TOKYO The riddle of what lies inside the pyramids has fascinated Egyptologists for centuries, yet the risk of damage and the complexity of excavating these ancient structures has meant we still only have a limited understanding of what is hidden at their core.
Now, intriguingly, elementary cosmic particles that hurtle through space may just provide scientists with a noninvasive way of peering inside these monolithic feats of engineering, in much the same way as X-rays are used to see inside the body.
Muography, as the technology is known, has also produced results in studying conditions of places that will always remain off limits.
The technique dates back to the late 1960s, when Nobel Physics laureate Luis Alvarez made the first attempts to use muons to see inside the pyramids, which are impenetrable to radar or X-rays. The elementary particles can pass through just about everything, including bedrock several kilometers thick.
Inspired by Alvarez's ultimately unsuccessful attempts to map the pyramids, Nagoya University assistant professor Kunihiro Morishima became driven to join an international team known as Scan Pyramids. The muon imaging technology used by Morishima and his colleagues showed that a previously unknown space may exist behind the northern face of the Great Pyramid of Giza.
With such an extremely small mass, muons can pass through even the thickest bedrock. As they encounter substances with varying densities, the number that successfully pass through changes. The ones that make it through are picked up on a special film known as a nuclear emulsion plate.
Beginning in September, Morishima and his team began taking readings from various locations and angles around the pyramid. By observing an area where more particles made it to the detector than expected, they got indications of an as-yet undocumented empty chamber.
The space is thought to be 1 to 2 meters in height and width. "This was an unexpected result," Morishima said.
NEW APPLICATIONS Analytical technology has moved on immeasurably in the 50 or so years since Alvarez's endeavors.
In the past, it was necessary to check the state of huge numbers of emulsion plates visually -- they turned dark where they had been struck by muons. But Nagoya University has developed a system that can automatically read and analyze muon tracks. Its processing capacity has "improved about a hundredfold in the past decade," according to Morishima. This has made it possible for muography techniques to be used in a variety of new fields.
The technique has now even been applied to study the insides of the nuclear reactors at the stricken Fukushima Daiichi nuclear power plant. Three of the units suffered meltdowns in the wake of the March 2011 disaster and nobody could be certain of the exact location of the molten nuclear fuel. This created enormous hurdles for the decommissioning work.
When researchers from the High Energy Accelerator Research Organization set up emulsion plates at unit No. 1, it became clear that the fuel was not in its original location but leaking out of the containment vessel. The No. 2 unit was studied by Nagoya University and Toshiba, revealing a high-density substance thought to be fuel that had melted down to the bottom of the pressure vessel. This year, a similar study is planned for unit 3.
A team from the University of Tokyo also began volcanic research using muography in the 1990s. Partnering with Nagoya University, in 2007 they took images of Mount Asama, in central Japan. Following that, they successfully captured images of the Showa-shinzan lava dome in Hokkaido, and Mount Stromboli in Italy.
Hiroyuki Tanaka of the University of Tokyo Earthquake Research Institute -- a core member of the group -- is now working to develop more compact and lightweight muon measurement technology. Nuclear emulsion plates require no power source and can be used underwater. However, they must be retrieved, developed and read for each individual reading. This makes it difficult to capture the insides of volcanoes, which are constantly in flux.
Tanaka has developed a system that combines a rod-shaped plastic detector with a photoelectric tube. This system captures light emitted when muons traverse the inside of the detector. With the ability to make multiple and continuous observations, this system was able to capture images of magma moving inside Satsuma-Iojima in Kagoshima, southern Japan.
He is also working on a system using gas detectors, in cooperation with institutions including the Hungarian Academy of Sciences. This approach would not use expensive photoelectric tubes, making it more compact and less costly -- and offer improved resolution.
NEW POSSIBILITIES Interest in the field is growing across the globe. In November, the University of Tokyo launched an international research organization that includes 19 institutions from seven European countries. The group aims to examine the condition of reinforcing bars and cracks inside buildings and expressways without having to demolish them. This would allow them to study the effects of shoddy construction and aging. The organization also plans to look for new applications, such as observing the interior of glaciers. "If we can reduce the cost of observation equipment and make it easier to use, there are enormous possibilities for it to spread into general use," Tanaka said.
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