The main fission products from the Fukushima Daiichi nuclear power plant (FDNPP) accident are 129mTe-129Te, 131I, 132Te-132I, 134Cs, 136Cs and 137Cs [1–4]. These radionuclides emit gamma rays and beta rays through β− decay. However, there are few studies about dose estimation from beta-ray irradiation following the FDNPP accident [5–7]. The beta-ray dose contributes to the whole-body dose among small biota, such as insects, plant leaves, and human skin. Therefore, beta-ray dose estimations are important for the risk assessment of the impact of the FDNPP accident (including on small biota) to clarify the effects of this large-scale radiological accident.
Retrospective dosimetry with brick samples has been used to evaluate the gamma-ray dose of the Hiroshima atomic bomb [8–10], the Chernobyl nuclear power plant accident [11–14], and the Semipalatinsk nuclear weapon testing [15, 16]. Recently, Stepanenko et al.  used retrospective dose evaluation of brick samples to estimate gamma-ray doses and perform beta-ray dose reconstruction for the FDNPP accident with a similar method to that used for a Hiroshima tile sample . They used a single-grain quartz optically stimulated luminescence (OSL) method (similar to that of Ballarini et al. , although layer-by-layer consequences for very thin layers of the sample’s aliquots were used for analysis, with separate dose calibration for each quartz grain) with brick samples taken in 2014 from Odaka, Minami-Soma City, Fukushima Prefecture, Japan . Dose enhancement near the surface of the brick was identified by the OSL measurements . Stepanenko et al. suggested that the enhancement was caused by the beta-ray dose from the deposited fission products .
To establish the cause of the dose enhancement near the brick surface, we performed a Monte Carlo simulation of a small brick building with radionuclides uniformly distributed on the ground surface. The calculated results were compared with the data measured by Stepanenko et al. . The depth profiles of the dose in the brick sample for beta rays and gamma rays were estimated separately, and the dose enhancement near the brick surface was discussed.
MATERIALS AND METHODS
Particle and Heavy Ion Transport code System calculation
The energy deposition as a function of depth in the brick wall of a small building was calculated using the Particle and Heavy Ion Transport code System (PHITS) Monte Carlo code Ver. 2.52 . The calculation geometries are shown in Fig. 1. The calculation regions were 1 m × 1 m for beta rays and 21 m × 21 m for gamma rays. The calculation regions consisted of ground, air, and the small brick building (red region: 0.5 m × 0.5 m square, 1.5 m high, wall thickness of 10 cm). The brick building was located in the center of the soil surface. Beta- or gamma-ray sources were uniformly distributed in the 5-mm-thick soil surface (brown region). To save calculation time, the previously reported mirror condition was used for these calculations . Figure 1a shows the geometry used to calculate the radiation that entered the calculation region (outer source calculation) via the mirror boundary. First, the histories for the particles were accumulated near the mirror boundary (green lines) without the brick building. Second, the particles were generated from the mirror boundary (back line) in Fig. 1b according to the accumulated histories. The generated particles were transported to the brick wall cells (yellow box) of the brick building. Third, radiation was generated from the surface of the 5-mm-thick soil layer (brown region) in the calculation region (inner source calculation) in Fig. 1b. The energy deposition in brick cell layers of 10 m × 10 cm and thicknesses of 0.1, 0.2, 0.3, 0.4, 0.5, 1, 3, 5, 7.5, 10, 20, 40, 60, 80 and 100 mm were obtained by summing the outer and inner source calculations corrected with the number of particles generated per unit area.
Beta and gamma rays from 129mTe, 129Te, 131I, 132Te, 132I, 134Cs and 137Cs were calculated separately. Beta-ray energy spectra were taken from the literature , and the internal conversion electrons of 137Cs were taken from the website of the National Nuclear Data Center . The gamma-ray energies and emission rates for the radionuclides were taken from the National Nuclear Data Center .
The elemental composition of the brick sample was Si: 28.9, Si: 50.4, Al: 17.5, Fe: 1.4 and Ti: 1.8 wt %, and those of soil and air were taken from the literature .
Air dose and tissue dose calculation
where Eijk is the energy deposition (J) at the i-th depth by beta or gamma rays from the k-th radionuclide, mb is the brick sample mass (kg), and aj is the area of the source (0.75 and 1 m2 for inner and outer beta calculations, 440.75 and 441 m2 for the inner and outer gamma calculations, respectively). Ij is the emission rate for beta or gamma rays per Bq and fc is the conversion factor of the stopping power ratio  for beta rays and the kerma ratio  for gamma rays between air or tissue and brick to convert from the brick dose to the air or tissue doses.
Cumulative dose estimation
where Tk is the half-life for each radionuclide of k = 129mTe, 129Te, 131I, 132Te, 132I, 134Cs and 137Cs (Table 1), and τ is the time period from deposition to the brick sampling date.
RESULTS AND DISCUSSION
Calculated dose rate for beta and gamma rays
A 137Cs deposition density of 308 kBq/m2 and the ratio of each radionuclide to 137Cs deposition density taken from the literature  were used to obtain Ak for each radionuclide. The deposition densities for the seven radionuclides are listed in Table 2. The beta-ray dose rates on the brick surface and gamma-ray dose rate at a depth of 0.5 mm in the brick at a height of 80 cm are shown in Fig. 2a and b, respectively. 129m, 129Te contributed less to the gamma-ray dose rate, and accounted for the third and fourth largest contribution to the beta-ray dose rate. This is due to the small gamma-ray emission rate per decay of 129m, 129Te of <10%. The gamma- and beta-ray doses decreased by ~10% and ~30%, respectively, over 1 month. The calculated beta-ray dose rate decreased slower than the calculated gamma-ray dose rate.
Beck reported conversion factors for various radionuclides to estimate the air dose rate at a height of 1 m from the unit deposition density of radionuclides . The initial gamma-ray air dose rates (15 March 2011) at a height of 80 cm from the ground for each radionuclide obtained by our calculations were compared with the values estimated by Beck conversion factors  interpolated at a relaxation depth of 0.65 g/cm2 (Table 2). The present dose rates were estimated to be 57% lower than those calculated by Beck conversion factors. The present dose rates were in-brick values in one of the walls of the brick building, whereas the Beck conversion factor values were free-in-air values. Therefore, the difference of 57% can be explained by shielding effects, whereby gamma rays from behind the building are neglected.
The cumulative dose over 3 years, from 12 March 2011 (Unit 1 explosion) to 19 March 2014 (brick sampling by Stepanenko et al.) and the dose rate change over time are shown in Fig. 3. The solid line shows the calculation result, the dashed histograms are the averaged calculation values for the measured sample thickness, and the open circles are Stepanenko’s data . The calculation agreed well with the data measured by Stepanenko et al. in the region deeper than 10 mm. The results indicated that the cumulative dose deeper in the brick was due to gamma rays, and that the dose enhancement at the surface was dominated by the beta-ray contribution. The difference between the calculated and measured doses at the surface was about 2 standard deviations. A possible explanation might be connected with the contributions of low γ emission rate radionuclides, such as 89Sr, 127mTe-127Te, 140Ba-140La, etc. However, the trend in the dose increase at the brick surface was supported by the calculations. Therefore, the single-grain OSL measurement by Stepanenko et al. shows the advantage of dose estimations not only the cumulative gamma-ray dose but also the cumulative beta-ray dose. Thus, we concluded that the single-grain OSL method is a good tool for retrospective beta-ray dose estimation.
The calculated tissue dose at a brick depth of 50 μm was assumed to be a skin dose, and would be similar to a 70-μm tissue dose. The skin dose was estimated to be 164 mSv for 3 years at the sampling location.
To confirm the cause of the dose enhancement near the surface of a brick sample taken from Odaka, Minami-Soma City, Japan, a Monte Carlo calculation was performed using PHITS code and the calculated results were compared with measurements. The calculated results agreed well with previously published measured data. The dose enhancement at the brick surface in the measured data was explained by the beta-ray contribution, and the gentle slope in the dose profile deeper in the brick was due to the gamma-ray contribution. The calculated result estimated the skin dose to be 164 mGy (164mSv) over 3 years at the sampling location.
On September 5, 2017, Minami-soma city made a statement on the city’s radiation levels compared to 3 cities in West Japan, which has been reported in several newspapers. It’s important to comment on this study because the statement is intended to persuade the population to return to live there.
We are publishing comments on the articles below after having discussed with M. Ozawa of the citizen’s measurement group named the “Fukuichi Area Environmental Radiation Monitoring Project“. For English speaking readers, please refer to the article of Asahi Shimbun in English. For our arguments we refer to other articles published in other newspapers – Fukushima Minyu and Fukushima Minpo – which are only in Japanese.
Here are the locations of Minami-soma and the 3 other cities.
Here is the article of the Asahi Shimbun
Fukushima city shows radiation level is same as in west Japan
By SHINTARO EGAWA/ Staff Writer
September 5, 2017 at 18:10 JST
MINAMI-SOMA, Fukushima Prefecture–Radiation readings here on the Pacific coast north of the crippled Fukushima No. 1 nuclear power plant are almost identical to those of sample cities on the other side of Japan.
The Minami-Soma government initiated the survey and hopes the results of the dosimeter readings, released Sept. 4, will encourage more evacuees to return to their home areas after they fled in the aftermath of the 2011 nuclear disaster.
A total of 100 portable dosimeters were handed out to 25 city employees from each of four cities–Minami-Soma, Tajimi in Gifu Prefecture, Fukuyama in Hiroshima Prefecture and Nanto in Toyama Prefecture. They were asked to take them wherever they went from May 29 through June 11.
The staff members were evenly dispersed with their homes in all corners of the cities they represented.
In addition, only those living in wooden houses were selected as different materials, concrete walls, for example, are more effective in blocking radiation.
In July 2016, evacuation orders for most parts of Minami-Soma were lifted, but not many residents have so far returned.
The city’s committee for health measures against radiation, which is made up of medical experts, analyzed the data.
The median value of the external radiation dosage of the 25 staff of Minami-Soma was 0.80 millisieverts per annum, while the average value was 0.82 mSv per annum, according to Masaharu Tsubokura, the head of the committee and a physician at Minami-Soma general hospital.
No significant difference was found in the three western cities.
Both figures were adjusted to include the natural radiation dose, and are below the 1-mSv per annum mark set by the national government as the acceptable amount of long-term additional radiation dosage, which is apart from natural radiation and medical radiation dosages.
The radiation doses in all cities were at levels that would not cause any health problems, according to Tsubokura.
“Making comparisons with other municipalities is important,” Tsubokura said. “I am intending to leave the survey results as an academic paper.”
1) The difference of life style between city employees and local agricultural population
As we see in the article, portable dosimeters were handed out to city employees. They spend most of their day time in an office protected by concrete walls which are efficient for blocking radiation as stated in the article. However, in Minami-soma, most of the population spends more time outside, very often working in the fields. Their life style is different and therefore the external radiation dose cannot be similar to those of city employees. The result of the comparison between the external radiation dose of city employees cannot be used as an argument to say that it is safe for the local population to live in Minami-soma.
2) In the article of Fukushima Minyu, it is stated that in Minami-soma the radiation dose has a wider range than in the other three cities. This means that there are hotspots, which leads to higher risks of internal irradiation.
3) The radiation dose expressed in terms of Sieverts is relevant for radioprotection when the source of radiation is fixed and identified. This is the case for most of the nuclear workers. However, in the case of Fukushima after the nuclear accident where the whole environment is radio-contaminated and the radioactive substances are dispersed widely everywhere, it is not a relevant reference for radioprotection. It is important in this case to measure surface contamination density, especially of soil.
4) 6 years and 6 months since the accident, cesium has sunk in the soil. It is thought to be between 6 and 10 cm from the surface. This means the top layer of soil from 0 to 5 cm is blocking the radiation, reducing the measures of the effective dose. However, this does not mean that the population is protected from internal irradiation, since cesium can be re-scattered by many means, by digging or by flooding, for example.
5) The reliability of individual portable dosimeters has already been raised many times. This device is not adequate to capture the full 360° exposure in radio-contaminated environments as described in point 3 above.
6) In the article, it is stated that background radiation is included in the compared values, but it does not mention the actual background radiation measurements in the 4 cities.
The Table of Fukushima Minyu
Radiation dose of the 4 cities
Values include the background radiation dose
To summarize, the sample study group does not represent the overall population. The study doesn’t include the risks of internal radiation, for which the measurement of contaminated soil is indispensible. The dosimeters are not adequate to measure the full load of radio-contaminated environments. So, the research method is not adequate to draw the conclusion to say that it is safe for the population to return to live in Minami-soma.
A few days ago Pierre Fetet learned of a map which immediately called his attention.
That map displays at the same time precise and unsettling measurements. Not knowing Japanese, Pierre Fetet asked Kurumi Sugita, the president of Nos voisins lointains 3.11 association, to translate for him the text. She immediately accepted and explained to him what it was:
“The project to measure environmental radioactivity around the Fukushima Daiichi nuclear power plant (“Fukuichi Area Environmental Radiation Monitoring Project“) is conducted by a team of relatively old volunteers (who are less radiosensitive than youth) to perform radioactivity measurements with a tight mesh size of 75 x 100 m for radioactivity in air and 375 x 500 m for soil contamination. Measurements of ambient radioactivity and soil radioactivity are carried out mainly in the city of Minamisōma and its surroundings. They try to make detailed measurements so as to show the inhabitants the real conditions of their lives, and also to accumulate data for the analysis of long-term health and environmental damages.”
Thanks to the Kurumi Sugita’s translation and with the agreement of Mr. Ozawa, author of the document, Pierre Fetet was able to make a French version of this map, which I translated into english here below:
Map of Mr. Ozawa’s team, “Fukuichi Area Environmental Radiation Monitoring Project” (translation first by Kurumi Sugita, then by Hervé Courtois)
In the context of the normalization of contaminated areas into habitable areas, the evacuation order of the Odaka district of the city of Minamisōma was lifted on 12 July 2016, except the area bordering Namie (Hamlet of Ohatake where a single household lives) classified as a “difficult return” area.
Situation of the study area
The contamination map examines the Kanaya and Kawabusa areas of the Odaka district, about fifteen kilometers from the former Fukushima Daiichi nuclear power plant. Mr. Ozawa, the engineer who launched this investigation, has chosen the precision of the measurements, that is to say laboratory scintillation radiometers are used to measure radioactivity: Hitachi Aloka TCS172B, Hitachi Aloka TGS146B and Canberra NaI Scintillation Detector.
The originality of this map is due as much to the quality of its realization as to the abundance of its informations: it can be read, for each of the 36 samples taken, measurements in Bq / m², in Bq / kg, in μSv / h at three different soil heights (1 m, 50 cm, 1 cm) And in cpm (counts per minute) at the height of 1 cm. For those who know a little about radioactivity, these informations are very valuable informations. Usually, measurements are given in either unit, but never simultaneously with 4 units. Official organizations should learn this way of working.
The measures revealed by the map are very disturbing. They show that the earth has a level of contamination that would make it a radioactive waste in any uncontaminated country. As Mr. Ozawa writes, these lands should be considered a “controlled zone”, that is to say a secure space, as in nuclear power plants, where the doses received must be constantly checked. In fact, it is worse than inside of a nuclear power plant because in Japan the inhabitants evacuated since five and a half years are now asked to return home, whereas it is known that they will be irradiated (Up to 20 mSv / year) and contaminated (by inhalation and ingestion).
This citizen research is remarkable in more ways than one:
- It is independent of any organization. There is no lobby to alter or play down this or that measure. These are just raw data, taken by honest people, in search of truth.
- It respects a scientific protocol, explained on the map. There will always be people to criticize this or that aspect of the process, But this one is rigorous and objective.
- It takes measurements 1 m from the ground but also 1 cm from the ground. This approach is more logical because until now men are walking on the ground no? The contamination maps of Japan often show measurements at 1 m from the ground, Which does not reflect reality and seems to be done to minimize the facts. Indeed, the measurement is often twice as high at 1 cm from the ground as at 1 m.
- It acts as a revealing map. Mr. Ozawa and his team are whistleblowers. Their maps say: Watch out ! Laws contradict each other in Japan. What the government claims, namely that a dose of 20 mSv / year will not produce any health effect, is not necessarily the truth. If you come back, you are going to be irradiated and contaminated.
France is preparing for the same forfeiture, namely that ‘it is transposing into national law the provisions of Directive 2013/59 / Euratom: the French authorities retained the upper limit of the interval: 100 mSv for the emergency phase and 20 mSv for the following 12 months (And for the following years there is no guarantee that this reference level will not be renewed). These values apply to all, including infants, children and pregnant women! ” (source Criirad)
The Japanese government is asking residents to return home and abolishing compensation for evacuees. The Olympics are coming, Fukushima must be perceived as “normal” so that the athletes and supporters of the whole world won’t be afraid, even if it means sacrificing the health of the local population. It is therefore necessary to make known the map of Mr. Ozawa so that future advertising campaigns do not stifle the reality of the facts.
Data on measurements at Minamisōma
Website of the measuring team: Fukuichi Area Environmental Radiation Monitoring Project
Address of the original map (HD)
Source : Article of Pierre Fetet
(Translation Hervé Courtois)