Muon scanning hints at mysteries within an ancient Chinese wall

Physicists have found lots of scanning uses for these particles that rain down all over Earth

A photo of the Xi'an fortress wall with the city skyline visible in the background.

Muon scans are helping to find flaws and archeological surprises inside an ancient fortress wall in the Chinese city of Xi’an, which has grown and modernized since the wall was built nearly 650 years ago.

Mariusz Kluzniak/Moment/Getty Images Plus

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

Now, thanks to their penetrating abilities, muons may be key to ensuring that the walls that once protected the treasures of the first Ming Dynasty — and are now a national architectural treasure in their own right — stand for centuries more.

A refined detection method has provided the highest-resolution muon scans yet produced of any archaeological structure, researchers report in the Jan. 7 Journal of Applied Physics. The scans revealed interior density fluctuations as small as a meter across inside one section of the Xi’an ramparts. The fluctuations could be signs of dangerous flaws or “hidden structures archaeologically interesting for discovery and investigation,” says nuclear physicist Zhiyi Liu of Lanzhou University in China.

An image showing the density of a section of the Xi'an fortress walls.
In this image, muons have revealed density anomalies in a section of the Xi’an fortress walls. All colors in this plot indicate places where densities are lower than average for the structure, with the bluer portions showing where density is lowest.Xi’an City Wall Management Committee, G. Liu et al/Journal of Applied Physics 2023

Muons are like electrons, only heavier. They rain down all over the planet, produced when charged particles called cosmic rays hit the atmosphere. Although muons can travel deep into earth and stone, they are scattered or absorbed depending on the material they encounter. Counting the ones that pass through makes them useful for studying volcano interiors, scanning pyramids for hidden chambers and even searching for contraband stashed in containers impervious to X-rays (SN: 4/22/22).

Though muons stream down continuously, their numbers are small enough that the researchers had to deploy six detectors for a week at a time to collect enough data for 3-D scans of the rampart.

It’s now up to conservationists to determine how to address any density fluctuations that might indicate dangerous flaws, or historical surprises, inside the Xi’an walls.

James Riordon is a freelance science writer and coauthor of the book Ghost Particle – In Search of the Elusive and Mysterious Neutrino.

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