?

No they’re just sitting there I haven’t fired it up with anything yet. The plan was to use them with the SiLi detector when I get a load of LN2 but the SDD is so much easier and nicer.

Can you run the comparison tests, I’m curious to see the difference.

Dud

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That should be the same one I have here a Kevex W micro focus. I ?don’t see why you need to have 2 positions or a detector collimator, it will work fine with just ?a close up detector and no collimator. For low energy work you run a 15 kVp beam. You want to set the beam voltage about 10 keV above the K edge. Run 2 test shots, no detector collimator close up and with collimator close up. Try a 15 or 20 kVp beam if it’s not filtered which I don’t think the Las Vegas one was. ?All XRF needs to be done close up to maximize signal and minimize air absorption. What is you close up and far distance? You should be able to see Si, K, and Ca as they are important in determining what type of rock you’re looking at.

Dud

?

?

Geo,

What is the high energy position and purpose for both? I’m seeing W in the spectra is this the high energy position run? What Is the purpose of the W 1mm collimator on a 25mm detector? The collimator and filter is used on the x-ray exit. ?Can you do the same run without it and close up detector.

The newer commercial units use a 45 deg setup with the detector the closest to the sample and it runs well down into the 1-2 keV range.

Dud

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?

?

?

?

?

?

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?

Back to Obsidian.?
One of my wife Betty's finds in New Mexico

A nice lump with a broken tendril the end of which is glass smooth. Perfect for micro beam XRFing.

1mm beam and 1mm hole in sensor's W collimator. 300 seconds, the sensor is moved back to the high energy position, which eliminates tungsten XRF activation in the collimator. It is moved much closer when lowest energies are to be observed, due to air attenuation in the path of those low energies. Normally they are studied in a vacuum for this reason.

High def pic of the setup and .mca attached.

Note the exciter beam path- .? This closer to conventional and the 45 degrees is the angle of the beam to target and of the XRF to the sensor for 90 degrees total. Commercial units are arranged with the exciter and sensor both facing nearly? the same way, more like the RAPCAP arrangement we use.

Geo

Are you using a medical grade xray setup?Recently, some??medical??x-ray manufacturers have produced machines that replace the conventional 60-cycle AC high-voltage current of the x-ray tube with a high?frequency power supply. This effect is an essentially constant potential between the anode and cathode. The result is that the mean energy of the x-ray beam produced by these x-ray machines is higher than that from a conventional half-wave rectified machine operated at the same voltage. This is because the number of lower- energy (nondiagnostic) x-rays is reduced. These photons are produced as the voltage across the x-ray tube rises from zero to its peak and then decreases back again to zero during the voltage cycle in the half-wave rectified machine.

So if you are using HF power supply, to get to the lower peaks cut down kvp?

This may or may not help depending on filter thickness.

Filters are not totally to blame for?missing low energy peaks here.

Taray

On Sunday, January 24, 2021, 12:49 PM, Dude <dfemer@...> wrote:

Geo,

Well this sucks. I think you’re right and they are hardening the beam by dumping the low energy components to reduce skin dose. What is your x-ray tube manufacturer and model number? Is this a micro focus tube?. I thought that’s what we were using up in Las Vegas. What’s the max operating voltage of the tube? I’m sure you’re right and is there is an Al filter in there knocking down the <15 keV portion. That may standard on medical x-ray tubes. Well that sucks.

The skimming set up as I understand it is used for thin film analysis not general XRF sample analysis and isn’t as efficient as a standard set up. It’d be interesting to see the difference between the two.

?

Geo, I don’t understand the logic on continuing to use a wrong instrument setup with the 93 keV energy span. Explain to me how it’s not directly comparable. It has half the energy resolution where an optimal 64 keV span will double that, both will show the XRF but one will have trouble resolving overlapping peaks. You’re needlessly degrading the instrument’s response and limiting the ability to see small important features. Just to be compatible with a bad set up? That doesn’t make sense. Correct the problem now you’ll be glad you did. If you look at the these spectra they are very noisy for a 30 sec x-ray tube shot and going to 4096 channels is going to make it worse for no gain due to the detectors resolution limit. A detector with a 0.2 keV FWHM is not going to resolve any peak separations close to that no matter how many channels you are using. You have a perfect peaking time / filter set up here but the system needs to be optimized for the energy range of interest or you’re just throwing out half your capability.

Fix the problem now and move on.

Dud

?

?

From: [email protected] [mailto:[email protected]] On Behalf Of GEOelectronics@...
Sent: Saturday, January 23, 2021 1:15 PM
To: [email protected]
Subject: Re: [XRF] Obsidian

?

That's correct. Same as the ratio chart I showed.

?

Nothing at all at 15 keV or lower.?

?

Looking at the beam directly with a sensor tells why. There must be a compound filter in or right at the output centered on tungsten L-XRF lines (~8.40/8.67keV). Like a high pass filter in electronics. I can't identify the small peaks from the filter, which is why it is probably a graded stack.

?

The two L line are there but weaker than the bremsstrahlung peak to the right .I often wondered where is the lower portion of the Brems continuum, besides the L peaks, it goes down to the floor at the low end. Turned al, the way open, there appears another continuum at the low end but still that gap between the two humps.

Makes sense, these tubes are manufactured to drive X-Ray image intensifiers, where the lowest energies are of no use and should be eliminated. They did a good job of doing so.

?

Like you mentioned, the most energy is in the mid point or "top" of the hump. Merely setting the cutoff point to a little above K-Edge does not give much output there as that is the right hand bottom of the hump, so that's why there is a lag between the K-edge and the HVpeak setting. So the top of the hump is where the K-edge is set and that does work as most of the impressive peaks are on the left hand slope of the hump.

?

This tube I've been using for this since the first is a unitized module, the X-Ray tube, HV generator, heat sink and X- shielding are all one sealed piece. Not much could be done even if I wanted to.

?

I don't do any background subtracting or software smoothing, so what my .mca's show is untampered with.

?

Right now the whole setup is turned on it's side instead of facing straight up like before. Doing so allowed an almost perfect 45 degrees between the beam and the detector, and the results are good, better than the 90 degrees I use with the chamber installed. But the convenience and safety features are not as good that way, so I'll stick with the chamber for most routine stuff, and the skimming for those Ca vs K determinations.

?

Meantime I'll stick with 93 keV top end and when finished messing around, switch back to 4096 channels. If I change the top end, all the serious testing I've done that way won't be directly comparable to future tests.

?

Geo

?

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----- Original Message -----
From: Dude <dfemer@...>
To: [email protected]
Sent: Sat, 23 Jan 2021 13:36:26 -0500 (EST)
Subject: Re: [XRF] Obsidian

?

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Geo,

Moist of these MCA files
have nothing in them

50kV, 40, 30 seem ok but
20 have very few counts and 15 has no counts same with all. You are getting no
signal below 30

The data are very noisy as
I think is due to the skimming and the collimator.

Rerun these using the
standard 45 degree shot and use a beam current around 40 or 50 and no narrow
collimator. Try for a spot size of about 6mm or larger

Dud

?

?

?

From:
[email protected] [mailto:[email protected]] On
Behalf Of GEOelectronics@...
Sent: Saturday, January 23, 2021
8:38 AM
To: [email protected]
Subject: Re: [XRF] Obsidian

?

Theory says Fe and Ca should XRF with quite low energy excitement =
"just above the K-edge".

. Fortunately the test was left as-is last night, so it was easy to redo and
save example .mca's. so here's a fresh set of .mca that show the reality of
this skimming arrangement.?


Note the Ca, Fe and Ba peaks as exemplars. Also remember the possibility of
stray "W" L-lines due to the Si-PIN collimator..?


The .mca marked as "ALLkVp" should contain the others too, if not
I've included each individual .mca's.


Once I'm able to strike this test, I'll take a repeat this test wiht the
standard 45 degree arrangement of exciter and detector angle.

Then a direct Gamma Scan of the beam itself to see how the internal tungsten
target characteristic L lines look. If they are being filtered internally that
would explain a lot.


Geo

?

?

?

?

?

?

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?

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Geo,

Moist of these MCA files
have nothing in them

50kV, 40, 30 seem ok but
20 have very few counts and 15 has no counts same with all. You are getting no
signal below 30

The data are very noisy as
I think is due to the skimming and the collimator.

Rerun these using the
standard 45 degree shot and use a beam current around 40 or 50 and no narrow
collimator. Try for a spot size of about 6mm or larger

Dud

?

?

?

?

Theory says Fe and Ca should XRF with quite low energy excitement =
"just above the K-edge".

. Fortunately the test was left as-is last night, so it was easy to redo and
save example .mca's. so here's a fresh set of .mca that show the reality of
this skimming arrangement.?


Note the Ca, Fe and Ba peaks as exemplars. Also remember the possibility of
stray "W" L-lines due to the Si-PIN collimator..?


The .mca marked as "ALLkVp" should contain the others too, if not
I've included each individual .mca's.


Once I'm able to strike this test, I'll take a repeat this test wiht the
standard 45 degree arrangement of exciter and detector angle.

Then a direct Gamma Scan of the beam itself to see how the internal tungsten
target characteristic L lines look. If they are being filtered internally that
would explain a lot.


Geo

?

?

?