The Chinese city of Xi’an has a deep, rich history. Remains from Lantian Man, belonging to the species Homo erectus, were discovered in the 20th century and dates to about 710,000 years ago, which is perhaps one of the oldest known occupations of the area. A 6,500-year-old Neolithic Village has also been discovered there, and centuries later it was the political center of the Zhou Dynasty in the 11th century. During various points in its history, the city served as the center for other dynasties, and therefore boats an impressive architectural history.
And now, thanks to new technologies, researchers are able to identify where structural damage might be and allow the walls to stand for centuries more.
A new study led by Guorui Liu from the Chinese Academy of Science used a scanning technique called muography to survey the defensive walls. Muography is a non-destructive technology which uses cosmic ray muons. A muon is an elementary particle that is similar to electrons, but weighs 207 times as much (that’s the difference in weight between an adult human and small elephant!). When a charged particle called a cosmic ray hits the atmosphere, muons rain down all over the planet. They can then travel deep into the earth, including into stone. Analysis centered on counting muons that pas through stone are particularly useful for archaeological or geological investigations—they’ve been used to study Egyptian pyramids where X-rays were unable to penetrate.
Because of both the natural presence of cosmic ray muons and their ability to deeply penetrate structures, various fields have been adapting muography. Due to its non-destructive nature, it is a particularly appealing methodology for archaeologists who investigate large-scale sites. In the current study, a portable muon imaging system (CORMIS, or Cosmic Ray Muon Imaging System) deployed up to six measurement locations to gather 3D muonraphy data of the Xi’an walls.
Scans performed on the Xi’an walls reveled interior density fluctuations about the size of a meter inside one section of the wall. Researchers speculate that the fluctuations could be a few things, including deterioration or an archaeologically important discovery. While more research must be done to determine what the density anomalies inside the walls mean, results are an important step towards adapting new technologies, especially for extremely culturally important archaeological sites.
Sources: Wikipedia, Journal of Applied Physics, U.S. Department of Energy, Science News (2022), Science News (2023)
