Author Topic: Setting dead times on Cameca SX50/SX100/SXFive  (Read 13375 times)

Paul Carpenter

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Re: Setting dead times on Cameca SX100
« Reply #15 on: December 19, 2014, 11:16:41 am »
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
« Last Edit: December 19, 2014, 11:57:06 am by John Donovan »
Paul Carpenter
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John Donovan

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Re: Setting dead times on Cameca SX100
« Reply #16 on: December 19, 2014, 11:55:24 am »
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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John Donovan

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Re: Setting dead times on Cameca SX100
« Reply #17 on: December 19, 2014, 02:12:01 pm »
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:

« Last Edit: December 19, 2014, 03:45:28 pm by John Donovan »
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Philipp Poeml

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #18 on: December 21, 2014, 05:26:21 am »
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
« Last Edit: December 23, 2014, 04:21:25 am by Philipp Poeml »

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #19 on: December 22, 2014, 01:31:59 pm »
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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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #20 on: December 23, 2014, 04:27:05 am »
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!

John Donovan

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #21 on: December 23, 2014, 08:30:14 am »
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.
« Last Edit: December 23, 2014, 08:39:25 am by John Donovan »
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Ben Vos

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #22 on: March 26, 2018, 08:00:51 am »

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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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #23 on: June 11, 2019, 01:49:23 am »
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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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #24 on: June 11, 2019, 08:30:57 am »
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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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #25 on: May 15, 2020, 06:04:53 pm »
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. which is consistent with set dead time to 3us. 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 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 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). With pile-ups in mind lots of surprises can be hidden on that "never seen side of the moon".

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).
« Last Edit: May 16, 2020, 09:45:12 am by John Donovan »

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #26 on: May 15, 2020, 06:18:08 pm »
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.
« Last Edit: May 16, 2020, 03:42:46 pm by John Donovan »

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Re: Setting dead times on Cameca SX50/SX100/SXFive
« Reply #27 on: May 16, 2020, 01:48:15 pm »
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.

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).
« Last Edit: May 16, 2020, 03:42:08 pm by John Donovan »