Author Topic: Setting dead times on Cameca SX50/SX100/SXFive  (Read 10895 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!

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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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