This weekend I tested a sample of Uranocircite from Andalusia, Spain.
The sample is nice to look at, and shows fluorescence under UV light. So far so good.
As usual the Geiger counter had the first say on the activity, 536 CPM, mostly of them alphas and betas.
I then tested the sample with the PDS 100G. Initial results told me in terms of equivalent dose and counts the sample was giving me about 5% of the background, and with these portable units you can place the sample very close to the crystal, so I knew right away I was in for a challenging measurement.
My new lead shield would have been pretty helpful with this one, but unfortunately the lead is not here yet so I had to make do without it. It will be interesting to repeat this measurement with the lead shield in place to check the improvement.
After recording a 12 hours background I started testing the sample with background subtraction and immediately it was clear I was getting very low counts, less than 6 CPS. Consider my background gives me 254 CPS so we are talking of less than 2.5% of the background.
I really didn’t know what I was going to get, luckily, after 22 hours, the expected U238 progeny peaks are there, but I think I can safely say Th232 progeny is there as well, you can tell by the fact the Bi214 peak is lower than 600 keV, which in my experience can happen only if Th208 is there too, mixing up its gammas with those from Bismuth, unless calibration is pretty bad, but that should not be the case around 600 keV.
Other clues of Th232 presence are the peaks from Ac228, which are not really crystal clear but seem to be there (I didn't label those at 409 and 463 but there something there too), and then there is the peak from Th208 at 2.6 MeV which settles the matter I think. The single escape peak is there as well.
Something funny goes on in the 1400 keV region. There’s a peak there, you could think it’s K40, but it’s centred around 1420 keV, a bit too low.
The background spectrum has K40 centred at 1465 keV, while the background+sample spectrum has the peak at 1452 keV, so the sample peak might be the result of a little excess of counts plus a slight calibration drift, but it’s not entirely convincing. So for now I left that peak unlabelled, which is a bit embarrassing being the peak with the most energy of the whole spectrum.
Repeating the test with the lead shield hopefully will make things clearer.
Spectrum is presented in both counts per Bin and Energy per Bin. In the Counts per Bin version I added the CPS of the highest peak. Yeah, you read right, it's 0.09 CPS.
In terms of exposure on the crystal we are talking about a very small number. You can see there’s small a discrepancy in the quantitative analysis. The exposure from the background is 0.107, that from the background + sample is 0.109 and that from the sample is 0.004, so something doesn’t quite fit. Actually I often have such discrepancies but usually the difference is negligible compared with the absolute value, here the to figures are comparable.
That's all for a very very weak sample, Glad I still pulled something out of it.
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