Hi guys
Ever since I got going with gamma spectrometry a few years ago, the aim has been to detect and quantify Cs137 in soil and other environmental samples. As I live in Sweden, I have the "fortune" of having acess to areas which got hit fairly bad from the Chernobyl accident.
My first "run-in" with Chernobyl fallout was with soil samples from the city of Gävle, where my wife's family is from. In a small forest area, I simply scooped up around 2 L of topsoil, dried it, and measured it with the detector I had at the time; a 3x3 " NaI detector. The 662 keV peak jumped out in around 10 seconds. Wow. For this detector setup, I also had a soil sample which was measured at a national reference lab, so I could get a good estimate of the Cs137 activity.
However, the 3x3" NaI was an excellent probe with great resolution but I could not provide enough lead shielding for it, and BG became a real issue for low(er) activity samples. So sadly, the 3x3" found a new loving home.
Now, thanks to the "lord of the Scintillators", Luuk Timmermann I got the opportunity to purchase a 2x2" well crystal. This meant two things; A: Sample geometry is optimized and B: It would fit my reinforced lead castle (See picture) which increased the sensitivity of the whole setup.
As before, I use a GS 2000-Pro and BeqMoni. By buying refurbished computer parts, I got a dedicated lab computer setup for well under 100€, and this I can recommend as a nice way to get dedicated equipment and be environmentally friendly. The lead castle consists of a stainless steel "bucket" into which lead has been poured, to a thickness of about 3 cm. Into this primary castle I could now fit what previosuly was my first lead castle, a 2 cm lead "roll" around a copper pipe. So effective lead shield thickness is now about 5 cm.
I am trying to give a good picture of the setup here, because I believe this will be important for the following posts on the measurement results as I realize it is interesting to see what can be detected using the tools we amateurs have at our disposal. Of course, the shielding is an integral part of the setup. I also know Steven gets quite a few inquirys about levels of detection in our setups, so this thread is an attempt at showing what can be done, and how it has been done. If people have objections or suggestions, feel free to put them forward as I am by no means an expert in the field and am most open to learning new things that will progress my efforts.
/Karl
Hunting Cs137
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brehwens
- Posts: 89
- Joined: 27 Apr 2015, 03:49
Hunting Cs137
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brehwens
- Posts: 89
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Re: Hunting Cs137
OK, so first out to take the field are two samples in the 2000-4000 Bq/kg activity range. One of these samples (Gävle soil) comes from the contaminated place in mid sweden. I am quite sure of the activity in this sample, as I have analyzed it previously with one of my other probes which I had "calibrated" against a sample of known activity, that had been measured at a swedish reference laboratory.
The other sample is an ash sample from Finland, where Cs137 has been enriched in the ash. Finally, if one looks at the Y-axis, the total number of counts in the 662 keV peak for the ash, it is around twice of that for the Gävle soil sample (and the counting time is 10 h for both) so around 4000 Bq/kg is a reasonable guesstimation.
The samples are in standard plastic scintillation vials with a volume of 20 ml, meaning they hold around 4-12 g of soil (depending on the density)
Gävle soil: Finnish ash sample: For both these samples, the 662 keV peak is clear and also the 32 keV characteristic X-ray. I would say that I can fairly easily detect Cs137 in samples of this activity magnitude. Note here that I differentiate "detect" and "quantify". Again, we are looking at less than 10 g of sample here.
The other sample is an ash sample from Finland, where Cs137 has been enriched in the ash. Finally, if one looks at the Y-axis, the total number of counts in the 662 keV peak for the ash, it is around twice of that for the Gävle soil sample (and the counting time is 10 h for both) so around 4000 Bq/kg is a reasonable guesstimation.
The samples are in standard plastic scintillation vials with a volume of 20 ml, meaning they hold around 4-12 g of soil (depending on the density)
Gävle soil: Finnish ash sample: For both these samples, the 662 keV peak is clear and also the 32 keV characteristic X-ray. I would say that I can fairly easily detect Cs137 in samples of this activity magnitude. Note here that I differentiate "detect" and "quantify". Again, we are looking at less than 10 g of sample here.
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brehwens
- Posts: 89
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Re: Hunting Cs137
Ok, increasing the stakes... these are two samples which both have been measured at a swedish referene laboratory. The first sample, "Finnish soil" has been measured to 253 Bq/kg:
As you can see, the 662 keV peak barely makes it above the BG sample. I should point out that from all of these samples, I have used soil from my garden patch. This soil has recently been put there, and mainly consists of compost material, and the soil is bought in bulk.
You can also see other things going on in the spectrum, and this is partly due to the fact that I have to some extent Radon in my basement (prominent 238 keV peak). So, things are getting complicated at this level of activity...
Next is a french soil sample, which clocks in at about 150 Bq/kg...
Again, it starts to become tricky regarding what is really above BG or not. Note here that I am measuring for 10 h, I have not done more extended analysis on these samples but it appears the biggest enemy is the background here...
As you can see, the 662 keV peak barely makes it above the BG sample. I should point out that from all of these samples, I have used soil from my garden patch. This soil has recently been put there, and mainly consists of compost material, and the soil is bought in bulk.
You can also see other things going on in the spectrum, and this is partly due to the fact that I have to some extent Radon in my basement (prominent 238 keV peak). So, things are getting complicated at this level of activity...
Next is a french soil sample, which clocks in at about 150 Bq/kg...
Again, it starts to become tricky regarding what is really above BG or not. Note here that I am measuring for 10 h, I have not done more extended analysis on these samples but it appears the biggest enemy is the background here...
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Sesselmann
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Re: Hunting Cs137
Karl,
Very nice work indeed, it seems like you are able to detect Cs-137 with a reasonable amount of confidence down to 200 Bq/kg which is quite acceptable and well within the maximum limit recommended by the WHO which is 1000 Bq/kg as shown in the table below.
Ref http://www.fao.org/fao-who-codexaliment ... e_2015.pdf
This means it is well within the amateurs capability to verify food safety in accordance with the limits set by the authorities (figure of speech only, not suggesting you are an amateur by any means :))
Thanks for posting...
Steven
Very nice work indeed, it seems like you are able to detect Cs-137 with a reasonable amount of confidence down to 200 Bq/kg which is quite acceptable and well within the maximum limit recommended by the WHO which is 1000 Bq/kg as shown in the table below.
Ref http://www.fao.org/fao-who-codexaliment ... e_2015.pdf
This means it is well within the amateurs capability to verify food safety in accordance with the limits set by the authorities (figure of speech only, not suggesting you are an amateur by any means :))
Thanks for posting...
Steven
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brehwens
- Posts: 89
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Re: Hunting Cs137
Hi Steven
You know, the word "amateur" actually means "one who loves" so I have no problem being an amateur :)
As for the results, yes, it appears to be no problem detecting 1000 Bq/kg. Of course, I still have a bit more work ahead of me if I am to reach some kind of limit of detection of my system. For the 200 Bq/kg samples, I still think there is more to be done as BM features functions to subtract overlapping peaks, such as the Bi peak at 609 keV which starts to interfere with the 662 keV peak. I might do two calibration setups; one for "hotter" samples where I don't subtract any peaks, and one "low activity" calibration where I subtract peaks and whatnot.
Also, I might try with longer measurement times. However, I recall that from a series of experiments done with a 1" well probe and measuring on the Gävle soil, measurement times longer than 3h did not yield that much more, and I don't think I have a less eficcient setup with the 2x2" well rig.
It should also be pointed out that I am comparing my measurements to a national reference setup; we are talking 15 cm of lead, lined with copper, HPGe detector with all the bells and whistles etc etc. So to be able to go out in Gävle and simply dig up some dirt, dry it and grind it a bit, and easily be able to pick out 30 year old fallout is pretty neat.
You know, the word "amateur" actually means "one who loves" so I have no problem being an amateur :)
As for the results, yes, it appears to be no problem detecting 1000 Bq/kg. Of course, I still have a bit more work ahead of me if I am to reach some kind of limit of detection of my system. For the 200 Bq/kg samples, I still think there is more to be done as BM features functions to subtract overlapping peaks, such as the Bi peak at 609 keV which starts to interfere with the 662 keV peak. I might do two calibration setups; one for "hotter" samples where I don't subtract any peaks, and one "low activity" calibration where I subtract peaks and whatnot.
Also, I might try with longer measurement times. However, I recall that from a series of experiments done with a 1" well probe and measuring on the Gävle soil, measurement times longer than 3h did not yield that much more, and I don't think I have a less eficcient setup with the 2x2" well rig.
It should also be pointed out that I am comparing my measurements to a national reference setup; we are talking 15 cm of lead, lined with copper, HPGe detector with all the bells and whistles etc etc. So to be able to go out in Gävle and simply dig up some dirt, dry it and grind it a bit, and easily be able to pick out 30 year old fallout is pretty neat.
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luuk
- Posts: 313
- Joined: 01 May 2015, 21:00
Re: Hunting Cs137
Hi Karl,
Good to see that your 2x2”well detector is working good.
Your detection levels are really low very impressive, your system works great, nice work.
Luuk,
Good to see that your 2x2”well detector is working good.
Your detection levels are really low very impressive, your system works great, nice work.
Luuk,
