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Muography and the benefits it will bring to society

November 08, 2017

In May 2017, the Earthquake Research Institute of the University of Tokyo, the Wigner Research Centre for Physics of the Hungarian Academy of Sciences, and NEC Corporation started the joint development of a system for visualizing the internal components of massive structures using muography, a visualization technique based on cosmic rays. We asked Professor Hiroyuki Tanaka from the Center for High Energy Geophysics Research of the Earthquake Research Institute of the University of Tokyo, the leading expert on muography, about what the technology is all about and what benefits it offers to society.

Muography: visualizes the interior of all kinds of structures

──What kind of technology is muography?

Professor Tanaka:
Muography is a visualization technology that uses muons—high energy particles that have high penetrating power—to create projection images (or simply projections) of the interior of objects, similar to X-ray images. Muons are particles generated when cosmic particles, which are plunged into the atmosphere by supernova explosions and other cosmic events occurring far out in space, reach the Earth and react with oxygen and nitrogen nuclei in the air. They are capable of travelling for long periods at the speed of light and have the power to penetrate rocks to a depth of a kilometer. In the same way that X-rays stop upon hitting human bones, muons also cannot pass through some rocks or objects, making it possible to create images of the internal structures using the difference in penetrability of objects. In other words, it is comparable to an X-ray for massive objects.

Professor Hiroyuki Tanaka
Center for High Energy Geophysics Research
Earthquake Research Institute
The University of Tokyo

──Tell us why muography has gained wide attention in recent years.

Professor Tanaka:
Muons were discovered by American physicists in 1936, were first used by Australian physicists to determine rock density in the 1950s, and were applied so searching for hidden chambers of pyramids in the late 1960s. And in 2006, our research team became the first in the world to successfully use muography to image the internal structure of a volcano (Mount Asama,Japan). Until that point, volcanoes have been considered as the most difficult imaging targets for muography. But revolutionary technologies of the 21st century have enabled overcoming a wide range of obstacles, such as imaging resolution, equipment size, and cost considerations, making it possible to use muography to visualize the interior of volcanoes, which are much bigger than pyramids. The results of this experiment proved that all massive objects smaller than volcanoes can be visualized, putting the spotlight back unto muography.

Since then, muography has been applied in various fields, such us in visualizing the meltdown of the Fukushima nuclear plant and the inner chambers of the Great Pyramids of Giza,Egypt.

Smoke rising from Mount Iwo-dake on Satsuma-Iwojima Island (© 2014 Hiroyuki Tanaka)
Muograph of Mount Iwo-dake on Satsuma-Iwojima Island. A larger-than-expected mass of low-density magma is seen about 300 meters below the volcano crater. (© 2014 Hiroyuki Tanaka)

──What’s the difference between objects that are penetrable and those that are not?

Professor Tanaka:
To be accurate, it’s not about being penetrable or not, but rather about the level of penetrability, i.e. whether objects are easily penetrable or not. For example, between an object that allows 80 out of 100 particles to penetrate and one that allows only 30 particles to penetrate, the latter can be considered to have high density, while the former to have low density. We can, therefore, visualize the density contrast from the degree of penetrability.

──Is there a correlation between the development of muography and the evolution of technology?

Professor Tanaka:
That’s a very important question. For example, X-ray images taken 100 years ago are essentially the same as those taken today. The commercialization of X-ray irradiation machines and the development of imaging plates and recyclable X-ray films, however, have made it now possible for any radiographer to perform X-ray imaging in hospitals, a task that before only radiology experts were able to perform. That is very important because previously only particle physicists had access to muography, but technological development has made it accessible to non-experts, making it available for use in a wide array of applications in society.

──If the technology of muography becomes generic and applied to the solution of social issues, what benefits would it bring to society?

Professor Tanaka:
For example, muography can be used to confirm the status of meltdown inside nuclear power plants, where it would be dangerous for people to enter, or it can be embedded in advance in expressways to monitor the state of degradation of roads, or it can be used to visualize the interior of blast furnaces, and a wide range of other possibilities. Our role, however, as scientists, is to develop the base technology, while working on the individual applications is the job of the different companies.

For the current project, we have chosen NEC as a partner in developing practical applications of muography in society because we believe that NEC does not develop technologies that address only particular problems, but the company has the ability to convert “seeds” into needs of different customers and enterprises and to apply muography to the solution of prevailing social issues.

AI is essential in the development of practical applications of muography in society

──For this joint research project, what are the respective roles of the Hungarian Academy of Sciences, the University of Tokyo, and NEC Corporation?

Professor Tanaka:
The Wigner Research Centre for Physics of the Hungarian Academy of Sciences developed a ground-breaking particle detector that is lightweight, inexpensive, and has superior resolution. The Earthquake Research Institute of the University of Tokyo forged an academic exchange agreement with the Hungarian Academy of Sciences in 2015 and signed an IP agreement the following year. Through these agreements, the Hungarian particle detectors were embedded in the muography measurement system we developed. For the current joint development project, the measuring system was lent to NEC, who is tasked to conduct demonstration experiments aimed at developing practical applications of muography in society.

NEC will be using muography in monitoring degradation and visualizing the interior of buried structures and other massive social infrastructures that are inaccessible to humans, in cautionary surveillance to determine activity of natural structures that lead to natural disasters, and other applications.

The muography measurement system was lent to NEC, who is tasked to conduct trials aimed at developing practical applications of muography in society.

──Other than lending of the system, will you also help in coming up with ways to use and apply it?

Professor Tanaka:
That’s right. By carrying out the project as a two-way rather than a one-way arrangement, we expect NEC to be able to help us categorize the applications of muography, such as figuring out how it can be used for particular industries and for investigating particular materials.

──Can AI be used in muography research?

Professor Tanaka:
AI is an integral component of the next-generation muography for two reasons: one is in quality improvement and the other is for forecasting.

In principle, muography is similar to sun printing, wherein image quality is determined by the amount of muons captured. You can obtain a precise image if you capture as many muons as possible by using a longer time of exposure. Time (length of exposure) and size (of the detection device) constraints, however, come to play when the technology is used for practical applications. Due to such limitations, only course resolution images could be obtained. This is where AI comes in; it can be used in enhancing the definition of the images. Through AI, pixels can be refined using a few good-quality images as baseline data as a means to improve image quality.

Another reason why AI is important in next-generation muography is in forecasting; machine learning can be used to forecast future phenomena. For example, we can take one muography image of a volcano every day and perform a time-series analysis of the images to grasp the volcanic activity as a means to forecast future activity.

──IoT seems to be also highly compatible with muography, is that correct?

Professor Tanaka:
IoT is also a very important technology for muography; and we have already started doing research on IoT at the university. The current project we are working on is displaying muographic images on smartphones by holding up your smartphone camera to a volcano. This is done by setting up a detection device on a volcano, uploading the images obtained by the device to a central server, and computing the location of a user that accesses the server based on compass angle and the GPS of the user’s smartphone, to send the relevant image to the user. This experiment is for volcanoes, but it can be applied to other uses like obtaining internal images of buildings and other structures in urban areas.

──Going forward, what will be needed to be able to use muography for practical applications in society?

Professor Tanaka:
One is the further advancement of the technology itself. The first-generation muography was like a sun printing image where real-time measurements could not be made. In the second-generation muography, it became possible to capture movement of magma inside volcanoes. And now, we are developing the third-generation muography system that is equipped with the device from Hungary. Although we think this is the final form of the system, most likely a fourth-generation system would come about eventually. Another thing is that the era where muography is the only tool we have is now over. As mentioned earlier, combining with AI, IoT, and other technologies, will make muography much easier to use and understand and will make it a truly practical tool.

In addition, it is also important to have more people become interested in the technology. That’s why we have to widely disseminate information and open up new avenues to reach new users.

──Finally, tell us about your expectations for NEC.

Professor Tanaka:
Being able to acquire concrete evidences for practical application of muography in society through demonstration experiments is a very welcome development for us. Also, since making muography practically applicable requires downsizing the sensor and reducing power consumption and costs, we are counting on NEC to address these issues, too. We would like to see NEC function as a hub for sharing the results of muography demonstration experiments to different companies and pave the way for it to be used in solving various social issues. Since this enables a different kind of advantage compared with simply accumulating results from joint research between scientists or bilateral research between industry and academia, I hope that NEC will actively take on that pivotal role for finding practical applications in society for muography.

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