The fortifications of Xi’an are an archaeological marvel and one of UNESCO’s World Heritage Sites. The fortress was built during the 14th century by Zhu Yuanzhang, the first Emperor of the Ming dynasty, and it has been kept and refurbished several times since its construction. But to see the areas that might need interventions, researchers today have employed a cutting-edge technique that requires celestial help. Using cosmic rays, they have found some peculiarities within the walls.
The approach is called muography, or muon tomography. It employs a subatomic particle, called a muon, that is produced in great abundance by cosmic rays. You can picture a muon as the heavy cousin of the electron. They have the same electrical charge, but muons are 200 times heavier. When cosmic rays (usually protons or atoms) hit the atmosphere at high speed, they can create showers of muons.
Even though they last for just 2.2 microseconds, about 10,000 muons reach the ground per minute before decaying into an electron and two kinds of neutrinos. That's roughly like getting one through your hand every second. When they get through solid objects, they scatter due to the interactions with atoms, and by measuring these scattered muons it is possible to map the internal three-dimensional structures of solid objects.
This technique has been applied to volcanoes, including the three most famous Italian ones: Stromboli, Etna, and Mount Vesuvius. It has been used to study glaciers, and even assess damage to the Fukushima nuclear power plant following the 2011 Tōhoku earthquake and tsunami. It has also had intriguing applications in archaeology, notably the discovery of a void inside the Great Pyramid of Giza.
“Due to the natural presence and deeper penetration of cosmic ray muons, scientists have performed various pioneer studies in fields, such as customs security, the internal imaging of volcanoes, scientific archaeology, and others,” the authors wrote in their paper. “With unique advantages, muography has gained increasing attention from archaeologists as a novel and innovative tool to investigate large-scale archaeological sites.”
When it came to the City Walls of Xi’an, researchers focused on a specific rampart and discovered density anomalies within the wall. They believe that these are safety hazards for the structure and that they ought to be eliminated to keep the walls standing.
The work from the team suggests that muography can be an invaluable tool, not just to study archaeological sites, but also to preserve them.
The findings are reported in the Journal of Applied Physics.