Machines that can ‘X-ray’ a volcano to get more affordable

A cutting-edge imaging technique used to see inside objects from buildings to volcanoes may become more accessible and affordable, thanks to a major innovation by Japanese and Hungarian researchers.

The researchers have found that using gas detectors can sharply reduce the size and cost of equipment used in muography — a form of radiography that uses subatomic particles called muons to view the internal structures of opaque objects. They believe the innovation will lead to a wide range of commercial applications, such as monitoring the internal conditions of buildings.

Muons, a type of elementary particle, are similar to electrons but more than 200 times heavier. The muons found at the Earth’s surface are created by collisions of cosmic rays with particles in the Earth’s atmosphere and can penetrate deep into the ground.

Just like an X-ray plate captures radiation passing through the body, special equipment is used to capture muons passing through volcanoes and other objects.

Joint development

An international conference held in Japan last year led to the joint development of prototype equipment for muography that uses gas detectors, by the University of Tokyo’s Earthquake Research Institute and the Wigner Research Centre for Physics at the Hungarian Academy of Sciences. In May this year, the two institutions signed an agreement on intellectual property cooperation in a ceremony at the Hungarian Embassy in Tokyo. The agreement focuses on joint efforts to explore commercial applications of the new technology.

The researchers will continue working together to create a commercially viable muography instrument that is small enough to be carried in a bag, according to Hiroyuki Tanaka of the University of Tokyo’s Earthquake Research Institute, who developed the prototype with Dezso Varga of the Hungarian research lab. The equipment currently used typically weighs several tons and costs more than 100 million yen ($973,000).

Muography has been used to detect the movements of magma inside volcanoes and to investigate archaeological monuments. It has also been used to monitor the conditions of nuclear fuel that melted inside the damaged reactors at the Fukushima Daiichi nuclear power plant. To facilitate commercial use in areas like inspecting the internal conditions of buildings, roads and bridges, it is vital to reduce the weight, size and price of the equipment.

The first generation of muography equipment featured nuclear emulsion plates that recorded the traces of charged particles passing through, just as photographic film records the traces of light. Second-generation equipment, now used widely, utilizes a combination of plastic detectors and photo tubes to capture light emitted by muons as they pass through — an improvement that enables the instruments to gather data continuously.

However, the second-generation technology requires many photo tubes, making instruments expensive — the tubes cost about 200,000 yen each — and difficult to downsize.

The gas detectors now being developed feature tubes filled with a mixture of argon and carbon dioxide. A grid of fine electric wires captures small bolts of lightning produced when muons pass through ionized gas. In addition to being smaller and cheaper, the new instruments also allow higher-resolution images by narrowing distances between electric wires.

Some technological challenges had to be overcome. Gas detectors are vulnerable to vibrations, which can cause a short circuit in the electrical wiring, and also to changes in temperature. The Hungarian Academy of Sciences, which has a reputation for developing sophisticated gas detectors, has contributed to overcoming these issues. By combining tubes, it has developed a prototype muon detector in the form of a plate that is 80cm in length and width.

The detector costs around 200,000 yen per square meter, less than 10% of the cost of second-generation equipment, and weighs about 10kg, or one-seventh the weight. It offers a resolution of 1cm, twice the performance of current instruments.

The new instruments may find a wide range of applications. They can be put to use, for instance, to detect signs of shoddy construction and aging inside buildings and highways. They can also be used to inspect conditions inside furnaces as part of quality control for steel products.

Tanaka has an ambitious goal of developing a muography instrument that costs only about 100,000 yen. If the project succeeds, the market for muography equipment could grow sharply.

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