would give very low concentration of Cr, with totals about 30% at 650nA, while 100% at 14nA). Modeled values agrees well with spectrometer count rate capping at different set dead time values.
I.e. at dead time set at 1µs the count rate caps at ~300kcps, while at 3µs it caps at about 200kcps, which this modeling predicts very well.
I am attaching modeling notebook (python ipython notebook, requires numpy and matplotlib for plotting), so this can be looked and experimented with. I am thinking about what kind of non-linear equation could precisely calculate to real counts.
I know most of you don't use such high currents, but our SXFiveFE is crippled with a bug, where recalling saved column conditions sets suppressor to hardcoded 300 V, ignoring value saved in conditions (and Our good as new FEG tip, which is running like new for more than 2 years is adjusted by me to 275 V suppressor). That (+ non-repeatability of C3 lens) makes multi-option approach not possible, these kind of currents does wonders for measuring trace elements for minerals which can withstand such harsh beam conditions (i.e. rutile).
Alongside python notebook, I am attaching few charts, few lines unfortunately completely overlaps, that is cps as would be calculated by cameca for 1µs DT and 3µs DT (blue line is under red - the same cps). The curves with "guessed" DT with used dead time as 2.56µs for software DT=1µs and 4.56µs for software DT=3µs overlaps (pink line is under dark yellow). So generally the theoretical DT is software DT + 1.56 µs (for crossing 1M(raw) vs 1M(counted) counts, it should be smaller for better fit at 100k count rates.
It also shows that diff flattens out much sooner than integral mode, and even goes into "paralizeable dead time" mode. However model does not predict it precisely as in real this is due to PHA shifting, but model follows decrease from single (non piled up ) pulses becoming rare, as most pulses are piled up at high count rates. In real with diff method, peaks shifts toward lower energies in PHA, and window will catch higher pile-up orders. For Cr Ka on LPET the PHA pulse (at low current) is normally centered at right, thus even with 650 nA current, the counts do not diminish, as all pulses those of first order (non-piled up) still is above background cutoff value. Count at Integral mode in both model and real instrument will stabilize and wont go down with increasing current.
The big surprise is that from model it is clear that setting software DT to 1µs is better than to 3µs, as that has better slope at 1M - 1.3M raw counts, thus recalculation to raw counts at such range will have less uncertainty.
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While comparing the model to real observed values on the machine, I had found some strange behavior in peaksight (but as PfS is using some dll, from cameca for communication and data transfer from machine it can be affected too) depending from set software dead time.
The dead time and count capping is nicely illustrated with comparison of WDS scans of same substance at different probe currents. It is clear that background is exactly same (As it is low count rate) when comparing with cps/nA, but high current peaks are capped.
From the beginning of SXFiveFE installation in 2014 I was always in impression that our spectrometer 5 has worse performance as amplitude in WDS scans always were lower. With low count rates the difference is not big, but with high current it is like half. While doing 660nA on Cr2O3 I spotted that such discrepancy is obviously not right as xray rate meter showed nearly identical intensity bars while at peaks.
When setting interpretor for do count for 1 second, the counts would result in the very close values. So Quanti is also not affected, but WDS scans are. I tried to acquire mapping of Cr Ka at 660, and the intensity for 5 spectrometer is reduced. As I did few WDS scan runs at different set dead times, I saw that there is no discrepancy when dead time is set to 1µs, The WDS amplitudes of peaks are near identical and intensity mapping is also near identical for both 2 and 5 spectrometers.
This is really enigmatic behavior, had you seen this on PfS? The results probably is differently acquired from machine when doing image and WDS scan (needs to be pushed to software constantly with WDS position/stage position movement), than when using hardware counters for measuring counts in few seconds for quantification.
I see that mappings should be done at 1 µs DT.
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This is really enigmatic behavior, had you seen this on PfS? The results probably is differently acquired from machine when doing image and WDS scan (needs to be pushed to software constantly with WDS position/stage position movement), than when using hardware counters for measuring counts in few seconds for quantification.
Hi SG,
I haven't looked at deadtime corrections in wavescans explicitly, but in PFE we apply the deadtime (and beam drift) corrections to all samples: standards, unknowns and wavescans (and of course X-ray maps). The method we use is described in the first page or so of posts in this topic.
For such high beam currents it seems to me that you should be utilizing the higher accuracy deadtime equation described here:
(https://probesoftware.com/smf/gallery/1_21_04_21_9_56_07.png)
The difference between the "normal" dead time correction expression (#1) and the "high precision expression" (#2) is quite significant at high count rates with Cameca GFPC detectors.
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Thank You for the formula.
I put it into the simulation and find that it is not satisfactory, the problem is that it merges two separate processes into one. Counting piled-up peaks as single peak is not depending from the dead time, but from count rate alone. I am coming up with some improved formula. Unfortunately, I find it works rather only on SXFive (or SX100 with new WDS board), as unfortunately I found out that SX100 (old electronics) counting electronics have paralysable behavior (flats out at about 250kcps (raw, not-dead-time-corrected)). I am trying to understand the process to make the equation universal.
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The deadtime correction expression is an infinite factorial of which only the first factorial term is generally utilized. This is because deadtime is a statistical event and the single factorial term is generally good enough at relatively moderate count rates. The expression #2 just utilizes a 2nd factorial term in this infinite series for better handling of extremely high count rates. It is not intended to deal with other electronic non-linearities.