Chinese researchers have discovered unusual density segments in the Ming Dynasty city wall built by Emperor Zhu Yuanzhang.
The Xi’an City Wall is an archaeological wonder and a UNESCO World Heritage site. This wall was constructed in the 14th century by Zhu Yuanzhang, the first emperor of the Ming Dynasty. The structure has undergone several fortifications since its construction. However, to determine which areas require intervention, researchers are now applying advanced techniques using cosmic rays to detect anomalies within the 12-meter high and 18-meter thick city wall.
Cosmic rays can penetrate the 12m high and 18m thick Xi’an City Wall. (Photo: Science News).
According to a study published in the journal Applied Physics in January 2022, nuclear physicist Zhiyi Liu from Lanzhou University, China, and his team created the highest resolution muon scan images of the archaeological site. The scans reveal density fluctuations inside the Xi’an City Wall. These fluctuations may indicate dangerous construction flaws or hidden structures of archaeological value.
The method used by the researchers is called muography, which utilizes subatomic particles known as muons, produced abundantly by cosmic rays. Although they carry the same charge as electrons, muons are over 200 times heavier. When cosmic rays (usually protons or atomic nuclei) penetrate the atmosphere at high speeds, they can generate a rain of muons.
Despite existing for only 2.2 microseconds, approximately 10,000 muons reach the ground every minute before decaying into electrons and two types of neutrinos. When passing through solid objects, they scatter due to interactions with atoms. By measuring the scattered muons, researchers can map the three-dimensional structure inside solid objects.
This technique has been applied to volcanoes, including the three most famous volcanoes in Italy: Stromboli, Etna, and Vesuvius. Muons have also been used to study glaciers, assess damage to the Fukushima nuclear power plant following the earthquake and tsunami in 2011, and even to discover voids inside the Great Pyramid of Giza.
- “Due to their natural persistence and the deep penetration capability of muons in cosmic rays, scientists have conducted pioneering research in various fields such as security, imaging inside volcanoes, and archaeology. With its unique advantages, muography is increasingly attracting the attention of archaeologists as a new advanced tool for exploring large-scale archaeological sites,” Liu’s team remarked.
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