Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

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Go-Figure
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Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

Post by Go-Figure » 06 Jan 2020, 08:32

Hello there, happy new year everyone.
As a Christmas present I bought myself a new laptop, so the old one went into retirement as far as work is concerned. That means he (sorry for not calling him “it”) can now stay at home doing gamma spectra when when I am in my office. That wasn’t the case before and that limited me as far as maximum acquisition time was concerned.

Normally a day and a half will suffice, I normally record a 12 hour long background and up to a 24 hour long spectrum, but when the sample is really weak that might be not enough.
Some time ago I tested a sample of soil from north-western Italy sent me by Cicastol where a Cs137 contamination due to Chernobyl is detectable, but I wasn’t very happy with my result. Cs137 peaks where there but not as clear as I hoped and there was a lot of noise pretty much everywhere else.
You can read about the first test here viewtopic.php?f=5&t=667

My obvious conclusion was that testing that sample went beyond the capability of my current shielding. I am still waiting for some more lead which will improve it substantially, but ever since then I’ve been thinking there must have been a way to do better with what I have which is 5 mm of plastic, 1 mm of copper , 4 mm of pewter and 13-15 mm of lead.
01 - DSC06984R.jpg
So I did it again. I tried to assemble the shield more carefully and thank to that my background went from 69 to 64 CPS. Not such a great improvement but still an improvement.
Then I extended the background measurement to 20 hours to smooth it up (in hindsight I should have make it even longer). As usual lead fluorescence is there but pewter (with a little help from copper) does a good job keeping it under control.
02 - Background - 20 Hours - Counts x Bin - Shield V2 - 0.040 Gauss - 27-28 - 12-19.png
02 - Background - 20 Hours - Counts x Bin - Shield V2 - 0.040 Gauss - 27-28 - 12-19.png (70.63 KiB) Viewed 97 times
Then it was time to deal with the sample. I tried to put as much soil as I could inside the “chamber” ad in the end I managed the fit in roughly twice as much as I did the first time around. In order to do so I put the detector vertically, It’s the first time I used this configuration. The result was 9 CPS from the sample versus 6 CPS of my first attempt.
03 - DSC07143R.jpg
And then I decided to leave it there accumulating counts for as long as it would take to have a good improvement on my previous measurement.
After the first 24 hours I extended it to 48, then to 72….in the end the accumulation time was 144 hours, meaning 6 days no stop. If my old laptop could talk he would probably have something to complain about his first week of retirement…
And here’s the result, both in counts per bin and energy per bin.
04 - Piemonte Soil - 144 Hours - ID - BG Subtraction - C x B - Graded Shield V2 - 0.040 Clean - 28-12-19 - 03-01-20.png
05 - Piemonte Soil - 144 Hours - ID - BG Subtraction - E x B - Graded Shield V2 - 0.040 Clean - 28-12-19 - 03-01-20.png
Now I am not going to say this is an exceptionally good looking spectrum, but if you compare it with the one I got back in October it’s a good improvement.
Cs137 gamma peak at 661.67 keV is much more prominent, and the X-Rays peak at 32 keV is now very visible (while last time there was merely a hint of it).
K40’s peak is a bit south of where it was supposed to be but still pretty clear, it’s roughly as strong as the Cs137 gamma peak.
There are then the other expected peaks from U238 and Th232 decay chain (actually it looks to me it's more Thorium than Uranium), albeit some of them are not very clear without the gaussian correlation on screen.
And then I have two peaks I haven’t labeled yet, one in the region of 70-72 keV and the other between 95 and 100 keV. The former could be from Lead, I am not entirely convinced but at the moment I have no better explanation. For the latter I am a bit in the dark right now. For a moment I wondered if they could be Th234 at 63 and 93 keV but it seems unlikely to me, plus calibration would be underestimating the energy of the first peak and overestimate that of the second.

Anyway, the goal was to detect Cs137 better than last time and this time both peaks are unmistakable.

And finally this is the quantitative recap of the measurement.
06 - Spectrum Analysis @Contact+1-001.jpg


I will still repeat this measurement with the improved shield…..sooner or later.
That’s it.
Until next time.
Massimo

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Steven Sesselmann
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Re: Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

Post by Steven Sesselmann » 06 Jan 2020, 10:11

Massimo,

Happy New year and thanks for posting, good work as always..

Steven

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Re: Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

Post by Sparky » 07 Jan 2020, 12:53

Massimo,

Very nice spectrums. Someday I hope to go "fallout hunting" in New Mexico and Nevada and this is interesting to me.

How do you contend with thermal drift with these long duration surveys?
With Theremino, It is difficult for me to do long superposition plots without drifting. I have gone to extreme methods to combat such drift (putting my HV supply and pulse amplifier on ice). This method works OK, but it is really too much trouble and maybe not good for the equipment.

Mike Loughlin
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Steven Sesselmann
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Re: Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

Post by Steven Sesselmann » 07 Jan 2020, 15:30

Sparky wrote:
07 Jan 2020, 12:53
With Theremino, It is difficult for me to do long superposition plots without drifting. I have gone to extreme methods to combat such drift (putting my HV supply and pulse amplifier on ice). This method works OK, but it is really too much trouble and maybe not good for the equipment.
Mike,

I don't know much about the theremino, but all my GS-PRO models have an MCP1541 precision voltage regulator which holds the voltage stable over a large range.

This leaves only the crystal and PMT both of which also are affected by temperature drift, your ice bucket method might well be a good solution, but anyone attempting this should take care, because the NaI(Tl) crystal is not only hygroscopic but can also crack if the temperature change is too rapid. Max 1C˚ per minute is a figure I have heard.

Steven

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Go-Figure
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Re: Chernobyl (Cs137) Contamination in Italian Soil - 6 Days Gamma Spectrum

Post by Go-Figure » 08 Jan 2020, 08:27

I confirm what Steven says, calibration held very stable over the whole acquisition time (which, including the background acquisition, was a week).
You can see it's a bit off when it gets to K40 but it was like that right from the start. Elsewhere it's pretty good, Cs137 peak is at 662 keV, right on the dot.
Actually there's an interesting thing about the K40 peak I omitted: In the spectrum with the background unsubtracted the K40 peak is at 1452 keV, and in the background itself it's around 1454 keV (an indication that calibration held stable for the whole week) but the counts excess left once the background was subtracted was centered at 1425-1430 keV.
I don't have an explanation for that, but it has nothing to do with calibration.

Temperature in the room where I do my measurements is pretty stable, it can go a bit up and down with the day/night cycle, but if acquisition time is a whole multiple of 24 hours such small thermal fluctuations tend to calcel out.

Massimo

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