Friday, 6 April 2012

Food 1

Hi again,






Cherry blossom at its peak in Tokyo but still cold here.
Goodnight all


  1. Hi, Anne --

    It's "he". :)

    Many years ago, I used to work for a company which made germanium detectors like the ones you're talking about. The less expensive one probably uses NaI (Sodium Iodide), which also allows isotope identification but with much less precision.

    It takes half an hour for your test. For really exacting measurements of the specific mix of isotopes in a sample, scientists using germanium detectors leave the sample in a much more heavily shielded container than the one in your photo for a full day or more. The shielding itself is made out of special "low background" lead which has very low levels of radioactive contaminants, usually salvaged from pre-WW II ships and buildings:

    By reducing the interference from natural radioactivity and cosmic rays, and counting for tens of hours, it is possible to identify specific isotopes in really tiny quantities (picograms, a millionth of a millionth of a gram).

    So, to put things in a bit of perspective, an average 70-kg person contains 8100 Bq of natural radioactivity:

    Scroll down about 60% of the page to "Human Body", and you'll see 4400 Bq of K-40 and 3700 Bq of C-14, ignoring all the minor isotopes. If we divide 8100 by 70, we get 116 Bq/kg. So you'll no doubt be pleased to know that the new food limit effectively bans cannibalism in Japan, because people are just too radioactive to eat!

    Of course, that's cheating a bit, because Cs-137 does not occur naturally and any amount ingested is additive to the natural internal dose. Still, it gives a sense of scale for how small 100 Bq/kg really is.

    Cs-137 is used as a test source to calibrate Geiger counters and other types of portable radiation detectors. For $125 USD, I can buy a little pellet with up to 10 microCuries of Cs-137:

    Scroll down to the "Radioactive sources" section. These are amounts which can be shipped anywhere by regular post and kept with no license from the NRC; in other words, it's not regarded as excessively hazardous by the US Government. But if we convert to SI units 10 uCi is... 370 thousand Bq! Eating it would be equivalent to consuming 3.7 metric tons of material contaminated at 100 Bq/kg, and would expose me to a 5 mSv dose (twice annual background in the US).

    Here's how I get that dose conversion. The NRC specifies annual allowed occupation exposures for all radionuclides. Here's the page for Cs-137:

    The oral ingestion ALI is 100 microCuries (recall this is an occupation exposure limit for radiation workers, not the general population). ALI is defined as the amount of ingested material per year which results in 50 mSv of annual dose:

    So we can scale down to 10 uCi at 5 mSv, or 1 uCi at 0.5 mSv. 1 uCi is still 37000 Bq, or 370 kg of food contaminated at the maximum allowed value. So a full kilogram a day of worst-case contaminated food would result in only 1 percent of the annual exposure in the "Red Zone" of the ground contamination map you published a while back.

    Sorry to be so long-winded here, but there are so many people actively trying to scare you that it's worthwhile getting a deeper understanding of what the numbers mean.

    What matters is the cumulative average, not any particular meal. So go ahead, enjoy the wild mushrooms once in a while -- just don't eat tens of kg of them. :)

  2. Hi Anne, I fully appreciate your concern for the farmers, and what has happened to them breaks my heart. But we do have to think about the children of Japan, and it is safer to stay on the conservative side. These poor children will be accumulating radioactive materials in their bodies for their entire lives, and no one really knows what the effect will be. It's all guesswork. I am married to a Japanese and have two young children, so this issue is something that affects me profoundly.

  3. Although the evidence seems to suggest that effects will be minimal (see comments by Diogenes NJ above and after the post 'Still Waiting', also my post 'Expert Advice') you are right when you say it's all guesswork. There is no data yet on long term low dose radiation and we are all part of a giant experiment. I sympathise with you for it must be very hard when you have children to think of.