Uranocircite from Andalusia, Spain. With Shielding This Time

[Go-Figure]

Hello there,
In the past few days a gave a “better look” to a sample I tested back in September when I still had no shield whatsoever, it’s Uranocircite from Andalusia, Spain. You can read about that first test here viewtopic.php?f=5&t=633

The sample is very weak, therefore an unshielded measurement was always going to give an unsatisfactory result and I long planned to give it another go.

Incidentally this was my last measurement with version 2-1 of my lead shield: 5 mm of plastic, 1 mm of copper, 4 mm of pewter and 13-15 mm of lead, plus 2 mm to close the “chamber”.

Version 3 will have more lead, but before doing that I upgraded it with 4 more mm of pewter which will allow me to further reduce fluorescence from shielding, I did some preliminary test and it looked pretty promising, but that’s for another time.
So here’s my shielded background, little less than 62 CPS. Accumulation time was 4 days, split into two 2 day “stints”.

04 - Background - Stanza Fondo - 96 Hours - Shield V2-1 - 22_23 + 29-05_01-06-20.png (42.84 KiB) Viewed 4514 times

Compared with my first attempts not only I added the lead shielding and extended accumulation time (4 days background + 5 days with the sample) but I also improved geometry, the vertical setup allowed me to increase the counts I got from the sample significantly, from less than 6 CPS to more than 9.
So it’s no surprise that the result looks a lot better than my first attempt. The measurement confirmed the presence of Th232 decay progeny as well as U238, and of course K40 always comes along with them.
Peaks from Th232 looks weaker than those from Uranium so I only added the Th232 labels where it seems their contribution was more significant, I should have probably added one or two more, but even like this the spectrum is pretty crowded.

Given the abundance of peaks I focused on energies below 1800 keV, the single escape peak and the Th208 peak at 2.6 MeV was there as well.
The better quality of the result allowed me to identify a peak that completely eluded me last time: Pb210 at 46 keV. There’s something else south of that, a 30-35 keV peak which showed up several times when dealing with Uranium but I don’t know what it is yet, I can’t find a decent candidate anywhere at the moment. I also thought it could be some back scatter peak, but the truth is I really don’t know at the moment.

And here’s the quantitative analysis.

The total duration of the test was 9 days, in hindsight maybe the sample deserved better, so who knows, maybe one day with the further improved shield…

Massimo

[Mike S]

Massimo,

That's a very cool sample and a nice spectrum.

I'm still learning all this stuff, but I had a couple thoughts and questions:

1. Are you sure the peak you attribute to your lead shield is actually from your shield? You seem to have adequate absorption for lead x-rays with 1mm copper and 4 mm pewter. I have 2mm copper and 3mm pewter, and I see essentially zero lead x-rays in my background (I don't think the extra 1mm of copper makes the difference, your extra 1mm of pewter is more effective). I do have a similar looking peak, but it is at a lower energy, around 29 keV that threw me off for a while. I now believe it is from the x-rays generated by iodine in the detector crystal. I only started seeing it when my shield was built-up enough to be effective. The attached picture shows how the peak looks on my shielded background.

2. Just after I made my first basic shield with just a 2mm copper liner, I started testing uranium ore samples and was very disappointed at the huge lead x-ray peak I saw in the spectrum. I thought the gammas from the sample were generating x-rays in the shield. That may have been true to a small extent, but it turned out that the largest source was x-rays from the lead in the sample itself.

3. Maybe I missed it, but what is your unshielded count rate with your 2x2 detector? I see about 135 counts per second unshielded with my 2x2" NaI. My current shield gets that down to about 16 cps, but even early basic shields were in the 25-30 cps range. I'm wondering if you have a naturally high background.

Thanks again for sharing!

Mike S.

[Go-Figure]

Hi Mike,
You are absolutely right, if you read this thread (viewtopic.php?f=5&t=795) you will see I did both a shielded and unshlieded measurement of the same sample in order to try to address this very issue: how much of the 77 keV peak was from the lead shiedling.

I did enough unshielded measurements to know that peak was going to be there regardless of the shielding, in my experience that is invariably the most prominent peak in terms of counts every time you test an uranium sample and it comes from the sample. But of course the shielding adds something to it, that's why in this and other shielded measurements I labeled that peak as Pb214 X-Rays from the sample + fluorescence from the shielding, because both contribute to it.

The normal background in the room when I do my measurement is 255-260 CPS with my 2''x2''. The latest version of my shielding which I just slightly improved (not adding more lead, which I still have to do, but adding more pewter in order to further reduce fluorescence) reduces it to about 59 CPS.

Massimo