Author Topic: LIF on low pressure spectrometer  (Read 620 times)

D.

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LIF on low pressure spectrometer
« on: October 08, 2018, 01:11:26 pm »
Hi All,

Question: I have seen a couple Camecas with LIF on low pressure. What would this be used for? In analogy to a JEOL sealed Xe detector, shouldn't LIF always be high-pressure?

EDIT: Are there some soft lines that are in LIF range on the 160mm Rowland Circle versus the JEOL 140mm?

Thanks,
Deon.
« Last Edit: October 08, 2018, 01:32:56 pm by D. »

neko

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Re: LIF on low pressure spectrometer
« Reply #1 on: October 09, 2018, 10:38:49 am »
I'm not sure, but maybe it has something to do with the size of the crystal? Our low-pressure spec with LIF is all small crystals, and our two LLIFs are our high pressure spectrometers. We don't use it that often (it's usually Tap/Pet depending on our needs), but when we do I haven't noticed any particular problems other than the typical ~50% reduction in count rate (well, 50% for Fe Ka, I don't know that I've cross-compared anything else besides the verification line).

Probeman

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Re: LIF on low pressure spectrometer
« Reply #2 on: October 09, 2018, 01:54:37 pm »
I would say that having an LIF crystal on a 1 atm detector is a configuration mistake.  There would be no advantage that I can think of and as Nick points out, a decided disadvantage. Typically PET/TAP are 1 atm detectors, but LIF/PET really should be a 2 atm detector for better absorption for LIF emission energies.

Someday we will have solid state detectors for our WDS spectrometers and then this won't matter.    8)
john
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Ben Buse

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Re: LIF on low pressure spectrometer
« Reply #3 on: October 15, 2018, 08:14:44 am »
Hi Deon,

Yes it gives a reduced count rate having a LIF on a low pressure counter, but this can be useful if you are measuring say trace elements in steel, high beam current for trace elements, but reduced count rate on major elements e.g. Fe, so don't run into detector deadtime. Also it lets you have more LIF crystals on your instrument if your aggregating counts from multiple spectrometers.

Probeman

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Re: LIF on low pressure spectrometer
« Reply #4 on: October 15, 2018, 10:31:12 am »
Hi Deon,

Yes it gives a reduced count rate having a LIF on a low pressure counter, but this can be useful if you are measuring say trace elements in steel, high beam current for trace elements, but reduced count rate on major elements e.g. Fe, so don't run into detector deadtime. Also it lets you have more LIF crystals on your instrument if your aggregating counts from multiple spectrometers.

Hi Ben,
Those are a fair points, though I would say these situations are less common than the general need to maximize efficiency to improve sensitivity.

But I would also point out that we should be able to have the best of both worlds, not only when we finally get decent solid state detectors in our WDS spectrometers, but even now we could have tandem detectors for each spectrometer allowing a mix of any crystals on all spectrometers as described here:

https://probesoftware.com/smf/index.php?topic=644.msg4827#msg4827

We should have detectors that can handle the full range of photon energies on every spectrometer, whether they are only solid state, combinations of gas and solid state or tandem gas detectors (Ar and Xe).  It should be mentioned that several WDS spectrometers for SEMs already utilize these tandem detector technologies, so when are Cameca and JEOL going to "get off the stick" and implement these better technologies for EPMA instruments?
john
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D.

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Re: LIF on low pressure spectrometer
« Reply #5 on: October 17, 2018, 06:20:56 am »
Hi Ben,

Avoiding detector overload from majors while analyzing traces on the same routine with high current is the only spinoff I could think of too. But if you need to aggregate counts from 2 LIFs to get a trace composition, I expect that the low-pressure LIF will provide an almost negligible signal contribution. But I use a JEOL, so maybe the low-P  efficiency is better on a Cameca than a JEOL...

Thanks,
Deon.


Hi Deon,

Yes it gives a reduced count rate having a LIF on a low pressure counter, but this can be useful if you are measuring say trace elements in steel, high beam current for trace elements, but reduced count rate on major elements e.g. Fe, so don't run into detector deadtime. Also it lets you have more LIF crystals on your instrument if your aggregating counts from multiple spectrometers.
« Last Edit: October 17, 2018, 06:22:53 am by D. »

JonF

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Re: LIF on low pressure spectrometer
« Reply #6 on: October 17, 2018, 10:56:54 am »
 For my SX100, there's about an order of magnitude difference in count rates between a LIF with 1atm P10 vs a LLIF with high pressure P10.

I do pretty much as Ben said: majors on small LIF and traces on LLIF in steels. Saves using multi conditions, and so analysis time.

D.

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Re: LIF on low pressure spectrometer
« Reply #7 on: October 18, 2018, 04:47:23 am »
Hi Jon,

Out of curiosity....you have 5 spectrometers...how many high-P LIFs do you have?

Deon.

For my SX100, there's about an order of magnitude difference in count rates between a LIF with 1atm P10 vs a LLIF with high pressure P10.

I do pretty much as Ben said: majors on small LIF and traces on LLIF in steels. Saves using multi conditions, and so analysis time.

JonF

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Re: LIF on low pressure spectrometer
« Reply #8 on: October 18, 2018, 09:49:10 am »
Two high P LLIF and one low P LIF. The low P LIF is usually set as a PET or TAP though (there's a PC0 in there, too)