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Fukushima Soil

Posted: 15 May 2019, 23:20
by Go-Figure
Here it is my spectrum of a 1.5 gram sample of Fukushima soil from a few weeks after the incident.
This is an unshielded test with background subtraction. The Cs 134 gamma peak is clearly visible although reduced to roughly 6% its of original activity after eight years.
Both expected Cs 137 peaks are there.
There's an hint of K40, barely visible in the count per bin spectrum, a bit more spottable in the energy per bin one. I attach both below.

I really want to thank Luuk for the opportunity to test this 2011 sample, something I never thought would be possible only a couple of weeks ago.

The test was done last night, I only had 12 hours between 8 PM yesterday and 8 Am this morning, and having no lead shield (yet) I always have to take a very long background before testing the sample, so a 7 hours measure was the best I coud do. Anyway this gives me the chance to see how the peaks from the sample compares with the background ones.
I will certainly repeat the test over the weekend over a longer period of time in order to have a smoother spectrum in the compton edge/back scatter peak region.

Massimo

Re: Fukushima Soil

Posted: 16 May 2019, 03:24
by luuk
Hi Massimo,
Thanks for posting the spectrum it looks good even without any shielding, you can still see on the left slope of the Cs-137(662KeV), a part of the Cs134 569KeV.
What is the resolution of your detector at 662KeV?
Luuk

Re: Fukushima Soil

Posted: 16 May 2019, 05:23
by pilgrim
Hi Massimo, for a comparison here https://www.gammaspectacular.com/phpBB3 ... ?f=5&t=414 there is another thread about Fukushima soil samples.

Re: Fukushima Soil

Posted: 16 May 2019, 07:47
by Steven Sesselmann
Very nice...

In this case would it be possible to use the ratio of Cs137/Cs134 to accurately date the soil sample?

Do we know the initial state ?

Steven

Re: Fukushima Soil

Posted: 16 May 2019, 08:28
by Go-Figure
Steven Sesselmann wrote:
16 May 2019, 07:47
Very nice...

In this case would it be possible to use the ratio of Cs137/Cs134 to accurately date the soil sample?

Do we know the initial state ?

Steven
I think we can ask Luuk about the initial state, he probably was in a position to test samples like this not long after the incident.
Thinking about it there's also another person I could try to ask the same question.

I was calculating that March 11, 2011 was 2988 days ago, which is 8.186 years.
If I am not mistaken since the half-lives of Cs134 and Cs137 are 2.06 and 30.17 years respectively, the ratio between the two today should be 12.93 times the ratio it was 8.186 years ago, with Cs134 reduced to 6.4% of its original activity and Cs137 reduced to 82.8% of its original activity.

I am not sure how to see the total number of counts for each ROI, I only visualise the counts per each bin. If I take the peaks' centre I read 13917 counts for Cs137 and 933 counts for Cs134. So the present ratio between the counts at the centre of the peaks (according to this particular test) is 14.92.
But I doubt I can use the the counts of the centre of each peak as a comprehensive indicator of present activities.
pilgrim wrote:
16 May 2019, 05:23
Hi Massimo, for a comparison here https://www.gammaspectacular.com/phpBB3 ... ?f=5&t=414 there is another thread about Fukushima soil samples.
Yeah, I checked your spectrum while I was taking mine, just in order to see how I was doing!
luuk wrote:
16 May 2019, 03:24
Hi Massimo,
Thanks for posting the spectrum it looks good even without any shielding, you can still see on the left slope of the Cs-137(662KeV), a part of the Cs134 569KeV.
Actually in the begninning it looked like a peak was starting to form around 600 keV. I never tested Cs134 before but I thought it could be the 604.72 keV peak. In the end it disappeared pretty soon.
What is the resolution of your detector at 662KeV?
Luuk
I read 6.149 %.

By the way, below you can see the spectrum without the background subtraction, so it's visible the "importance" of Cs 134 and Cs 137 peaks compared with background peaks, which are the usual ones, Pb214 and Bi214 with the latter barely visible at 609 keV given the proximity to Cs137 gamma peak.

Re: Fukushima Soil

Posted: 16 May 2019, 09:35
by Steven Sesselmann
Massimo,

In PRA you can read the total net counts in the ROI. To select the R.O.I. you select a bin to the left of your peak and press the "b" key (beginning) then select a bin to the right of your peak and press "e" for end. The ROI should show in yellow. Now along the top of your window you should be able to see the Gross and Net counts in the ROI. the gross counts are the total counts in the highlighted region and the Net counts should be the Gross counts less the baseline counts (imagine drawing a straight line between the first and last bin in the ROI.
ROI.png
Regions of Interest
ROI.png (82.12 KiB) Viewed 317 times

Taking this a step further, the formula for half life is,

\[ N_t = N_0 (\frac{1}{2})^{t/t_h} \]

Where N(t) is the amount of material you have now and N(0) is the amount of material at t zero. Since the half life of Cs134 is 2.06 years, you can take the net counts in your Cs134 peak as N(t) and work out what the count rate would have been when it was released 8.2 years ago, if we assume the count rate of your sample today is 1 cps, then;

\[ 1 cps = X (\frac{1}{2})^{8.2y/2.06y} = 15.78 cps \]

So the intensity of Cs134 has dropped to 1/16 whereas the Cs137 count rate has only fallen by 1/1.2.

Hope I got that right, let me know if I didn't :p

Steven

Re: Fukushima Soil

Posted: 17 May 2019, 00:26
by Go-Figure
Steven Sesselmann wrote:
16 May 2019, 09:35
Massimo,

In PRA you can read the total net counts in the ROI. To select the R.O.I. you select a bin to the left of your peak and press the "b" key (beginning) then select a bin to the right of your peak and press "e" for end. The ROI should show in yellow. Now along the top of your window you should be able to see the Gross and Net counts in the ROI. the gross counts are the total counts in the highlighted region and the Net counts should be the Gross counts less the baseline counts (imagine drawing a straight line between the first and last bin in the ROI.

ROI.png


Taking this a step further, the formula for half life is,

\[ N_t = N_0 (\frac{1}{2})^{t/t_h} \]

Where N(t) is the amount of material you have now and N(0) is the amount of material at t zero. Since the half life of Cs134 is 2.06 years, you can take the net counts in your Cs134 peak as N(t) and work out what the count rate would have been when it was released 8.2 years ago, if we assume the count rate of your sample today is 1 cps, then;

\[ 1 cps = X (\frac{1}{2})^{8.2y/2.06y} = 15.78 cps \]

So the intensity of Cs134 has dropped to 1/16 whereas the Cs137 count rate has only fallen by 1/1.2.

Hope I got that right, let me know if I didn't :p

Steven
Steven,
The tiny difference between your result and mine comes from approximations.

The formula for explonential decay is the same of course.
Considering 8.2 years, as you did, you got that the activity of Cs134 dropped by a factor 15.78 and that of Cs137 dropped by a factor 1.207. So after 8.2 years the ratio between the two (Cs137/Cs134) is 13.07 of what was originally.

Considering 8.186 year (a difference of five days compared with 8.2 years), as I did, the activity drop for Cs134 is 15.60 times while for Cs137 it stays at 1.207 (actually 1.2069) which gives the number 12.93 I mentioned above as the ratio change factor.

Thanks for the clarification on how to get the total number of counts inside a ROI. It's been in front of my very eyes every time I highlightened a ROI but I never saw it! I will check it this evening when I am at home.

Massimo

Re: Fukushima Soil

Posted: 17 May 2019, 18:32
by elemental
Is it possible to get a sample of Fukushima soil in Australia?

Gary
Willunga south

Re: Fukushima Soil

Posted: 18 May 2019, 00:05
by luuk
Hi Massimo,
I just see a dumb mistake that I made, but you already noticed it the low Cs-134 peak is not 569KeV but 605KeV just visible on the left edge from the 662KeV.
Don't know why I came up with 569KeV I must be getting old I guess, or just a little to much Belgian trappiest beer :-)
Luuk

Re: Fukushima Soil

Posted: 18 May 2019, 00:25
by luuk
Almost forgot but here is a link to the location were the soil was harvest.

https://www.google.com/maps/place/37%C2 ... 140.750785

Luuk