XRF on a gamma spectrometer!

[bstefanov]

Hello,

I have been trying to squeeze the most out of my beginner CsI(Tl) gamma spectrometer and looking for things to do with old smoke detectors.
I was too impatient to design a proper setup, so I set up a piece of polystyrene foam with a hole at 45 degrees away from the spectrometer and a small area for sample placement on a piece of cardboard above it.
The Am-241 source is pressed into a bullet-shaped lead fishing weight which has exactly the right nest for the source. I wrapped it into a 2mm lead sheet to make a nice shielded cylinder with an opening for the Am-241 59.5keV to get through. It turns out with this set-up, after subtracting the background from the sample-less set-up, I can get nice XRF of the K lines of Ba through Er. Noise limited below Ba and source limited above Er. In-between is OK. Attached is the proof.

There is a little lead left in the spectrum and also the spectrometer makes a lot of noise just under 29keV but all in all not bad for my 3 cubic centimeter CsI crystal...

Boris Stefanov, Hellertown, PA

[bstefanov]

Next, I got excited about checking this on some rocks.
First one shows some Er peaks, OK, possible since it was from some magnetite with fergusonite (YNbO3) with known 3% erbium content. So, I check another - roughly the same effect.
I test a fossil (!) - same thing. Luckily, some of that rusty physics training kicked in as well as some common sense.
1. Conservation of energy, hopefully, says something like E(source) ~ E(XRF) + E(inelastic x-ray scattering)
2. The most common elements in rocks are Si, Fe, O, Ca, K, etc.
Here is when the element collection comes in handy again.
Some numbers, assuming scattering on K electrons:
The case for Fe: 59.5 keV (Am-241) = (6.7+/-0.3) keV XRF + (52.8+/-0.3) keV IXS
And some limit cases (for my spectrometer):
for Mg: 59.5 keV(Am-241) = (1.75+/-0.25) keV XRF + (58.25+/-0.25) keV IXS
for Sb: 59.5 keV(Am-241) = (28+/-2) keV XRF + (31.5+/-2) keV IXS

And indeed:

Fe_IXS.png (15.4 KiB) Viewed 36605 times

Mg_IXS.png (12.95 KiB) Viewed 36605 times

Sb_IXS.png (7.85 KiB) Viewed 36605 times

Fe and Sb are consistent. Mg is a bit off. May be there are some other absorption effects. Also tried it (not shown) on Ag (works) and on Cu and Zr (does not). Cu and Zr must have, like Mg some other absorption going on or maybe there is not much scattering at -135 degrees to detect. Does anyone in the forum have knowledge or experience with this?

So, the good news, is one more effect to play with and ponder, the bad news - the rare-earth XRF using a gamma spectrometer is probably not usable for any analytical purpose.

Cheers,

Boris Stefanov, Hellertown, PA

[luuk]

Hello Boris,
Interesting experiment, but I curious how does you crystal looks is it a disc(picture)? And if so how thick is it?
Can you show a direct spectrum from Am241, because than it is possible to see if your detector can be improved a little or not.
Luuk

[bstefanov]

Hi Luuk,

The crystal is 1x1x3cm.
This is the Am-241 spectrum.


Note that the previous XRF/scattering data as only at 10k pulses.

Boris

Am241.png (68.48 KiB) Viewed 36591 times

[bstefanov]

And here is the scattering off a piece of polycrystalline Si after background subtraction. Basically the same FWHM as the source.

Si_IXS.png (58.74 KiB) Viewed 36589 times

[luuk]

Hello Boris,
To be honest your Am-241 spectrum looks not to good, I did expect to see more peaks from your Am source.
It will be very difficult to do some "serious" xrf with your detector.
Your noise is very high, and that does not help getting a better performance, the lower energies from Am-241 are not visible.
When I am at work on Monday I shall try to made a Am-241 spectrum with CsI(Tl) disc and a pmt to show what is possible with just a simple disc crystal and a pmt.
I think that way it is more easy to make a much better xrf detector, I expect a noise from around 10-12 keV.
Greetings ,
Luuk

[Peter-1]

Hello,
for a comparison I have my Am241 spectrum with a 2 "NaI crystal and a EMI PMT.
Peter

[bstefanov]

OK, here is Am-241 with the expanded range.

Am241_ext.png (82.93 KiB) Viewed 36569 times

[luuk]

Hi guys,
Well I am not sure because I don’t use software mca but only hardware mca but I still think the results should be much better the noise I see now is way to high.
Tomorrow I shall try to do a simple test and let you know the results, I shall make the test simple and show exactly how I did it , I think it will be easy to copy for everyone but you will need a good pmt and a CsI(Tl) preferrable a CsI(Tl) disc crystal.
But I expect it must be much better than what I see now.
Lets wait till tomorrow and talk about the results.
Luuk

[Sparky]

Boris,

I really like scattering experiments with your CsI(Tl) spectrometer. Some folks at the University of Malaysia did something similar with a HPGe detector. They could go to much lower energies where elastic scattering is more of an issue, particularly with the many Np peaks from Am241 decay.

https://inis.iaea.org/collection/NCLCol ... 041560.pdf

Is there any way to modify your detector to go lower than Ba K peaks? My NaI probe can go down to Silver. The attached spectrum is of Am241 excitation of Ag, Cd, and Rum (OK I was playing around).

I would suggest a crazy experiment for you. The detection of Ni, either through xrf or chemically, is used to distinguish iron Meteorites from terrestrial iron minerals (meteorwrongs). Attached is a SiPin spectrum of a Canyon Diablo iron meteorite showing the K lines for Iron and Nickel. The scattering peak around 50 is too broad to be of much use, the SIpin is not sensitive here. If you superimposed a scattering spectrum for a meteorite with mild steel, would there be a enough of a difference in shape between the two spectrums to indicate Ni? I.E. to tell the difference between a meteorite and a meteorwrong?

Mike Loughlin