No way to find hot spots with dosimeter at 1m from the ground via Fukushima 311 Voices

We have published several articles in this blog saying that to protect the population the Japanese goverment should take into account the soil contamination as well as the radiation dose in the air.  The policy to open the evacuation zones and encourage the population to return to live there (with the end of financial compensation and relocation aid) is based only on the airborn radiation dose measurements (the evacuation order is lifted when the radiation dose is under 20mSv/year).  We have been saying that this is very dangerous, even  criminal, for the air radiation dose rate (indicating the amount of radioactive dose received by a person within a certin period time) is useful with a well-identfied fixed source of radiation, but is not adequate to reveal the overall environmental contamination after a nuclear accident. It doesn’t account for the internal radiation exposure induced health hazards (note 1).

Now we would like to point out another problem related to hotspots: it is nearly impossible to find hotspots by the usual measuring practice of the airborn radiation dose rate (in sieverts per unit of time).  To illustrate this difficulty, we are translating here a Facebook post of Mr Yoichi Ozawa of the citizen’s measurement group named the “Fukuichi Area Environmental Radiation Monitoring Project“.




As we can see from the graph above, the value in terms of Sieverts decreases drastically with the distance from the ground. At 1m, which is the usual reference height to measure the radiation dose rate, the value becomes very small even with the soil of over 4 million Bq/kg, which is absolutely enormous (note 2).

前出のグラフに見られるように、シーベルトで表される数値は地面からの距離とともに激減していきます。通常、空間線量率が測定される地表1mでは、直下にキロ当たり400万ベクレル以上という驚異的な汚染があっても、それほど高い線量率になりません。この驚異的汚染レベルの測定対象は悲しくも有名な「黒い物質」です (注2)。


Note 2: This extremely high level of contamination is understandable, for what is measured here is the infamous “black substance” or “black dust”, a kind of Cyanobacteria, about which we invite you to listen to podocast of Marco Kaltofen with English transcription.

注2:「黒い物質」(藍藻類)についてはMarco Kaltofenさんのインタビューと英文書き起こしがあります。日本語では“黒い物質”を追う:志葉玲・小出裕章に動画と書き起こしがあります。専門的になりますが、福島第一原子力発電所事故に絡む: 環境アクチニド元素諸核種も参考になります。

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