Muon scanning hints at secrets of China’s ancient Great Wall, scientists study

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Physicists have found many applications for scanning these particles that rain down across the Earth.

For almost 650 years, the fortress walls of the Chinese city of Xi’an served as a powerful barrier around the central part of the city. At 12 meters high and up to 18 meters thick, they are impenetrable to almost everything except subatomic particles called muons.

Now, thanks to their penetrating abilities, muons may be the key to keeping the walls, which once protected the treasures of the first Ming dynasty and are now a national architectural heritage, standing for centuries to come.

An improved detection method has provided the highest-resolution muon scan of any archaeological structure, researchers report in Journal of Applied Physics from January 7. The scan revealed internal density fluctuations of up to a meter within one section of the Xi’an shaft. Fluctuations could be signs of dangerous defects or “hidden structures that are archaeologically interesting to discover and explore,” says nuclear physicist Zhiyi Liu of Lanzhou University in China.

In this image, muons have detected density anomalies in part of the walls of the Xi’an fortress. All the colors in this graph indicate places where the density is lower than the average for the structure, and the blue parts show where the density is the lowest.XI’AN CITY WALL MANAGEMENT COMMITTEE, H. LIU ETC. / JOURNAL OF APPLIED PHYSICS 2023

Muons are like electrons, only heavier. They pour across the planet, formed when charged particles called cosmic rays hit the atmosphere. Although muons can penetrate deep into earth and rock, they are scattered or absorbed depending on the material they encounter. Counting what passes makes them useful for studying the interior of volcanoes, scanning pyramids for hidden cameras, and even finding contraband hidden in x-ray-proof containers.

Although the muons stream in continuously, their numbers are small enough that the researchers had to use six detectors over the course of a week to collect enough data for a 3-D scan of the shaft.

Conservationists must now determine how to respond to any fluctuations in density that may indicate dangerous flaws or historical surprises in Xi’an’s walls.

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