Autunite from Nevada

[Go-Figure]

Hello there!
It’s been a while since my last post, but I used the lockdown here in Italy, which ended this past week (albeit the emergency is far from over), to record a very long Trinitite spectrum which I am going to write about in the coming days, there’s still one thing I need to check before posting it.
That’s an easier one, an Autunite sample. It’s pretty small, I can’t tell you the exact weight because it’s fixed to its little box, but it must be less than 5 grams.
It’s small but beautiful, and of course it glows under UV light, which is always a plus.

While the GS 2’’x2’’ was busy with Trinitite in a 28 days measurement (first a 7 days background, then 14 days with the sample, and finally more 7 days background) I first tested the sample with my Ranger and then with the PDS.
The Ranger gave me a respectable 3000 CPM.

The PDS told me the amount of gamma radiation from the sample was pretty low, my background is around 0.10 µSv/h and highest read I got at contact was 0.16-0.17 µSv/h, background included.
Over a 9 hour time the average was 0.14 µSv/h (always including background). In the spectrum from the PDS you cannot really tell the Autunite contribution from the background because peaks are the same, what you can see is that it makes all of them a bit more “bumpy”.

All this happened on April 4, and then I had to wait until last Friday, May 8 for the next step. Since I just finished accumulating the background for the Trinitite measurement I decided to use that as a background for this measurement as well. Calibration was several weeks old and some thermal drift was inevitable but as you can see below it held pretty well, it’s a bit off only at high energies.
I wanted this to be a shielded measurement because I will repeat it with no shield in order to see the difference and understand how much the XRF from the shielding is contributing to the 77 keV peak.
As usual I first recorded a 24 hours background. My shielding was basically the same as I my last experiments, 5 mm of plastic, 1 mm of copper, 4 mm of pewter and 13-15 mm of lead. My shield improvement has been postponed for obvious reasons.

So here’s the background spectrum accumulated in two distinct 7 day sessions for a total of two weeks. My shielded background was 61 CPS, one of my best result with this setup.

08 - Background - 336 Hours - Counts x Bin - Shield V2-1 - 0.036 Gauss - 31-03_07-04+01-05_8-05-20.png (20.49 KiB) Viewed 8736 times

And here’s the result. Accumulation time with the sample was 40 hours.

The sample gave me roughly 43 CPS on average, which should be good enough to repeat the measurement with no shield and a simple background subtraction. Nothing strange for an uranium ore, the 77 keV peak is the highest in terms of counts, and this is often the case regardless, but it remains to be seen how much the fluorescence from shielding contributed to it.
Peaks from U235 are well visible, while those from Th234 are a bit overshadowed by the 77 keV peak, but still clearly there (and the gaussian correlations are unmistakable).

The K40 peak is a bit off, but again, the calibration for this measurement was done about a month ago, so I wouldn’t complain! And there’s more to say about that, but that’s for the next post.

Last, probably the most interesting peak of the spectrum is that at 46 keV which is clearly Pb210, it must be only the second or third time that I “catch it” in one of my spectra, so I was happy it showed up!
A few more bumps are visible at lower energies but for now I decided to leave them unlabeled.

Quantitative analysis summarizes what’s above.

In the end it was just another uranium ore, but there’s always something interesting to look for and this one in particular will have another close encounter with my scintillator in the coming days!

Hope you are all well.
Until next time.

Massimo

[Svilen]

Hi Massimo,
Thank you for sharing this measurement. You may have also Bi-214 at 935keV visible, but you said you want to leave the other bumps unlabeled, so you may have a reason. Is the unlabeled bump before Bi-214 at 609keV from Compton interactions?

Greetings,

[Go-Figure]

Hi Massimo,
Thank you for sharing this measurement. You may have also Bi-214 at 935keV visible, but you said you want to leave the other bumps unlabeled, so you may have a reason. Is the unlabeled bump before Bi-214 at 609keV from Compton interactions?

Greetings,

Yeah, Bi214 at 934.06 keV is definitely there and the label should have been there too.
I not always put the compton edge label, also around 500 keV there's the annihilation peak (511), but the whole zone is a bit "blurred"

The unshielded spectrum is almost done and I'll be able to post it shortly.

Massimo

[Go-Figure]

Hello there,
This is the second post about my “Autunite from Nevada” test.
Why doing another post about it? Because I wanted to do a comparison of a shielded and an unshielded measurement of the same sample, both with background subtraction.

When I started doing gamma spectroscopy it took me about six months to set up a decent lead shielding, it was a limitation but I was mostly testing pretty active samples so generally the result didn’t suffer too much because of that.
And there was a plus, when I had a x-rays peak around 77-78 keV (which not only happened all the time when there was Uranium progeny involved, whether it was an Uranium ore, pure background or rain, but it also was consistently the most prominent peak of the whole spectrum in terms of counts) I knew it was the result of lead in the sample, not XRF from the shielding.
Before starting doing gamma spectroscopy on my own I remember checking spectra online and often the 77-78 keV peak was just labelled as XRF from the shielding, but operating with no shield whatsoever for months allowed me to see that when there’s uranium involved that peak is pretty much there anyway, fluorescence from shielding merely contributes to it, but how much? That’s the question I want to start to address with this test.

So I did something I had in mind for a while, checking the difference between a shielded and an unshielded measurement with background subtraction of an uranium sample in order to see how much XRF from the shielding was contributing to the 77-78 keV peak of this particular sample.

The shielded spectrum is in the first post, here I show results from the unshielded test.

So here’s the setup, the only part of my shielding left in place is the plastic tube, merely to help keeping the scintillator upright. As you can see there’s nothing at the bottom, just paper.

First, the background, 48 hours of unshielded background, which for this test was 261 CPS, a few CPS up compared with the 255-257 I usually get in my “experiments room”. In 48 hours I accumulated 45 million counts and a pretty smooth spectrum.

03 - Background - 48 Hours - Counts x Bin - NO Shield - 0.036 Gauss - 11_12-05-20+14_15-05-20.png (59.87 KiB) Viewed 8630 times

And then I put the sample back in and repeated the measurement above with no shielding in place for 48 hours.

And here’s the spectrum both in counts per bin and energy per bin. Unsurprisingly it’s pretty much the same as above in terms of peaks, with a few small differences I am not going to focus on here because this is not the point of the post.

The sample gave me a little less than 39 CPS versus nearly 43 CPS I got in the shielded measurement, which is roughly a 10% difference.

So where is this difference coming from? Is it going to be all from the XRF?
Actually if you compare the centroid CPS of each peak of the two spectra you see the difference is pretty much spread out across the peaks, particularly in the low energy peaks.
The 77 keV peak centroid of the shielded spectrum has 22.34% more CPS than in the unshielded one, but all low-medium energy peaks around it show a similar difference, fluctuating between a +17% and a +24%. So in the low energy zone of the spectrum the ratio between peaks doesn’t chance that much.
Moving above 200 keV things get a bit less clear, you still have a +15% for the Pb214 peak at 353 keV, but other peaks show little to no difference.

We are talking about two different experiments, so no matter how hard you try you will never get the exact same result, even trying your best to keep things the same, as soon as you take the sample away and them put it back the geometry will never be exactly the same.
All considering I would say results from this specific test with this specific sample show that XRF from lead shielding contribution to the 77-78 keV peak is not really anything you can notice at first glance once background is removed and even looking at the numbers the difference appears to be comparable with the margin of error (or margin of uncertainty) that you can expect when trying to recreate the same setup twice.

I will repeat this comparison with different samples, this time without removing and putting back the sample but merely removing the shielding.

That's it for now. Have a nice Sunday and stay safe.

Massimo

[Svilen]

Nice study and interesting to know, Massimo. I also thought the XRF contribution is greater.

Greetings,

[Sparky]

Massimo,

That is a very interesting table. I think most of the increase on the low end is back-scatter noise, in fact I believe the extra increase at 143 is a 180 degree reflection from the 352 keV Pb214 peak. The calculator below puts this back-scatter peak at around 148 keV. The spectrum attached is of a Radium watch hand, with and without a back-scatter target (which happens to be the bottom of my shield). It mimics a pretty good peak for U235 (144 keV), but there is no U235.

I think back-scatter explains why sometimes a weaker source give a better spectrum.

http://www.sciencecalculators.org/nucle ... cattering/

[Go-Figure]

Nice study and interesting to know, Massimo. I also thought the XRF contribution is greater.

The 5 mm of pewter added to my shielding really do a good job in limiting the incidence of XRF and I am in the process to add some more.

I think most of the increase on the low end is back-scatter noise, in fact I believe the extra increase at 143 is a 180 degree reflection from the 352 keV Pb214 peak. The calculator below puts this back-scatter peak at around 148 keV. The spectrum attached is of a Radium watch hand, with and without a back-scatter target (which happens to be the bottom of my shield). It mimics a pretty good peak for U235 (144 keV), but there is no U235.

That's a really interesting comparison indeed.
In my case the counts increase extends all the way to Pb210's peak and it's still detectable in the 350 keV Pb214 peak so it's a bit less straightforward.

Massimo