An Explanation about Radiation

07 Apr

Today, I found an interesting article explaining how to understand the risk of radiation, but unfortunately, it was in Japanese and I can’t find a translated post in English. I am going to translate it into English as much as possible. There is anything you found, my mistakes or anything you feel it difficult to understand, please let me know.

Qestion1) Radiation decreases inverse proportion  to square of the distance. (right) Then, does it become 1/4 you move from 10 km point to 20 km point?

Answer) Radiation decreases like that in a vacuum. If it is filled with air ,the space between 10 km point to 20 km point, it works like a wall to block the radiation. That’s why it is impossible to reach  the surface of Earth for Gamma ray from Space. About radiation in Tokyo right now, it is provided by radioactive materials flying from Fukushima Nuclear Power Plant. In this case, the amount of radioactive materials / a square will decrease in distance, but those materials keep providing radiation.

Question 2) It is 8 days a half-life of Iodine-131. Then, is it safe 8 days later?

Answer) Both yes and no. Iodine131 lost its half power in 8 days. That’s true. But, if there are 4 times of safe limits for Iodine-131, there are still twice of safe limits 8 days later. So, you need 16 days to have safe limits of Iodine-131 in this case. And, Iodine-131 changes to Xenon-131m while providing Beta ray, and Xenon-131m changes to Xenon-131 while providing Gamma ray. A half-life is 11.84 days. So, it is impossible to say something is safe 8 days later.

Question 3) Is it more dangerous to be exposed a little every day from inside of your body by a radioactive material which comes into your body than to be exposed to more radiation only once.

Answer) It depends on how much radiation you are exposed in a moment. It is similar to the risk of having salts. In Japan, it is OK with an adult to have 10 g of salt / a day. Then, it won’t cause a problem on your health if you had 10 g of salt / a day for 20 days, but it is not good for you if you had 200 g of salt at a time. About Radiation, it is less effective for your health to be exposed to 6 micro SV of radiation for 42 days (1000 hours) than to be exposed to 6000 micro SV of radiation by 1 time CT scan.

Question 4) Experts say, “it doesn’t mean to cause a problem to your health right now.” Does it mean it will be dangerous later?

Answer) It is again similar to the risk of salt. It is safe to have 10g / a day, and it won’t be dangerous to have 20g if it is once or so. But, it will be dangerous for your health if you keep having 20g of salts everyday for a long time. It is the same with those vegetables or tap water that show much radioactivity than the safe limit. If you ate those vegetables or drank such tap water once or so, it won’t be dangerous just like 20g of salts won’t be dangerous if it was once or so. It will be dangerous if you eat/drink much everyday for a long time, again just like the salts, then, Japanese government announced the warning not to eat/drink as much as possible.

Those questions and answers are very understandable and seems rational for me. I hope I translated well.


Posted by on April 7, 2011 in Disaster, Science, Uncategorized


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5 responses to “An Explanation about Radiation

  1. Mark Foreman

    April 17, 2011 at 12:34 am

    Regarding question one.

    The inverse square law should only be used for the radiation emitted by an object (point source), the radioactivity emitted by a leaking reactor is different. The radioactivity may disobey the inverse square law, for example if it is released at 0 km, it passes through the air to 10 km where rain occurs then the radioactivity level at 5 km will be much lower than the activity at 10 km

    Always bear in mind that radiation and radioactivity are different but releated things.

    • scarlet^^

      April 17, 2011 at 7:23 am

      Thank you for your explanation.
      I think I understand what you said. I know radiation is provided by radioactive materials and radioactivity is a possibility of a radioactive material to provide radiation. (At least I found such explanation in Japanese.)
      As we have troubled nuclear power plant here in Japan and the plant keep leaking radioactive materials from time to time, those materials are flying in the air or falling down to the ground by rain, so I can understand it could be happen to be exposed at 10 km than at 5 km.
      Anyway, I think it is not so dangerous to stay in Tokyo even now, at least about radiation? I hope so.

  2. Paul

    June 30, 2011 at 12:12 pm

    Hi Scarlet,
    There is a lot of information on the Fukushima disaster on the AboveTopSecret forum. The thread there ( ) is over 960 pages long at the moment and still growing. Since you live in Japan, it would be useful if you could add some comments on the thread about life in Japan and what your news says about the situation. If the information is incorrect, you can help to correct it too. Good luck. 🙂


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