In March of 2011, there was a massive earthquake off the northeast coast of Japan. The quake caused a tsunami that flooded the Fukushima Daiichi Nuclear Power Plant on the coast of the Fukushima Prefecture. Three reactors melted down and there was a huge explosion in Unit 3 that destroyed Unit 4. Radioactive materials were ejected into the atmosphere and fallout rained down all over Japan. Detailed maps of contamination are important to the cleanup but existing methods of creating such maps were not sufficient. Recent work at Kyoto University seeks to improve the making of contamination maps.
Toru Tanimori is a professor of Physics and Astronomy in the Graduate School of Science at Kyoto University. His latest research has just been published in Scientific Reports. He says that the "... best methods we have currently are labor intensive, and to measure surface radiation accurately complex analysis is needed."
Tanimori team "... constructed an Electron Tracking Compton Camera (ETCC) to detect nuclear gamma rays quantitatively. Typically this is used to study radiation from space, but we have shown that it can also measure contamination, such as at Fukushima." His analysis reveals that there are "micro hot spots" of radiation from cesium-134 and cesium-137 around the Fukushima Daiichi Nuclear Power Plant in places which have already been decontaminated and were thought to be free of radioactive materials. As a matter of fact, even existing methods would have been sufficient to show that the cleanup in those areas was not adequate to ensure the safety of the citizens who live there.
Tanimori and his team are employing gamma-ray imaging spectroscopy that is more versatile and robust than previous gamma-ray detection cameras, resulting in a clearer image. Previous use of gamma-ray imaging was error prone and had difficulty pinpointing the exact location of the gamma-ray sources. He said that the key to better images was to take a color picture that included the direction and the level of energy of all gamma-ray sources in the vicinity.
Tanimori says that "Quantitative imaging produces a surface radioactivity distribution that can be converted to show dosage on the ground. The ETCC makes true images of the gamma rays based on proper geometrical optics." This distribution can then be used to relatively easily measure ground dosage levels, showing that most gamma rays scatter and spread in the air, putting decontamination efforts at risk. "Our ETCC will make it easier to respond to nuclear emergencies," continues Tanimori. "Using it, we can detect where and how radiation is being released. This will not only help decontamination, but also the eventual dismantling of nuclear reactors." The latest ETCC available for field measurements is built in the 16 inch × 16 inch × 20 inch base frame with the weight of 88–110 lb and operated with a single PC with 24 V portable battery.
Tanimori's work will have application far beyond Fukushima and the radioactive contamination of the Japanese countryside. There are contaminated areas all over the world that need to be cleaned up and Tanimori system will be useful in that work.
