No way to find hot spots with dosimeter at 1m from the ground

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“.



Here are the radiation dose rates, captured vertically above a highly radioactive substance (“black substance” or “black dust”) of 4,120,000 Bq/kg, measuring 79 μ Sv/h.

鉛直下での点線源場の線量、下には 412 万 Bq/kg 、79 μSv/hの高線量物質(「黒い物質」)。

Below are measurements at different distances from the ground.

5cm:9.112 μSv/h
50cm:0.630 μSv/h
1m:0.251 μSv/h

graph new Eng

Conclusion: it is impossible to discover micro-hotspots right under your feet when you walk around measuring radiation doses at 1m of distance from the ground.

Measuring device:
Aloka TCS172B
Measurements carried out by Mr Yoichi Ozawa.

For 0cm from the ground, Aloka TCS172B, which cannot measure over 30µSv/h, was replaced by Polimaster PM1703M and Radex RD1706. The value is the average of the measurements of these two devices.  

graph new1mの高さで測定して歩いても足元のマイクロホットスポットは探索できない。


距離 0cm に関してはアロカTCS172Bでは最大30µSv/hまでしか測定できないので、ポリマスターPM1703MとラデックスRD1706を使用、その平均値を採用。

Here is the video of the measurement. こちらが測定の動画です。


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)。

Some readers might be familiar with the image of a Japanese citizen measuring  radioactivity with a device at about 1m from the ground. This practice, almost unknown before the Fukushima Daiichi nuclear accident, has become widespread among citizens, although it has become rather a rare practice nowadays as many people have more or less become accustomed to live with radiation. Besides the fact that it is hard to live a life worrying about radiation around the clock and some people prefer to stop thinking about it, this “normalisation” of radiation is strongly enforced by a governmental security campaign. One of the methods employed is to focus on the external irradiation risk, neglecting the internal irradiation risk, by spreading the knowledge and data only in terms of the radiation dose in the air (measured by Sieverts), at the expense of other measurements such as the radiocontamination density in soil (surface contamination density in terms of Becquerel/unit of surface).


One of the now well-known problems of radiocontamination of the environment is that the contamination is not homogeneous, but dispersed with what is called a hot spot. This is a serious problem for the population, as the absorption of radioative particles contained in these hot spots can cause internal irradiation related health damage. And as we see above, it is extremely difficult to detect these hotspots, from 1m and above, even with the extremely highly contaminated substance such as “black dust”.


It is widespread belief among the public that if the value of the airborn radiation dose at 1m from the ground is under 0.23µSv/h (note 3), it is safe. This value, diffused by the authorities as well as by media, is indeed applied as the lower limit to carry out decontamination work.  Yet, as we have seen, even with the extremely highly contaminated substance such as “black dust”, at 1m, the radiation dose is only 0.25µSv/h, that is to say, only slightly over the limit of the 0.23µSv/h, which is believed to be the “safety level”.


It is unfortunate to say that for most of the residents taking the measurements of the airborn radiation dose by themselves, the values they observe have become rather an “encouraging” factor to continue living there or to return to live, than an alarming factor, as these values do not reveal but rather conceal the presence of hotspots which can cause internal radiation exposure induced health damage.


It is difficult to find hotspots anyway.  So when the soil contamination is high (see the concentration maps in this blog, for Namie, Minamisoma), it is better to keep the zone closed, continuing to aid the evacuated people.

いずれにせよ、ホットスポットを全て見つけるのは非常に困難です。結論として、土壌汚染密度が非常に高い場合は(このブログの浪江町南相馬市 の汚染地図をご覧ください)、避難指示解除をせず、汚染地帯の立ち入りを制限して住民の避難を支援し続けるべきです。

Note 1: In opposition to the external radiation exposure which occurs when the human body is exposed to an external source, the internal radiation exposure is an exposure from inside the body due to the incorporation of radioactive particles through ingestion, inhalation or adhesion to skin.

注1: 外部被ばくとは、放射線を発するもの源が体の外にあって、外から放射線が体に当たって被ばくすることです。内部被ばくとは、体の中で発射された放射線で被ばくをすることです。放射性原子を吸い込んだり食べたり、または皮膚から吸収することによって、体の中に取り入れてしまい、体の中で放射線が発射されます。

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。

Note 3: In fact, the 0.23µSv/h value is problematic in itself.  This is based on the 1mSv/year value following the ICRP (International Commission on Radiological Protection) recommendations on the public health.  However,  the 0.23µSv/h value is not the result of a simple division of 1mSv by 365 days x 24 hours. The calculation of 0.23µSv/h presupposes that people stay inside for 16 hours/day and that the radiation is reduced by 60% because of the building structure.  Then, the background of 0.04µSv/h is added. (1000µSv÷365÷(8 + 〈16×0.4〉) + 0.04  But in the real life in rural areas such as Fukushima, people spend more time outdoors.  Besides, some recent research has shown that in some cases the radiation dose can be higher indoors than outdoors because of the infiltration of hot particles. Thus, the reality is much more complex to apply uniformly the value of 0.23µSv/h as a safety threshhold. Lastly, many people in Fukushima were victimes of the initial exposure right after the accident. For such population, any exposure, whatever the quantity is, is to be avoided.

注3: 毎時0.23µSvという数値自体が問題をはらんでいます。この数値はICRP(国際放射線防護委員会)の勧告による年間1mSvの線量限度に基づくものです。しかし毎時0.23µSvという数値は1mSvを365日x24時間で割ったものではありません。人々は毎日16時間屋内で生活し、かつ屋内の遮蔽効果が0.4として計算し、それにバックグラウンドの0.04µSv/hを加えたものです。しかし、福島県のような地方では屋外でもっと多くの時間を過ごす生活形態の人々が多いのです。また、最近の研究によると、場所によっては放射性粒子の侵入により、屋外よりも屋内の方が線量が高い場合もあります。このように現実は複雑で、毎時0.23µSvという数値を安全の閾値として均一に当てはめることはできません。最後に事故当時多くの住民は初期被ばくに晒されています。そのような人々には数値に関わらず、追加被ばくは避けられるべきです。

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