Question about Cesium conversion factors

Submitted by Anonymous on Fri, 2011-06-03 13:42

I have a question to the team about Cesium conversion factors used. When comparing to air travel time do you take into account that Cs-137 has a biological half time of about 6 months, i.e. stays and radiates that long in the body while a transcontinental flight lasts only 6 hours? I.e. there is almost a three order of magnitude difference in the duration of the exposure. If this weren't taken into account, your numbers (say, eating a kilogram of strawberries equals a thousand transcontinental flight) would look very different (i.e. eating a kilogram of strawberries equals a thousand transcontinental flights).

Thanks for clarifying this.

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Hi, that's a great question.

Submitted by bandstra on Fri, 2011-06-03 14:25.

Hi, that's a great question. You're absolutely right that the exposure from an airplane flight lasts only while you are flying, while ingested radioactive material can give you a dose for months or years afterward.

The conversion factors we use for each isotope take into account several factors, including:

  • the type of radiation emitted
  • the energy of the radiation
  • the specific tissues that the isotope might concentrate in
  • the susceptibility of the tissues to cancer
  • the radioactive half-life of the isotope
  • the biological half-life of the element

For internal exposures, the quantity that accounts for all this is called the "committed effective dose equivalent (CEDE)," and it quantifies the total dose someone will receive from ingesting a quantity of radioactive material, integrated over the rest of one's life. So yes, the exposure time is included in those numbers.

If you would like to read more about this, you can check out these FAQs: "What is total effective dose equivalent?" and "Is it valid to compare doses from these radionuclides to a cross-country plane flight?".

Mark [BRAWM Team Member]

Wonderful, thanks Mark. I

Submitted by Anonymous (not verified) on Fri, 2011-06-03 15:57.

Wonderful, thanks Mark. I guess I might go buy some organic CA-grown strawberries now :-)

Unrelated question: do you guys have any idea about what fraction of the low-level Cs-137 you're finding in some fruit and vegetables as well as in topsoil might be left-over deposition from Chernobyl and bomb tests?

Great :) Regarding the

Submitted by bandstra on Fri, 2011-06-03 18:17.

Great :)

Regarding the Cs-137 levels — since we started these tests after the fallout arrived, we don't have a good pre-Fukushima baseline. However, by continuing our testing of soil, grass, and strawberries, we hope to establish a baseline. This baseline will likely be our minimum detectable activity (MDA).

However, another line of evidence would point to the majority of Cs-137 we're measuring being from Fukushima. This logic relies on the measurement of Cs-134, which is chemically identical to Cs-137 and thus should "track" with Cs-137 in the environment. In most of our measurements, we see nearly equal activities of Cs-134 and Cs-137, and this is consistent with the amounts of each isotope that would have been produced by the reactors. But Cs-134 has a 2 year half-life and should only be from Fukushima. If a considerable fraction of the Cs-137 were from beforehand, then we would be seeing less Cs-134 relative to Cs-137 than we expect. So this would lead me to believe that nearly all of the Cs-137 is from Fukushima as well. Does that make sense?

Mark [BRAWM Team Member]

Mark, makes perfect sense.

Submitted by UCB physicist (with no association with BRAWN) (not verified) on Fri, 2011-06-03 19:50.

Mark, makes perfect sense.

I remember seeing a study by Lawrence Livermore lab that hinted at bay area fallout post Chernobyl that seemed large-ish compared to what landed here from Fukushima. That's why I thought this might be a significant systematic. But the Cs-134 and Cs-137 correlation clearly argues against this.

Btw, I'd be curious to see studies about the "agricultural half life" of Cs-137 in Europe post Chernobyl. Let us know if you know of any. Thanks.

"Agricultural" half-life

Submitted by bandstra on Fri, 2011-06-03 22:30.
I'm trying to get more information on this, too. For now, someone posted this on the forum a couple weeks back, about dating wine in Bordeaux by measuring the Cs-137 content: http://www.nuc.berkeley.edu/node/4118 They link to this plot of the Cs-137 activity per liter of wine as a function of the vintage:
Original paper: "From the mass of the neutrino to the dating of wine," Ph. Hubert, F. Hubert, V. Raffestin The activity in the wine should be proportional to the activity in the soil when the grapes grew. So you can see from the plot that the amount of Cs-137 in the soil decays away much faster than the 30 year half-life of Cs-137 — it looks to me more like a 2 year half-life. So that would be the timescale for dispersion in the soil of that region. Oh, and Chernobyl is that tiny blip on the plot in 1986. The huge spike is due to atmospheric weapons testing, which ended in 1963. Mark [BRAWM Team Member]

"France continued

Submitted by Anonymous (not verified) on Fri, 2011-06-03 22:41.

"France continued atmospheric testing until 1974, China continued up until 1980."

http://en.wikipedia.org/wiki/Nuclear_weapons_testing

Nothing compared to what had been done before, but still...

Thanks, I wrote that too

Submitted by bandstra on Fri, 2011-06-03 22:45.

Thanks, I wrote that too quickly ;)

Mark [BRAWM Team Member]