Could cosmic rays unlock the secret tomb of China’s Qin Shi Huang guarded by terracotta warriors?

[Mista]

Could cosmic rays unlock the secret tomb guarded by China’s terracotta army?

A Chinese government study is assessing the feasibility of using advanced detectors to pinpoint the remains and treasures of the first emperor of China.

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Cosmic rays could help archaeologists pinpoint the secret chamber holding the remains and treasures of Qin Shi Huang, the first emperor of China, according to a Chinese government-funded study.

The Mausoleum of the First Qin Emperor in Xian, Shaanxi province, was built by hundreds of thousands of labourers over nearly four decades and finished around 208BC, according to Han dynasty historian Sima Qian, who lived soon after that period.

With a total area more than 70 times the size of the Forbidden City, it is the biggest tomb ever built for an individual in the world.
The tomb’s surface buildings are no longer standing, but its underground structures are mostly still intact. Some archaeologists believe the central chamber that housed the emperor’s coffin and most valuable treasures remain undisturbed after they combed the entire field and found no holes indicating thieves had been at work.

The study, funded by the central government to evaluate the feasibility of the cosmic ray project, found at least two cosmic ray detectors would be needed, to be planted in different locations less than 100 metres (328 feet) under the surface of the tomb.

These devices, each about the size of a washing machine, could detect subatomic particles of cosmic origin piercing the ground.

The data would allow scientists to identify hidden structures unseen using other detection methods in high detail, said Professor Liu Yuanyuan and her colleagues at Beijing Normal University in a paper published in Acta Physica Sinica, the official journal of the Chinese Physical Society, on Monday.

In the 1970s, more than 8,000 terracotta warriors – an army to protect the emperor in his afterlife – were found in the necropolis, buried in a pit away from the centre of the tomb.

After decades of surveying, archaeologists have confirmed the existence of an underground palace more than 30 metres tall. They also found trace evidence supporting descriptions by Sima that had been disregarded as fairy tale, such as pools and waterways filled with mercury to mimic China’s major rivers and the sea.

But the palace’s detailed structure and the exact location of the emperor’s chamber remained uncertain. Sima’s other descriptions – such as traps armed with arrows and crossbows to shoot anyone who enters the tomb – were not verified.
Using cosmic rays in archaeology is a concept that dates as far back as the 1960s. Astrophysicists discovered that cosmic rays could hit air molecules and produce a particle known as a muon that could penetrate almost anything.

Muons have a higher chance of being absorbed when going through denser materials. By comparing the number of muons a detector received from various angles, archaeologists could discover hollow structures, such as hidden chambers or passages in a building.

But the idea remained largely theoretical because muons were not easy to detect. And for decades scientists had to rely on bulky devices as big as a room, making field application difficult.

In recent years, thanks to the advances in particle physics, the size of cosmic ray detectors has shrunk significantly. In 2017, an archaeological team in Egypt discovered a 30-metre-long chamber in a 4,500-year-old pyramid using a portable device.

“As an ancient civilisation with a long history, our country has a large number of cultural relics in urgent need of study. Muon imaging can be an important supplement to traditional geophysical methods,” Liu and her colleagues said in the research paper.
Qin Shi Huang conquered all kingdoms and ended the Warring States of China with one of the largest, most technologically advanced armies in the ancient world. His exploits inspired many Hollywood blockbusters, such as The Mummy: Tomb of the Dragon Emperor, and Chinese epic films like Hero, both starring Jet Li.

Emperor Qin launched the construction of the Great Wall, built a nationwide highway network and standardised writing, units and currency. But he was also criticised for brutal policies, such as burning unauthorised books and prosecuting Confucian scholars.

When the construction of the tomb was completed, all workers and craftsmen were trapped and killed to keep the secrets inside, according to Sima.

Yang Dikun, assistant professor of geophysics with the Southern University of Science and Technology in Shenzhen, who was not involved in the project but is familiar with the technology, said the project was feasible.
“The muon detector that we built and used for fieldwork nowadays has become so small, it can be carried around by a child,” he said.

Yang said archaeologists had tried to map the tomb using other methods. Gravity anomaly detectors could detect substances of different densities but its range was limited to a small area and its accuracy could easily be affected by environmental disturbances.

Electromagnetic signals were most sensitive to structures containing metals, and ground-penetrating radar had limited depth.
However, the cosmic ray approach is not without challenges. The detectors must be planted at appropriate depths without affecting the building or artefacts above, according to Yang.

And unlike other detection methods that could get results almost instantly, the muon detectors must be in place long enough to gather enough particle counts for analysis.

“You need to be patient,” Yang said.

Computer simulation conducted by Liu’s team suggested it could take a year to gather sufficient data to produce a clear image.

It is not clear whether – or when – the project will get the nod from the government to go ahead. Liu and her colleagues said some technical details, such as the exact number and location of the detectors, required further evaluation and optimisation.

Most imperial tombs in China have been robbed or damaged, but a few remain intact, including the tomb of Empress Wu Zetian of the Tang dynasty (618-907), who ruled when ancient Chinese civilisation reached its zenith.

Chinese archaeologists opened an imperial tomb of the Ming dynasty (1368-1644) in Beijing in the 1950s and witnessed precious materials such as silk and paper disintegrating soon after being exposed to the open air.

Since then China’s government has had a strict policy forbidding access to these sites.

China is a latecomer in cosmic ray research, but its activities in the field have increased rapidly in recent years.
Liu’s team, for instance, was one of the research teams working in the Jinping laboratory, the world’s deepest cosmic ray detection facility more than 6,000 metres underground, to study dark matter.

In May, Chinese scientists detected the strongest light ever recorded in the universe with the world’s largest, most sensitive cosmic ray detector on a 4,410-metre-high mountaintop in Daocheng, Sichuan province.