Normal U spectrum from an unusual specimen

[Geoff]

I recently acquired an unusual specimen of pitchblende and native gold that came from Anna Adit, Mitterberg District, St Johann im Pongau, Salzburg, Austria. Spectrum wise its nothing special, just a really interesting combo.

U Au.PNG (14.44 KiB) Viewed 4636 times

[kotarak]

Truth be told, the combination of Uraninite and Native Gold is not that unusual. In my hobby of collecting Radioactive Minerals, I've encountered this exact combo numerous times and from a variety of localities. I might even have a specimen or two.
Uranium and Thorium tend to accompany REE, platinum group metals and other heavy metals such as Gold and Silver in mineralization. Uraninite is discovered in many gold and silver mines.
A more interesting spectrum would be to look at it with low-energy X-Ray spectrometer. The Gamma coming from the Uranium decay chain serves as an exciter and brings out gold's XRF K and L lines in the spectrum below 100 keV. If your detector has a good low-energy response in that region you should be able to see at least the XRF Ka1 line at 68.7 keV and Kb1 line 77.9 keV (dont use lead shielding in this case as the Pb XRF will mask it). To see the La1 and and Lb1 lines you'll really need a SiPIN detector or similar as they are all way down below 12 keV

[Geoff]

I’m not really set up for xrf but I’ll give it a go.

[Sparky]

Andrey,
I have studied the lower end (< 100 keV) of the Natural Uranium spectrum of a few minerals with both NaI and SiPin detectors. It is interesting to me.

The best NaI spectrum I have done in this region was from a Mosasaur Tooth. I think this one turned out better because the U appears to be concentrated in the thin enamel of the tooth, so the lower energy end has a better chance of being seen than with an ordinary rock. I think the U precipitates directly out of the seawater during fossilization.

Do you have the Gordon Gilmore Gamma Ray Spectrometry book? Appendix C lists x-rays associated with nuclides. I interpret this list as those x-rays most commonly seen with NaI. Most of these are the characteristic x-rays associated with the progeny of a specific decay. I believe the mechanism is that particles or photons exiting the nucleus excite the electron shells on their way out of the now progeny atom. I would describe these as “decay x-rays”. Gilmore also lists some x-rays as “Florescent” which I believe are just incidental x-rays from shielding etc. I think the decay x-rays predominate in most situations.

X-rays from gold in the present case would be of the “Florescent” or incidental variety.

Because the U238 decay chain has beta decays to both lead and bismuth, you will always see a strong combined (NaI) peak in the range 75-85 keV whether or not you are using lead shielding or your Uranium mineral contains lead. The attached SiPin spectrum (no excitation) is of a Uranium mineral (Euxenite?). It shows Ka1 lines for both Pb and Bi, even though the detector efficiency is not good at these energies. This combined peak will interfere with seeing the Au Kb1 peak (77.97 keV) with a NaI detector.
I don’t think there would be much chance of seeing the Au Ka1 line (68.79 keV) with NaI because there is a Th234 gamma peak at 63.3 keV which is difficult to see by itself next to the large combined peak for Pb/Bi x-rays.

Mike Loughlin