Wednesday 31 July 2013

Airborne Radiation 2

Hello again,
Last time I talked about the survey meter that I borrowed for a week from the city office. This time I'll share the results of the dosimeter that I borrowed from the same place for a month. It's a bit like a blue thermometer that you carry around in a pocket or on your belt.and it calculates accumulated radiation. The city asks you to record the results every day along with an estimate of how many minutes you were outside and where you were.

For me, it was clocking 2 μSv/day. At the end of the month this was worked out to 0.65 mSv/year which after subtracting natural radiation in Koriyama of 0.35 mSv/year left me with only 0.3 mSv/year in additional radiation as a result of the accident, well below the government target of 1 mSv./year. When I handed the dosimeter back I was told that it was low probably because I live and work in concrete buildings 7 and 8 floors up and I don't go outside much (usually less than an hour a day). I was told that many people are recording more than 1 mSv/year.

Incidentally, this 1 mSv/year does refer to radiation over and above natural radiation. (The term in Japanese is  年間追加被ばく線量 nenkan tsuika hibaku senryo.) But even so, natural radiation is so low here that even with the additional radiation, levels are still lower than many places in the world.

This is a point that the authorities are keen to stress and the Fukushima prefecture website now has banners and links showing comparative levels in other parts of the world. And you might be interested to check out Safecast, an independent organisation which monitors levels throughout the world and also here in Fukushima.
Next time, I'll look at the decontamination that's going on in Koriyama at the moment.
Bye for now


  1. Anne,

    Thanks for these informative posts. Lots of people and groups have pointed out discrepancies between what their handheld detectors show and what the official monitoring posts show. Safecast spent time investigating, and discuss what we found here:

    Lots of people have noted the same kinds of discrepancies you did, and we wanted to try to find out if the readings were being intentionally skewed, or if it was just the result of normal variations in radiation, the differences between devices, etc.. We came to the conclusion that it's basically normal variation, but fault the govt for not making it very clear how the systems work, their limitations, what kinds of quirks people should expect to see, etc.. And also for locating too many of them in places which have noticeably lower radiation levels than the immediate surroundings.

    When levels are in the relatively low range were seeing in many places in Fukushima, it's normal to see a lot of variation, so it's important to average readings over time, like 10 mins. The monitoring posts (we call them "droids") seem to use a 10-min averaging window.

    Azby Brown
    Safecast Volunteer

  2. Food for thought - I'm very grateful for independent analysis.