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Becquerel versus Sievert

  1. Aug 23, 2014 #1
    To anyone new to the field of nuclear physics, the units are positively nonplussing. For example, I have just understood that the Becquerel (desintegration per second) is not very significant if you do not tell what kind of Becquerel you are talking about, since different desintegrations yield different quantities of energy.

    Furthermore, we do not absorb different products the same way. For example,if we compare Cs137 (512 keV) and I131 (606 keV) beta decay energies they are similar. However, this 2011 article (http://www.bloomberg.com/news/2011-03-21/japan-sets-safe-limits-for-consuming-radiation-contaminated-food-table-.html [Broken]) tells us "The prescribed safe limit for I-131 in vegetables is set at 2,000 Becquerel (Bq) per kilogram and 500 Bq/kg for radioactive cesium."
    I do not know why cesium is supposed to be more dangerous, but I guess it accumulates more in the body. It probably has something to do with the different half-lifes too (about a week for Iodine and 3 decades for Cs 137). I guess that if your body incorporates I 131, you are radioactive for a little while, if it incorporates Cs 137, you are radioactive for decades.

    So my conclusion is, wherever I read "Becquerel" in an article about Fukushima or Tchernobyl, I do not really know what it means. I have never read any article that tried to make me aware of the fact that some Becquerels are more equal than others.

    The energy absorbed by an object is measured in Grays (joules per kg). In the case a living organisms, alpha particle are considered 20 times more harmful thant alpha or beta. So if you recieve 10 grays of alpha and 10 grays of beta, you get 210 sieverts of "effective dose". Why is the multiplication factor 20 and not 19 or 21? 20 is a round figure. We are talking about a rough estimate. So any measurement in Sieverts is by definition not very precise. For example, it does not make any sense to talk of "161 millisieverts", just say "160" to account for the lack of precision.
    In the wikipedia article about the most radioactive town in the world, Ramsar, Iran, I read:"Record levels were found in a house where the effective radiation dose due to external radiation was 131 mSv/a". Such a figure is puzzling, since no one stays home 24h a day and I do not think the effective "home time" of the inhabitants of the building could be measured with very accurate precision.
    The more I read things about radioactivity, the more I come across figures in sieverts that I have some reason to doubt. When you live in a radioactive environments, you might ingest, drink or inhale radioactive elements, or get affected by gamma rays that can travel even through walls. So estimates of the sievert result are often very difficult.

    In conclusion, I think Becquerel measurements are precise but not very informative, while sievert measurement are informative, but not precise.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 23, 2014 #2

    Vanadium 50

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    Bq and Sv measure different things. Like miles driven and gallons of gasoline used. They are proportional and both well-defined, although the constant of proportionality depends on the system: like driving an econo-box vs. a truck.
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