Fishermen express fury as Fukushima plant set to release radioactive material into ocean

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Local residents and environmental groups have condemned a plan to release radioactive tritium from the crippled Fukushima nuclear plant into the Pacific Ocean.

Officials of Tokyo Electric Power Co., the operator of the plant, say tritium poses little risk to human health and is quickly diluted by the ocean.

In an interview with local media, Takashi Kawamura, chairman of TEPCO, said: “The decision has already been made.” He added, however, that the utility is waiting for approval from the Japanese government before going ahead with the plan and is seeking the understanding of local residents.

The tritium is building up in water that has been used to cool three reactors that suffered fuel melt-downs after cooling equipment was destroyed in the magnitude 9 earthquake and tsunami that struck north-east Japan in March 2011.

Around 770,000 tons of highly radioactive water is being stored in 580 tanks at the site. Many of the contaminants can be filtered out, but the technology does not presently exist to remove tritium from water.

“This accident happened more than six years ago and the authorities should have been able to devise a way to remove the tritium instead of simply announcing that they are going to dump it into the ocean”, said Aileen Mioko-Smith, an anti-nuclear campaigner with Kyoto-based Green Action Japan.

“They say that it will be safe because the ocean is large so it will be diluted, but that sets a precedent that can be copied, essentially permitting anyone to dump nuclear waste into our seas”, she told The Telegraph.

Fishermen who operate in waters off the plant say any release of radioactive material will devastate an industry that is still struggling to recover from the initial nuclear disaster.

“Releasing [tritium] into the sea will create a new wave of unfounded rumours, making all our efforts for naught”, Kanji Tachiya, head of a local fishing cooperative, told Kyodo News.

http://www.telegraph.co.uk/news/2017/07/14/fishermen-express-fury-fukushima-plant-set-release-radioactive/

 

Fukushima’s Radiation Will Poison Food “for Decades,” Study Finds

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Three of the six reactors at Japan’s Fukushima-Daiichi complex were wrecked in March 2011 by an earthquake and tsunami. The destruction of emergency electric generators caused a “station blackout” which halted cooling water intake and circulation. Super-heated, out-of-control uranium fuel in reactors 1, 2, and 3 then boiled off cooling water, and some 300 tons of fuel “melted” and burned through the reactors’ core vessels, gouging so deep into underground sections of the structure that to this day operators aren’t sure where it is. Several explosions in reactor buildings and uncovered fuel rods caused the spewing of huge quantities of radioactive materials to the atmosphere, and the worst radioactive contamination of the Pacific Ocean ever recorded. Fukushima amounts to Whole-Earth poisoning.

Now, researchers say, radioactive isotopes that were spread across Japan (and beyond) by the meltdowns will continue to contaminate the food supply for a very long time.

According to a new study that focused on “radiocaesium” — as the British call cesium-134 and cesium-137 — “food in japan will be contaminated by low-level radioactivity for decades.” The official university announcement of this study neglected to specify that Fukushima’s cesium will persist in the food chain for thirty decades. It takes 10 radioactive half-lives for cesium-137 to decay to barium, and its half-life is about 30 years, so C-137 stays in the environment for roughly 300 years.

The study’s authors, Professor Jim Smith, of the University of Portsmouth, southwest of London, and Dr. Keiko Tagami, from the Japanese National Institute of Radiological Sciences, report that cesium-caused “radiation doses in the average diet in the Fukushima region are very low and do not present a significant health risk now or in the future.”

This phraseology deliberately conveys a sense of security — but a false one. Asserting that low doses of radiation pose no “significant” health risk sounds reassuring, but an equally factual framing of precisely the same finding is that small amounts of cesium in food pose a slightly increased risk of causing cancer.

This fact was acknowledged by Prof. Smith in the June 14 University of Portsmouth media advisory that announced his food contamination study, which was published in Science of the Total Environment. Because of above-ground atom bomb testing, Prof. Smith said, “Radioactive elements such as caesium-137, strontium-90 and carbon-14 contaminated the global environment, potentially causing hundreds of thousands of unseen cancer deaths.”

No less an authority than the late John Gofman, MD, Ph.D., a co-discoverer of plutonium and Professor Emeritus of molecular and cell biology at the University of California, spent 50 years warning about the threat posed by low doses of radiation. In May 1999, Gofman wrote, “By any reasonable standard of biomedical proof, there is no safe dose, which means that just one decaying radioactive atom can produce permanent mutation in a cell’s genetic molecules. My own work showed this in 1990 for X rays, gamma rays, and beta particles.”

The Fukushima-borne cesium in Japan’s food supply, and in the food-web of the entire Pacific Ocean, emits both beta and gamma radiation. Unfortunately, it will bio-accumulate and bio-concentrate for 300 years, potentially causing, as Dr. Gofman if not Dr. Smith might say, hundreds of thousands of unseen cancer deaths.

https://www.counterpunch.org/2017/06/22/fukushimas-radiation-will-poison-food-for-decades-study-finds/

 

 

Fukushima Farmers Struggle

The technology to fully decontaminate a contaminated land has not yet been invented. Despite of all their efforts and hopes, those farmers’ struggle is just beginning and will last for ages…

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Farmers in Fukushima are struggling to revive their livelihoods. The 2011 nuclear accident and subsequent evacuation devastated farms — the area’s main source of jobs.

Some areas, like the village of Iitate, have lifted most of their evacuation orders. But getting back to normal is taking some time.

More than 200 farmers used to raise cows in this region. But 2 months after authorities lifted their evacuation order, few farmers have tried to return to raising animals.

Six cows were released into a paddy field to graze. It’s a step to revive the farm work that was widely seen in Iitate village.

One farmer is using his cows as an experiment that could bring hope to others.

After the animals eat these fields for 2 months, they’ll have their blood tested to check if they have been influenced in any way by radioactive material.

“It’s finally starting. For those who are worried or not confident about resuming cattle raising, I hope what I’m doing will encourage them,” says the farmer, Takeshi Yamada.

Before the accident, farmers in Iitate used to cultivate some 2,300 hectares of land. But this year, only 20 are being used to grow rice and buckwheat.

Some 60 farmers plan to resume farming this year — a small fraction of the previous total.

A major concern behind the slow uptake is the uncertainty farmers have about being able to sell their produce. Surface soil in the area was removed to help decontaminate the ground, but doing that also lowered its fertility.

Another challenge according to farmers is weakened community bonds.

“We used to work together. We were ready to take on whatever tasks we had. But it’s been 6 years, and the motivation to work is low. Nobody now says ‘let’s work together,'” says farmer Koichi Aoki.

To counter their plight, farmers are doing small things.

They formed a group of volunteers to remove weeds. They’re planting flower seeds to beautify the land and keep weeds from coming back. And there’s an even bigger benefit.

“We’ve been protecting our farmland. We want to keep it from turning to wasteland. And by working together, we’ll be able to form human bonds again. That’s our main goal,” says farmer Masuo Nagasho.

It will take time, but people here are hopeful these small steps are just the beginning.

https://www3.nhk.or.jp/nhkworld/en/news/editors/3/fukushimafarmersstruggle/

 

In Fukushima, a land where few return

The evacuation orders for most of the village of Iitate have been lifted. But where are the people?

1.jpgThe build-up of contaminated bags is slowly changing the landscape of Iitate, Fukushima Prefecture.

IITATE, FUKUSHIMA PREF. – Some day when I have done what I set out to do, I’ll return home one of these days, where the mountains are green, my old country home, where the waters are clear, my old country home.

— “Furusato,” Tatsuyuki Takano

A cherry tree is blooming in the spring sunshine outside the home of Masaaki Sakai but there is nobody to see it. The house is empty and boarded up. Weeds poke through the ground. All around are telltale signs of wild boar, which descend from the mountains to root and forage in the fields. Soon, the 60-year-old farmhouse Sakai shared with his mother and grandmother will be demolished.

I don’t feel especially sad,” Sakai says. “We have rebuilt our lives elsewhere. I can come back and look around — just not live here.”

A few hundred meters away the road is blocked and a beeping dosimeter begins nagging at the bucolic peace. The reading here is a shade over 1 microsievert per hour — a fraction of what it was when Sakai’s family fled in 2011.

The radiation goes up and down, depending on the weather, Sakai says. In gullies and cracks in the road, and up in the trees, it soars. With almost everyone gone, the monkeys who live in the forests have grown bolder, stopping to stare at the odd car that appears instead of fleeing, as they used to.

A cluster of 20 small hamlets spread over 230 square kilometers, Iitate was undone by a quirk of the weather in the days that followed the nuclear accident in March 2011. Wind carried radioactive particles from the Fukushima No. 1 nuclear power plant, which is located about 45 kilometers away, that fell in rain and snow on the night of March 15, 2011. After more than a month of indecision, during which the villagers lived with some of the highest radiation recorded in the disaster (the reading outside the village office on the evening of March 15 was a startling 44.7 microsieverts per hour), the government ordered them to leave.

Now, the government says it is safe to go back. With great fanfare, all but the still heavily contaminated south of Iitate, Nagadoro, was reopened on March 31.

2.jpgA radiation monitoring post is installed in the village of Iitate on March 27, ahead of the lifting of an evacuation order for most areas of the village. The post bears the message ‘Welcome home.’

The reopening fulfills a pledge made by Mayor Norio Kanno: Iitate was the first local authority in Fukushima Prefecture to set a date for ending evacuation in 2012, when the mayor promised to reboot the village in five years. The village has a new sports ground, convenience store and udon restaurant. A clinic sees patients twice a week. All that’s missing is people.

Waiting to meet Kanno in the government offices of Iitate, the eye falls on a book displayed in the reception: “The Most Beautiful Villages in Japan.” Listed at No. 12 is the beloved rolling patchwork of forests, hills and fields the mayor has governed for more than two decades — population 6,300, famous for its neat terraces of rice and vegetables, its industrious organic farmers, its wild mushrooms and the black wagyu cow that has taken the name of the area.

The description in the book is mocked by reality outside. The fields are mostly bald, shorn of vegetation in a Promethean attempt to decontaminate it of the radiation that fell six years ago. There is not a cow or a farmer in sight. Tractors sit idle in the fields. The local schools are empty. As for the population, the only part of the village that looks busy is the home for the elderly across the road from Kanno’s office.

3.jpgA school sits deserted in Iitate, Fukushima Prefecture, in April.

The village will never return to how it used to be before the disaster,” Kanno says, “but it may develop in a different way.”

Recovery has started, Kanno says, wondering whether returnees will be able to start building a village they like.

Who knows? Maybe one day that may help bring back evacuees or newcomers,” Kanno says. “Life doesn’t improve if you remain pessimistic.”

Even for those who have permanently left, he adds, “it doesn’t mean that their furusato can just disappear.”

The pull of the furusato (hometown) is exceptionally strong in Japan, says Tom Gill, a British anthropologist who has written extensively about Iitate.

Yearning for it “is expressed in countless sentimental ballads,” Gill says. “One particular song, simply titled ‘Furusato,’ has been sung by children attending state schools in Japan since 1914.”

The appeal has persisted despite — or perhaps because of — the fact that the rural/urban imbalance in Japan is more skewed than in any other developed nation, Gill says; just 10 percent of the nation’s population live in the country.

This may partly explain the extraordinary efforts to bring east Fukushima back to life. By one study, more than ¥2.34 trillion has been spent decontaminating an area roughly half the size of Rhode Island.

There has been no official talk of abandoning it. Indeed, any suggestion otherwise could be controversial: When industry minister Yoshio Hachiro called the abandoned communities “towns of death” in September 2011, the subsequent outrage forced him to quit a week later.

Instead, the area was divided into three zones with awkward euphemisms to suggest just the opposite: Communities with annual radiation measuring 20 millisieverts or less (the typical worldwide limit for workers in nuclear plants) are “being prepared for lifting of evacuation order,” districts of 20-50 millisieverts per year are “no-residence zones” and the most heavily contaminated areas of 50 millisieverts or more per year, such as Nagadoro, are “difficult-to-return.”

In September 2015, Naraha, which is located 15 kilometers south of the Fukushima No. 1 nuclear plant, became the first town in the prefecture to completely lift the evacuation order imposed after the triple meltdown. Naraha has a publicly built shopping street, a new factory making lithium batteries, a kindergarten and a secondary school.

A team of decontamination workers has been sent to every house — in some cases several times. Of the pre-disaster 7,400 residents, about 1,500 mainly elderly people have returned, the local government says, although that figure is likely inflated.

In Iitate, the cost of decontamination works out at about ¥200 million per household. That, and the passage of time, has dramatically reduced radiation in many areas to below 20 millisieverts a year. However, Kanno says, the cleanup extends to only 20 meters around each house, and three-quarters of the village is forested mountains. In windy weather, radioactive elements are blown back onto the fields and homes.

All that money, and for what?” asks Nobuyoshi Itoh, a farmer and critic of the mayor. “Would you bring children here and let them roam in the fields and forests?”

4.jpgNobuyoshi Itoh walks through a forest by his land in Iitate, Fukushima Prefecture.

Itoh opted to stay in one of the more heavily toxic parts of the village after everyone fled, with little apparent ill effect, although he says his immune system has weakened.

One of the reasons why Iitate was such a pleasant place to live before the nuclear crisis, he recalls, was its unofficial barter system. “Most people here never bought vegetables; they grew them,” he says. “I would bring someone potatoes and they would give me eggs. That’s gone now.”

At most, he says, a few hundred people are back — but they’re invariably older or retired.

They alone will not sustain the village,” Itoh says. “Who will drive them around or look after them when they are sick?”

As the depth of the disaster facing Iitate became clear, local people began to squabble among themselves. Some were barely scraping a living and wanted to leave, although saying so out loud — abandoning the furusato — was often difficult. Many joined lawsuits against the government.

Even before disaster struck, the village had lost a third of its population since 1970 as its young folk relocated to the cities, mirroring the hollowing-out of rural areas across the country. Some wanted to shift the entire village elsewhere, but Kanno wouldn’t hear of it.

Compensation could be a considerable incentive. In addition to ¥100,000 a month to cover the “mental anguish” of being torn from their old lives, there was extra money for people with houses or farms. A five-year lump sum was worth ¥6 million per person — twice that for Nagadoro. One researcher estimates a rough figure of ¥50 million for the average household, sufficient to leave behind the uncertainties and worries of Iitate and buy a house a few dozen miles away, close enough to return for work or to the village’s cool, tranquil summers.

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Masaaki Sakai stands outside his home in Iitate, Fukushima Prefecture.

Many have already done so. Though nobody knows the true figure, the local talk is that perhaps half of the villagers have permanently left. Surveys suggest fewer than 30 percent want to return, and even less in the case of Nagadoro.

Yoshitomo Shigihara, head of the Nagadoro hamlet, says many families made their decision some time ago. His grandchildren, he says, should not have to live in such a place.

It’s our job to protect them,” Shigihara says. He lives in the city of Fukushima but returns roughly every 10 days to inspect his house and weed the land.

Even with so much money spent, Shigihara doubts whether it will bring many of his friends or relatives back. At 70 years of age, he is not sure that he even wants to return, he says.

I sometimes get upset thinking about it, but I can’t talk with anyone in Fukushima, even my family, because we often end up quarreling,” he says. “People try to feel out whether the others are receiving benefits, what they are getting or how much they received in compensation. It’s very stressful to talk to anyone in Iitate. I’m starting to hate myself because I end up treating others badly out of frustration.”

Kanno has won six elections since 1996 and has overseen every step of Iitate’s painful rehabilitation, navigating between the anger and despair of his constituents and the official response to the disaster from the government and Tokyo Electric Power Company Holdings (Tepco), operator of the crippled nuclear plant.

6.jpgGround Self-Defense Force members decontaminate areas tainted with radioactive substances in Iitate, Fukushima Prefecture, in December 2011.

He wants more money to complete decontamination work (the government claims it is finished), repair roads and infrastructure. Returnees need financial support, he says. However, it is time, he believes, to end the monthly compensation, which, in his view, induces dependency.

If people keep saying that life is hard, they will not be able to recover,” he says. “What we need is support for livelihoods.”

A new system gives seed money to people who voluntarily come back to start businesses or farms.

We don’t want to give the impression that we are influencing people’s decisions or forcing them to return,” the mayor says, using the phrase “kokoro ni fumikomu,” which literally means “to step into hearts.”

Yet, next year, thousands of Iitate evacuees will face a choice: Go back or lose the money that has helped sustain them elsewhere for six years. Evacuation from areas exposed to less than 20 millisieverts per year will be regarded as “voluntary” under the official compensation scheme.

This dilemma was expressed with unusual starkness last month by Masahiro Imamura, the now sacked minister in charge of reconstructing Tohoku. Pressed by a freelance reporter, Imamura tetchily said it was up to the evacuees themselves — their “own responsibility, their own choice” — whether or not to return.

The comment touched a nerve. The government is forcing people to go back, some argued, employing a form of economic blackmail, or worse, kimin seisaku — abandoning them to their fate.

Itoh is angry at the resettlement. For him, politics drives the haste to put the disaster behind.

It’s inhuman to make people go back to this,” he says. Like the physical damage of radiation, he says, the psychological damage is also invisible: “A lot of people are suffering in silence.”

Itoh believes the government wants to show that the problems of nuclear power can be overcome so it can switch the nation’s idling nuclear reactors back on. Just four are in operation while the fate of 42 others remains in political and legal limbo. Public opinion remains opposed to their restart.

Many people began with high hopes in Iitate but have gradually grown distrustful of the village government, says Kenichi Hasegawa, a farmer who wrote a book titled “Genpatsu ni Furusato o Ubawarete” (“Fukushima’s Stolen Lives”) in 2012. Right from the start, he says, the mayor desperately tried to hide the shocking radiation outside his office.

Villagers have started losing interest,” Hasegawa says.

Meetings called by the mayor are poorly attended.

But they hold meetings anyway,” Hasegawa says, “just to say they did.”

Kanno rejects talk of defeatism. A tourist shop is expected to open in August that will attract people to the area, he says. Some villagers are paving entrances to their houses, using money from the reconstruction budget. As for radiation, everyone “has their own idea” about its effects. The lifting of the evacuation is only the start.

Itoh says he once trusted public officials but those days are long gone. By trying to save the village, he says, the mayor may in fact be killing it.

7.jpgBags filled with contaminated waste sit in a field in the village of Iitate, Fukushima Prefecture, in March 2016.

http://www.japantimes.co.jp/news/2017/05/13/national/social-issues/fukushima-land-return/

Fukushima village farmers plant rice for 1st time since nuclear disaster

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Farmers have begun planting rice in a village in Fukushima Prefecture, Japan, for the first time since the 2011 nuclear disaster contaminated the soil with radiation, leading to forced evacuations.

Eight farms in the village of Iitate plan to resume rice-growing this year in a combined area of about 7 hectares, the Japan Times reported. That area is significantly smaller than the 690 hectares available to farmers before the Fukushima disaster.

It marks the first time since the area was evacuated that farmers have been able to plant rice for commercial sales. Evacuation orders were lifted at the end of March for parts of the village.

The farmers will conduct radiation tests on the rice before shipping it to retailers. However, no rice grown in Iitate has shown radioactivity levels beyond the safety standard since experimental planting began in 2012.

Meanwhile, the government also took steps to encourage evacuees to return to the area on Wednesday, with an upper house committee passing a bill aimed at boosting governmental support so that displaced individuals can return to their homes earlier than planned.

The bill, which is expected to soon be passed by the upper house plenary session, will allow the government to fund more infrastructure rebuilding in the area, including roads and removing radioactive substances.

Also on Wednesday, the mayor of Minamisoma, Fukushima Prefecture, called on Prime Minister Shinzo Abe to introduce an advanced medical care system in the city, which is located north of the Fukushima Daiichi nuclear plant.

Minamisoma is also developing a system which will give residents access to doctors online, in an effort to quell health concerns.

In March 2011, an earthquake and subsequent tsunami led to the meltdown of three nuclear reactors at Fukushima Daiichi, making it the worst nuclear disaster since the Chernobyl catastrophe in 1986.

https://www.rt.com/news/387907-rice-farmers-fukushima-nuclear/

Fukushima village begins sowing rice for first time since nuclear crisis

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A farmer plants rice at a paddy for commercial sale in Iitate, Fukushima Prefecture, on Wednesday for the first time since the meltdowns at the Fukushima No. 1 nuclear power plant in 2011. In the forefront is a sign warning against an electric fence set up for wild boars.

FUKUSHIMA – Rice planting for commercial sales began on Wednesday in a village in Fukushima Prefecture for the first time since the 2011 disaster at the Fukushima No. 1 nuclear power plant.

A total of eight farms in Iitate plan to resume growing rice this year in a combined area of about 7 hectares after evacuation orders were lifted at the end of March for large parts of the village.

With much of the area contaminated by radiation following the nuclear crisis, the total arable area has shrunk from around 690 hectares before the disaster, according to the village.

The farmers will conduct radiation tests before shipping their rice. No rice grown in the village has shown levels of radioactivity exceeding the safety standard since experimental rice planting began in 2012.

(I feel) comfortable. We want to get back even a step closer to the village of six years ago,” said Shoichi Takahashi, 64, while working a rice planting machine.

The municipality has supported farming efforts, including installing electric fences around the area to protect the rice fields from wild boar and working the soil after decontamination.

Measures to encourage evacuees to return to Fukushima are also slowly underway.

On Wednesday, an Upper House committee passed a bill aimed at boosting government support so evacuees can return to their homes earlier in areas which are off-limits in principle in the wake of the March 2011 nuclear meltdowns.

The Upper House plenary session is expected to clear the bill soon, allowing the government to fund more infrastructure rebuilding such as roads and get rid of radioactive substances in the area.

The bill already cleared the House of Representatives on April 14 but deliberations in the upper chamber stalled after Masahiro Imamura, who served as reconstruction minister, sparked outrage following a series of gaffes and ultimately resigned on April 26.

Minamisoma Mayor Katsunobu Sakurai called on Prime Minister Shinzo Abe on Wednesday to help introduce an advanced medical care system in the city north of the crippled Fukushima No. 1 nuclear plant.

Sakurai made the plea during his meeting with Abe at the Prime Minister’s Office.

The evacuation order was lifted last July in one part of the city but medical institutions and clinics had been on the decline even before the natural disasters and nuclear crisis.

In a bid to ease residents’ health concerns, the city office is developing a system where residents have access to doctors online.

Goichiro Toyoda, head of Medley Inc., which provides the remote medical care system, asked the government to revise regulations to allow a broader reach for the program.

Abe said he will do his best.

http://www.japantimes.co.jp/news/2017/05/10/national/fukushima-village-begins-sowing-rice-first-time-since-nuclear-crisis/

First On-Site True Gamma-Ray Imaging-Spectroscopy of Contamination near Fukushima Plant

Abstract

We have developed an Electron Tracking Compton Camera (ETCC), which provides a well-defined Point Spread Function (PSF) by reconstructing a direction of each gamma as a point and realizes simultaneous measurement of brightness and spectrum of MeV gamma-rays for the first time. Here, we present the results of our on-site pilot gamma-imaging-spectroscopy with ETCC at three contaminated locations in the vicinity of the Fukushima Daiichi Nuclear Power Plants in Japan in 2014. The obtained distribution of brightness (or emissivity) with remote-sensing observations is unambiguously converted into the dose distribution. We confirm that the dose distribution is consistent with the one taken by conventional mapping measurements with a dosimeter physically placed at each grid point. Furthermore, its imaging spectroscopy, boosted by Compton-edge-free spectra, reveals complex radioactive features in a quantitative manner around each individual target point in the background-dominated environment. Notably, we successfully identify a “micro hot spot” of residual caesium contamination even in an already decontaminated area. These results show that the ETCC performs exactly as the geometrical optics predicts, demonstrates its versatility in the field radiation measurement, and reveals potentials for application in many fields, including the nuclear industry, medical field, and astronomy.

Introduction

Following the accident in Fukushima Daiichi Nuclear Power Plants on 11 March 2011, a huge amount of radionuclides was released to the atmosphere. As in 2016, 137Cs and 134Cs, which radiate gammas mainly from 600 keV to 800 keV, still remain in Fukushima, and many areas are still contaminated as a result1. Operations of decontamination are called for in a wide area in Fukushima and its surroundings to satisfy a legal limit for the maximum exposure of 0.23 μSv/h at any publicly-accessible open spaces2. An effective method to measure and monitor gamma-ray radiation is essential for efficient decontamination work, and as a result there has been a surge of demand for gamma-ray instruments with a wide field of view (FoV) which quantitatively visualize Cs contamination.

Many gamma cameras have been developed to make imaging observations to help decontamination, based on the Compton camera (CC)3,4,5,6,7, pin-hole (PHC)8, and coded-mask technologies. However, none of them has detected more than a limited number of hot spots, or has reported any quantitative radiation maps, let alone imaging spectroscopy. The CC is the most advanced among these three, yet has an intrinsic difficulty in imaging spectroscopy, which is related to its Point Spread Function (PSF)9,10.

So far, the most successful evaluations for the environmental radiation in contamination areas have been made by backpacks11 and unmanned helicopters12,13. Although these methods are, unlike gamma cameras, non-imaging measurements, in which measurements at each point are made with either a spectrometer or conventional dosimeter, quantitative and reliable 2-dimensional distributions of radiation have been successfully obtained after several measurements with overlapping fields of view are combined. The downside is that they require a considerable amount of time and efforts, and thus are not practical to be employed in a wide area.

Another fundamental problem with all these methods is that they do not directly measure the radioactivity on the ground, but measure the dose at 1 metre high from the ground (hereafter referred to as “1-m dose”) instead, and hence require complex analyses to convert the measured dose to the actual radioactivity on the ground. Indeed, we show that the 1-m dose does not always agree well with that measured immediately above the ground, which suggests an intrinsic difficulty in obtaining an accurate radioactivity distribution on the ground from the 1-m dose.

After a few pilot experiments of decontamination were conducted in Fukushima, it turned out that the amount of reduction of the ambient dose by decontamination was limited. The reduction ratios, defined by the dose ratio compared between before and after decontamination, were approximately 20% only in lower ambient-dose areas (<3 μSv/h)2, while >39% in higher ambient-dose areas (>3 μSv/h). When a (high) dose is measured at a point, gammas that contribute to the dose can originate anywhere a few radiation lengths away (~100 m) from the point. The goal of decontamination is to somehow identify and remove those radiation sources. However, none of the existing instruments can identify them, i.e., none of them can tell where or even in which direction the radiation source is located. To untangle the sources of a dose of contamination, the directions of all the gammas, as well as their energies if possible, must be determined. It means that the brightness distribution around the point must be obtained.

To address these issues of existing methods and visualize the Cs contamination, we have developed and employed an Electron-Tracking Compton Camera (ETCC). ETCCs were originally developed to observe nuclear gammas from celestial objects in MeV astronomy14, but have been applied in wider fields, including medical imaging15 and environmental monitoring16,17. An ETCC outputs two angles of an incident gamma by measuring the direction of a recoil electron and hence provides the brightness distribution of gammas with a resolution of the PSF9,10. The PSF is determined from the angular resolutions of angular resolution measure (ARM) and scatter plane deviation (SPD)9,18. The ARM and SPD correspond to a resolution of the polar and azimuthal angles of an incident gamma, respectively. Since a leakage of gammas from their adjacent region to the measured point is correctly estimated with the PSF, quantitative evaluation of the emissivity anywhere in the FoV is attained.

The most remarkable feature of the ETCC is to resolve the Compton process completely; the ETCC does not only provide the direction of a gamma, but also enables us to distinguish correctly reconstructed gammas from those mis-reconstructed9. Thus, the ETCC makes true images of gammas based on proper geometrical optics (PGO), as well as energy spectra9 free of Compton edges10. The PGO enables us to measure precise brightness (or emissivity) at any points in an image using an equi-solid-angle projection, such as Lambert projection, without the information of the distance to the source, as shown in Fig. 1. The obtained emissivity can be unambiguously converted into the dose on the ground (hereafter the E-dose), of which the procedure is identical with that described in the IAEA report19, but without need of the fitting parameters. We find the E-dose to be consistent with the dose independently measured by a dosimeter, and thus confirm that remote-sensing imaging-spectroscopy with the ETCC perfectly reproduces the spatial distribution of radioactivity10.

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Results

We performed the field test of gamma measurement in October, 2014 in relatively high-dose locations with the averaged ambient dose ranging from 1 to 5 μSv/h in Fukushima prefecture, using the compact 10 cm × 10 cm × 16 cm ETCC with a FoV of ~100°ϕ17. The SPD and ARM of the ETCC were measured to be 120° and 6° (FWHM), respectively, for 662-keV 137Cs peaks, which correspond to the PSF (Θ~15°), i.e., the radius of the PSF of 15° for the region that encompasses a half of gammas emitted from a point source9. It uses GSO scintillators and has an energy resolution of 11% (FWHM) at 662 keV. We chose the three different kinds of locations for measurements: (A) decontaminated pavement surrounded with not-decontaminated bush, (B) not-decontaminated ground, and (C) decontaminated parking lot. Figure 2a,c and d show their respective photographs.

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We have found that the doses at 1-m and 1-cm measured with a dosimeter do not agree with each other, as demonstrated in Fig. 2a and b in the location (C). The 1-m dose, which is practically the emissivity averaged over the adjacent region of ~10 m, is the standard in the radiation measurement, presumably because it is useful to estimate potential health effects to the human body. The 1-cm dose, on the other hand, better reflects the emissivity on the ground at each grid point, of which the size is likely to be similar to the spatial resolution of the ETCC, and hence is useful to locate radioactivity on the ground for decontamination work. For these reasons, we adopt the 1-cm dose to compare with the emissivity measured with the ETCC in this work.

Figure 3a shows the photograph of FoV, overlaid with 1-cm dose at nine points and the E-dose map by ETCC, where the brightness (equivalent to the E-dose) is defined as the count rate of reconstructed gammas per unit solid angle (here 0.014 sr), corrected for the detection efficiency including the angular dependence of the ETCC9. Figure 3b shows the energy spectrum accumulated for the entire FoV, whereas Fig. 3c–e display those accumulated for the sky, the decontaminated pavement, and the not-decontaminated bush, respectively. The E-dose at the maximum brightness in Fig. 3a is estimated to be 2.6 μSv/h, which is consistent with the average of the 1-cm dose (0.9–4.3 μSv/h) around the centre of the FoV.

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The spatial distribution of the E-dose is found to be consistent with that of the 1-cm dose, which was independently measured. The spectrum in Fig. 3e shows prominent peaks of direct gammas of Cs, which implies the contamination from the bush area, whereas the spectrum of the decontaminated pavement (Fig. 3d) shows much weaker Cs peaks, which implies the effect of the decontamination. The latter is dominated with low-energy scattered gammas, which emanate from inside of the ground and the adjacent areas. The spectrum of the sky (Fig. 3c) is clearly dominated with Compton-scattered gammas from Cs peaks (with the expected energy ranging from 200 to 500 keV) in the air. We should note that the spectra free of Compton scattering components enable us to make the unambiguous identification of the sources of radiation.

The results of imaging-spectroscopy in the two contrasting locations (B and C), in which no and thorough decontaminations, respectively, have been conducted, are shown in Figs 4 and 5. The exposure times are 80 min and 100 min, respectively. The ETCC gives spatially-resolved spectra, and accordingly the detailed condition of contamination at each point, similar to Fig. 3. In the contaminated location (B), although the energy spectrum of the FoV shows strong and direct gamma emission from Cs, Cs is found to be concentrated in the limited area of spot 1 (Fig. 4e), whereas little Cs is found in the other regions in the FoV (Fig. 4f). As such, imaging-spectroscopic measurement is a reliable method to unravel the state of contamination quantitatively. Even in the decontaminated location (C), both the image (Fig. 5a) and spectrum (Fig. 5f) reveal the existence of a “micro hot spot”, where some Cs remains on the ground and the spectrum has the dominant Cs peak (Fig. 5f), whereas the spectra for other regions (Fig. 5e) show that the main component is scattered low-energy gammas. Both the maps of 1-cm dose (Fig. 5a) and E-dose (Fig. 5b) show a hint of a small enhancement originating from a micro hot spot, although it is at a similar level to the fluctuation of the scattered gammas. The E-doses at the points of the maximum brightness in (B) and (C) are 5.0 and 1.3 μSv/h, respectively, which are also consistent with the 1-cm dose at the corresponding points.

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Finally, we check consistency about a couple of properties of the ETCC and conventional dose measurements. First, we plot the total gamma counts obtained with the ETCC as 1-m doses at the position of the ETCC in Fig. 6a, and confirm a good correlation. Then, we plot the correlation between the 1-cm dose measured by the dosimeter and by the ETCC (E-dose) at the locations (B) and (C) in Fig. 6b. Except ~3 points adjacent to the hot spots in (B), the discrepancy between them is limited within ± ~30%. Considering the difference in the conditions, such as the size of the measured areas (~100 cm2 for a dosimeter and ~1 m2 for ETCC) and the energy range (>150 keV for a dosimeter and 486–1000 keV for ETCC), as well as the fact that a large dispersion in the accuracy of commercial dosimeters (±several 10%) has been reported, this amount of discrepancy is more or less expected. We conclude that good consistency between them is established for the wide range of the dose (0.1–5 μSv/h), and this is another proof that the ETCC achieves the PGO. In addition, the PGO gives the brightness of the sky over the hemisphere, and we find it to be comparable with that from the ground, after the difference in their solid angles is corrected (see the bottom row in Table 1). This means that roughly a half of the 1-m dose at any points originates from the sky. It then implies that the wide-band energy balance of gammas between the ground and the sky is in equilibrium and contribute to the ambient dose, presumably because the air is thick enough to scatter most of gammas emanating from the ground. It is consistent with the fact that the spectra of the sky (Figs 3c, 4c and 5c) are dominated with Compton scattering for Cs gammas (200–500 keV). This could not have been identified without spectra free of Compton edges. Our results also explain the reason why the amount of the reduction of the ambient dose was limited to often no more than 50% after decontamination work2 had been conducted in Fukushima, it is because a significant amount of radiation still comes from the sky in equilibrium.

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Discussion

Firstly, let us convert the emissivity to the 1-cm dose, using only the brightness measured by the ETCC. Figure 1b schematically shows the dosimeter configuration for the measurement of 1-cm dose. Since the top and the upper sides of the dosimeter are shielded with tungsten (W) rubber, it detects gammas emanating from the ground to the lower hemisphere only. The count density of the gammas which pass through the plane of the dosimeter (indicated as P in Fig. 1b) is estimated to be approximately 2πΣ · (1 − cos(θ = 80°)) = 5.2Σ, where Σ is emissivity on the ground. Then we convert the count density of gammas at the dosimeter position into doses in units of μSv/h with the conversion factor of 1 μSv/h = ~100 counts · sec−1 · cm−2 for 662-keV gammas in the dosimeter, based on the IAEA report19 (in page 85).

In the not-decontaminated location (B), 135 gammas were observed with the ETCC (dB) at the maximum brightness point in Fig. 4a, where the unit solid angle is 0.014 sr. The brightness of the gamma is calculated to be 135 counts · sec−1/(0.014 sr · 100 cm2) = 96 counts · sec−1 · sr−1 · cm−2, and then we get, from the relation Σ = dB, 5.2Σ = 500 counts · sec−1 · cm−2, which corresponds to the dose of 5.0 μSv/h (the two points indicated as 5.0 and 5.7 [μSv/h] in Fig. 4a). For the location (C), 35 gammas were observed at the maximum brightness point in Fig. 5a, and then dB (=Σ) = 35 counts · sec−1/(0.014 sr · 100 cm2) = 25 counts · sec−1 · sr−1 · cm−2 and 5.2Σ = 130 counts · sec−1 · cm−2, which corresponds to 1.3 μSv/h. The 1-cm dose at this point is found to be roughly equal to the average of 1.0–2.2 μSv/h in Fig. 5a. For the location (A), dB is calculated in the similar manner to be dB = 70 counts · sec−1/(0.014 sr · 100 cm2) = 50 counts · sec−1 · sr−1 · cm−2 and 5.2Σ = 260 counts · sec−1 · cm−2, which corresponds to the dose of 2.6 μSv/h. The 1-cm dose at this point is ~3 μSv/h, and is roughly equal to the average of 1–4.3 μSv/h in Fig. 3a.

For comparison, we also applied the simple method described in pages 96–101 in the IAEA report19, calculating the doses with a conversion coefficient of 8.7 × 10−3 (μSv/h)/(Bq/cm2) for θ~80° for the 1-cm dose, which is estimated by accumulating gamma-flux at each point from the ground with the tungsten rubber shield. This method is the one described in pages 96–101 in the IAEA report19. For the location (A), a gamma flux on the ground is calculated to be 2πΣ/0.85 = 369 (Bq/cm2) and then the dose is 369 × 8.7 × 10−3 = 3.1 μSv/h. For the locations (B) and (C), the doses are estimated to be 5.9 and 1.6 μSv/h, respectively. Thus, we confirmed that the results deduced by the two independent methods are consistent with each other.

Decontamination work in Fukushima faces serious difficulty; it is hard to pin down which region is badly contaminated from which radiation source without investing massive resources like wide-scale backpack measurements. The capability of the ETCC to measure the emissivity (or dose) independently of the distance would enable us to propose a novel approach to it. If a mapping of the brightness of 137Cs on the ground was carried out over the wide area with the ETCC by aircraft with the similar way conducted in 20122, we could visualize variation of the doses across the area, and could tell where decontamination work would be required most and how much.

As a different application, if multiple ETCCs are installed at various places in a nuclear plant to carry out a continuous three-dimensional brightness monitoring, we could not only detect, for example, a sudden radiation release by accident, but also make a quantitative assessment of where and how the release has happened. This would provide vital initial parameters to computer simulations to estimate the later dissemination of radioactivity over a wide area after an accident. In fact, simulations for this purpose faced a great difficulty in the past due to lack of reliable observed parameters of radio activity, because radiation monitoring was performed solely by repeated simple dose measurements. These simple dose measurements are unable to provide sufficient information over the wide area where the gamma radiation comes from, unless a huge amount of resources of manpower and hence budget are invested. Given that governments in many countries are confronted with the reactor dismantling issue, detailed and quantitative mapping of the radiation emissivity on the surfaces of reactor facilities, which would be well achievable with the ETCC, would be beneficial. The ETCC has immense potentials for immediate applications to various radiation-related issues in the environment.

Prospects

Some scientists assert that the detection efficiency of gas-based gamma detectors would be too low. However, we have found that some types of gas have sufficient Compton-scattering probability with the relevant effective areas of 110 cm2 and 65 cm2 at 1-MeV gammas with a 50-cm-cubic ETCC using CF4 gas and Ar gas at 3 atm, respectively9. Our prototype 30 cm-cubic ETCC with the effective area of a few cm2 at 300 keV was proved to perform expectedly well in MeV gamma-ray astronomy.

Now, we are constructing two types of more advanced ETCCs: one is a compact ETCC with the similar size and weight to the current model, but having a 20 times larger effective area (0.2 cm2 at 662 keV; type-A) and the other is a large ETCC aimed to be completed in 2018, which has a 1000 times larger effective area (10 cm2 at 662 keV; type-B). The details of Type-B are described elsewhere10.

Type-A has the similar size to the current ETCC, but has an increased TPC volume from 10 cm × 10 cm × 16 cm (rectangular solid) to 20 cm ϕ (in diameter)× 20 cm (cylinder), installed in the similar-sized gas vessel. It has a 5 times larger gas volume and 2.5 times wider detectable electron energy band with the TPC than the current model. In addition, if the mixed gas with Ar and CF4 (50%: 50%) at 2 atm is used, as opposed to the current Ar gas (~90% and some cooling gases) at 1.5 atm, the detection efficiency will be improved by a factor of 29. Then, the resultant detection efficiency (or effective area) will become 20 times larger than that of the current model, while keeping the similarly compact size and weight. The development of Type-A will be completed in 2017.

Type-B will provide the same detection limit for 6 sec exposure. If we perform a survey with Type-B from some aircraft at the altitude of 100 m, we will be able to make a spectroscopic map of a 1 km2 area with a 10 m × 10 m resolution for 1200 sec exposure to achieve the same detection limit, taking account of the absorption of the air. An unmanned airship is a good candidate for the aircraft, it flies slowly for an extended period and hence would enable us to do the precise imaging-spectroscopic survey. Then, the whole contamination area in Fukushima prefecture (roughly 20 km × 50 km) can be mapped with the same resolution as mentioned above in a realistic timescale of ~2 months, assuming 8 hours of work per day. Some of the spectra obtained in our aircraft-based survey might be found out to be generated by the gammas scattered by something, such as trees in woods, within the grid. Our survey will efficiently detect a hint for those areas, which can be then studied in more detail with on-site measurements, such as ones by backpacks11. No successful large-scale survey has been yet performed to monitor the radioactivity in Fukushima. Our upgraded ETCC will be capable of revolutionizing the decontamination work and more. We summarized the specifications of the current ETCC, type-A and type-B in Table 2.

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Methods

Instruments and Measurements

The ETCC was mounted at 1.3 m high from the ground at its centre, tilted 20° downwards beneath the horizontal plane. The average distance to the ground in the FoV is ~4 m, which corresponds to the spatial resolution of ~1 m at the ground for its PSF. As a reference, we also made a mapping measurement of the dose at two heights of 1 m and 1 cm with every 1-m square grid in the FoV (except for the location (A), where the points of the measurements were sparser and irregular) with the commercial dosimeter (HORIBA, Radi PA-1100, http://www.horiba.com). In the dose measurement at the latter height (~1 cm), the top and four sides of the dosimeter were covered by tungsten rubber to shield it from the downward radiation (Fig. 1b).

We have developed a compact ETCC with a 10 cm × 10 cm × 16 cm gas volume, based on the 30-cm-cubic SMILE-II for MeV astronomy9. The ETCC is, like CCs, equipped with a forward detector as a scatterer of nuclear gammas and a backward detector as a calorimeter for measuring the energy and hit position of scattered gammas. The forward detector of the ETCC is a gaseous Time Projection Chamber (TPC) based on micro-pattern gas detectors (MPGD), which tracks recoil electrons. The TPC of the ETCC is a closed gas chamber, and thus can be used continuously for about three weeks without refilling with the gas5. The backward detector is pixel scintillator arrays (PSAs) with heavy crystal (at present we use Gd2SiO5: Ce, GSO). It is noted that, at the time of writing in 2016 after the survey work presented in this paper, we have been developing the Ethernet-based data handling system to replace the existing VME-based system. The latest ETCC available for field measurements is much more compact, which is built in the 40 cm × 40 cm × 50 cm base frame with the weight of 40–50 kg, and operated with a single PC with 24 V portable battery.

The contamination area in Fukushima is the similar environment to the space in the background dominated condition, where the radiation spreads ubiquitously. It is understandable that gamma cameras with the Compton method became the first choice to be employed for the decontamination work in Fukushima, following the precedents in MeV astronomy, even though it is clearly not the ideal instrument especially in the background-dominated environment.

Analytical method for deriving an emissivity from the measured distribution of gammas

Here, we explain how we measure the emissivity (or brightness) based on the proper geometrical optics (PGO) by the ETCC and how we estimate the dose on the ground from the emissivity measured by the ETCC. The following are the reason why no gamma camera but the ETCC can take a quantitative nuclear gamma image with the similar principle to that of optical cameras. According to the well-known formulas in PGO, the relation between emissivity Σ on the ground and detected brightness of the gamma in ETCC (dB) for solid angle Ω is given as Σ · A1 · dΩ1 = dB · A2 · dΩ2. and the relations dΩ1 = A2/D2, dΩ2 = A1/D2 hold, where A1 and A2 are the observed areas on the ground and the detection area in the ETCC (A2 = 100 cm2), respectively, and D is a distance between the ground and the ETCC. Figure 1a gives a schematic demonstration of it. These relations are then reduced to Σ  = dB, which means that the emissivity is equal to the obtained brightness and is independent of the distance D in this optics. In practice, dB is calculated simply from the number of the detected gammas per unit solid angle corrected for the detection efficiency9. We should note that when the distance between a source and the ETCC (L) is comparable with, or longer than, the radiation length in the air (~70 m), dB in a unit solid angle must be corrected for the expected absorption, using the absorption coefficient (α) in the air for gammas with the relation dBcorrect = dB/(1 − exp(−L/α)).

Estimation of the emissivity and the detection limits

We estimate the detection limit using the sensitivity from the calibration data with a point source (137Cs, 3 MBq) in the laboratory17. We detected 662-keV gammas from the point source with a significance of 5σ at a distance of 1.5 m with the exposure time of 13 min. The point source increases the dose at the detector front by 0.015 μSv/h from a background dose. If the same amount of gammas entered the ETCC over the whole FoV, the significance would decrease by  = 0.5σ, assuming that the background gamma increases proportionally from 1 to 100 to the number of pixels. The current ETCC comprises 100 pixels and one pixel is defined as an area of the unit solid angle in the FoV. In the case of a 100 min observation under the dose of 2 μSv/h at the detector front (assuming the case of Location (C), i.e., low dose), the total number of gammas increases by . The expected significance per pixel is then calculated to be 16σ/ = 1.6σ, which is consistent with the observed significances of (1.2–2.5σ) in the low-dose area (see the error bars in Fig. 6b). Similarly, the expected significance for the high-dose area is calculated and is found to be also consistent with the observed values of (3–5σ). Thus, our results of the on-site measurements are well consistent with the expected significances estimated from the calibration in the laboratory.

We also estimate the emissivity within the PSF and the detection limit to check consistency with the calibration data. As shown in Fig. 7 the covered area by the PSF for the distance L between a target and the ETCC is given by L · sinΘ. Since the number of gammas (brightness) within the PSF is conserved along the line of sight, the sensitivity in the PSF is independent of the distance L if absorption in the air is not taken into account. For example, for the distances L of 10 m and 100 m, the sizes of an area corresponding to a detector pixel are estimated to be 1 m and 10 m, respectively, when the same detection limits for both the distances are used. The detection limit for the ~2σ level of the ETCC is 0.5 μSv/h at a unit solid angle for an exposure of 100 min (see the distribution of red points in Fig. 6b). Note that the limit is proportional to , and hence can be easily scaled for different exposures and effective areas.

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Additional Information

How to cite this article: Tomono, D. et al. First On-Site True Gamma-Ray Imaging-Spectroscopy of Contamination near Fukushima Plant. Sci. Rep. 7, 41972; doi: 10.1038/srep41972 (2017).

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Source: https://www.nature.com/articles/srep41972