Greetings from Pennsylvania!

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bstefanov
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Greetings from Pennsylvania!

Post by bstefanov » 27 Jul 2021, 07:46

Hi,

My name is Boris Stefanov and I live in Eastern Pennsylvania, USA. Originally from Bulgaria. Studied Physics but too long ago and only basic nuclear physics.

I come into gamma spectroscopy from an interest in geology and minerals. I found a few mildly radioactive minerals in a nearby quarry. They are in a metamorphic rock, so they are a bit more difficult to identify. I spent a lot of time with a handheld dosimeter trying to identify and separate the parts that were more active, finally identifying the sources. Based on some rough estimates of activity, some seemed more U, others - more Th. What I found also seemed to contradict what was previously published about this quarry about U:Th ratios based on my very rough estimates.

So, I bought an entry level small CsI(Tl) gamma spectrometer and have been playing with it for a while. Even an uncalibrated measurement of the minerals from the quarry confirmed my estimates which gave me a solid ego boost :)
I have now the system calibrated with Co-60, CS-137, and the Tl-208 2614keV peak and it is humming. Added a few mm of lead shielding (5x reduction of background at the expense of a little added lead x-ray). Maybe I will improve it in the future with thicker lead, cadmium, and copper, but I expect diminishing returns for this specific device. For now, with background subtraction, it behaves very well.

Tested with Am-241, K-40, Lu-176, Ra-226, Th-232, U-238. All nice and clean and agree with literature. I will probably also try La-138 after getting a larger La chunk.

So, now my goal is to get to a quantitative estimation of U:Th ratios based on spectral data. It looks doable so far with the caveats that I need to make sure the samples have not lost Rn (or Ra by dissolution) and are small enough not to have any self-shielding. I am hoping for about 5-10% accuracy. I know most of the methods in use are based on 1764 vs 2614 keV measurements but with my detector, this is definitely not doable. I am working on 294&352 vs 238 keV comparisons.

Has anyone in the forum tried something like this?

Boris Stefanov, Hellertown, PA

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Sesselmann
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Re: Greetings from Pennsylvania!

Post by Sesselmann » 27 Jul 2021, 07:54

Boris,

Welcome to the forum and thanks for your post.

Quantitative analysis of natural minerals is not my expertise, but someone here on the forum may have some suggestions for you. The general approach is to use a calibration source of known activity for comparison. Working out the count rate from first principles involves many factors which will have a larger or lesser affect on the result. Somewhere here I recall a paper detailing all the factors, I can't recall where it is. Will post if I find it.

Cheers..

Steven

bstefanov
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Re: Greetings from Pennsylvania!

Post by bstefanov » 28 Jul 2021, 02:06

Hi Steven,

Thank you for the welcome and for having this blog.
Can you recommend a good (gamma) spectrum analysis software (preferably free)? I am using PRA for acquisition and then looking at the histograms in excel. I could probably write some analysis code myself but would rather use something that is already available and tested.

Be well,

Boris Stefanov, Hellertown, PA

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Sesselmann
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Re: Greetings from Pennsylvania!

Post by Sesselmann » 28 Jul 2021, 08:09

Boris,

If you are looking for a good post analysis program for your spectra, check out Interspec, and yes it's free.

https://www.gammaspectacular.com/blue/s ... c-software

Steven

bstefanov
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Re: Greetings from Pennsylvania!

Post by bstefanov » 28 Jul 2021, 22:06

Hi Steven,

I started using InterSpec. It is indeed very good. The peak fitting is a bit shaky, especially on close or overlapping peaks. I also wish it had a way to save a file with all the analysis. But the isotope data in there is very useful and it is generally a great program.

Thanks for the advice,

Boris

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ColoRad-o
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Re: Greetings from Pennsylvania!

Post by ColoRad-o » 06 Aug 2021, 14:49

Hi Boris,
I too am using InterSpec with the intention of extracting quantitative information. As Steven noted, it will be necessary to get an 'internal efficiency' curve for your detector system, using reasonably well-calibrated sources. (You may be more familiar with the process than I--if so , my apologies.) This isn't easy because well-calibrated sources are expensive. You may know that InterSpec comes with some 'detector response function' files for a number of detectors but, alas, no CsI(Tl). While I find good correlation between tabulated (aged) branching ratios and peak counts from InterSpec, it's clear that the energy dependence of the detector is very important. Here's an example of a 232Th decay chain spectrum (lantern mantle from the 1980s).
Where do you live in eastern Pennsylvania? --I grew up in the Philadelphia area...

Best wishes,
DMW
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D. M. Wood, retired physics professor
Arvada, Colorado (USA)
SAFECAST member (bGeigie Nano)

bstefanov
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Re: Greetings from Pennsylvania!

Post by bstefanov » 11 Aug 2021, 03:20

Hi David,

Thank you for the reply and advice.

I was thinking I could avoid worrying about efficiency issues by constraining the problem as follows:
1. Only estimate U:Th ratios on the basis of 238:(295+352)keV emissions
2. Small samples (avoid self-shielding and geometry issues)
3. Old samples (>10^6 years old, to ensure Ra226 equilibrium)
4. Use Ra226 and at least a few years old thorium as check limit cases
5. Un-weathered, dense samples to avoid Ra-226 leaching issues.

Is that a reasonable approach in your opinion?

Best regards,

Boris

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ColoRad-o
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Re: Uranium/thorium

Post by ColoRad-o » 11 Aug 2021, 07:26

Hi Boris,

My meager understanding of, for example, uranium-thorium DATING is that "it calculates an age from the degree to which secular equilibrium has been restored between the radioactive isotope thorium-230 and its radioactive parent uranium-234 within a sample.' (*Wikipedia). Thus all measurements are of nuclides within a specific decay series (238U).

Thus I'm curious about precisely which isotopes you have in mind--I presume you'd like to estimate the ratio 238U to 232Th?

According to NIST measurements, around here about 2 pCi/g of soil radioactivity comes from 232Th itself and about 1 pCi/g from 238U. Thus BOTH decay chains are important in understanding soil radioactivity. Their measurements of the daughters of each isotope suggest both series are in secular eequilibrium. I mention this because you have to be specific about which U and Th isotopes you're interested in.

DMW
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D. M. Wood, retired physics professor
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bstefanov
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Re: Greetings from Pennsylvania!

Post by bstefanov » 11 Aug 2021, 09:45

Hi David,

No, I am not interested in dating. That would be difficult (at least with my equipment), since most of the gamma spectrum of old uranium rocks is Ra-226 (from U-235). U-238 is practically invisible, except in rocks that have the Ra-226 leached.
What I would like to know is if a mineral is a uranium mineral, a thorium mineral, or if in-between, then what is the approximate ratio. For example, is it uraninite, thorite, uranothorite, thorian uraninite, etc. So, essentially, it ends up being U235:Th232 in gamma spectra.0

So, I assume that U235:U238 is roughly constant in old rocks. Then use the Ra-226+daughters (Pb-214,Bi-214) as indicator of overall uranium content. The Th-232 daughters are different (Ac-228, Bi-212, Pb-212, Po-212, Ra-224, Tl-208) , so there is no issue.

Boris

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ColoRad-o
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Re: Greetings from Pennsylvania!

Post by ColoRad-o » 11 Aug 2021, 11:17

Got it. Thanks.
D. M. Wood, retired physics professor
Arvada, Colorado (USA)
SAFECAST member (bGeigie Nano)

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