26 January 2009

Tomb raiders

Cosmos Online
Archaeologists are turning to particle physics to probe deeper into ancient structures for hidden knowledge and treasure. The key to it all is a curious particle, the muon.
Tomb raider

What lies beneath: Muon detectors are unleashing the secrets of what lies beneath Egyption and Mayan pyramids, as well as other ancient archaeological ruins. Credit: Photolibrary

IN THE DENSE JUNGLES of northwestern Belize, the sound of metal hitting rock startles flocks of tropical birds and troops of black howler monkeys. Archaeologist Norman Hammond and his team stop digging. They have hit a stone wall three metres below ground; beyond it is a royal tomb. After almost 100 test excavations at La Milpa, an ancient Mayan city that once housed about 50,000 people, Hammond has unearthed something big.

Within the tomb lies the remains of a man wearing a jade pendant in the form of a vulture’s head. He is thought to be either the Maya king Bird Jaguar, the fifth-century ruler of La Milpa, or one of his successors. And finding him was an incredible stroke of luck.

Hammond, an expert on pre-Columbian archaeology, made his famous discovery more than 10 years ago. But repeated efforts to find other tombs at the site have come up empty-handed. Lacking an ancient map of La Milpa or blueprints for its five pyramid mounds and buried plaza, archaeologists rely mostly on instinct. But they run the risk of piercing priceless relics with their shovels or digging fruitlessly for years.

The ground beneath the site “could be Swiss cheese, for all we know,” riddled with burial chambers, tunnels and hidden entrances, says Hammond, who is based at Boston University in Massachusetts, USA.

Now researchers hope to find those hidden spaces with a little help from a most unexpected quarter: the particle physics community.

The key to the new approach is the muon, a hefty cousin of the electron that’s created when cosmic rays hit the atmosphere. Muons pass harmlessly through people and buildings; around 600 of them have flown through your body since you started reading this article. They fascinate scientists because they’re one of the few high-energy particles raining down from the sky that can be examined for clues to the nature of the cosmos. But it’s the ability of the muon to penetrate deep into rock and water that has archaeologists excited.

Muons travelling through rock or other dense material will slow and eventually stop, while those flying through empty spaces keep going at full speed. The idea is to catch the muons after they’ve passed through an archaeological site and measure their energies and trajectories. This allows researchers to reconstruct their paths and compile a three-dimensional (3-D) image that reveals hidden chambers or other voids.

If you think of the archaeologists as like surgeons probing a patient, particle physicists are like radiologists whose non-invasive X-rays reveal what’s going on inside the body. Just as X-rays leave patients unscarred, muons offer a way to explore ancient ruins without disturbing them.

A team led by Arturo Menchaca Rocha, head of the physics institute of the National Autonomous University of Mexico (UNAM), in Mexico City, plans to place a muon detector beneath the Pyramid of the Sun in Teotihuacán, northeast of the capital.

Meanwhile, particle physicist Roy Schwitters of the University of Texas at Austin, USA, is making plans to install muon detectors in wells dug on opposite sides of a mound at La Milpa where Bird Jaguar was found. The detectors will quietly gather muons for about a year, slowly building a picture of the interior of each pyramid.

MUON DETECTORS HAVE a rich history of revealing the unusual and the unseen. Sixty years ago, scientists in Australia used them to measure layers of mountain snow. Today, Japanese scientists are testing them as a way to track magma rising in volcanoes. Border patrol agencies in the U.S. see the technology as a potential way to uncover radioactive materials hidden inside cargo containers and trucks.

It was about 40 years ago that archaeologists tapped muons for the first time. Luis Alvarez, a Nobel Prize-winning physicist at the University of California, Berkeley, wondered if muons might reveal chambers in the Second Pyramid of Chephren, one of the three great pyramids of Egypt that had somehow escaped the notice of archaeologists and looters for 4,500 years.

Alvarez and his team put a detector in the Belzoni Chamber, near the centre of the pyramid’s base, and left it to collect muons for two years. They concluded that no additional chambers were hidden in the limestone above, although the scan could distinguish the four edges of the pyramid and what remained of its smooth limestone facing. While it would have been more exciting to discover a new chamber, this information still proved valuable for Egyptologists and archaeologists studying the pyramid – and it showed that the technique worked.

There are other high-tech ways to explore a ruin. Ground-penetrating radar reflects off buried features, while electrical probes measure the increased resistance to electrical flow due to the presence of stone and brick. Though useful, neither of them probes as deeply or takes as wide a view as a muon detector does.

In the early 1970s, a pair of archaeologists cleared stone and gravel out of a well at the base of the world’s third-largest pyramid, the Pyramid of the Sun at Teotihuacán in Mexico. Beneath the rocks they found a stairway leading to a tunnel 100 metres long and eight metres below ground. It opened up an extraordinary opportunity for the physics community, Menchaca Rocha says: “It is the key that will allow us to carry out an experiment similar to that of Alvarez” by placing a muon detector directly below the pyramid.

Menchaca Rocha published a preliminary paper on the method in the journal Nuclear Instruments and Methods in Physics Research in February, and hopes to begin collecting data by the end of 2008. The detector contains six gas-filled chambers, and when a muon travels through one of them, it collides with particles in the gas and gives off light. By recording those light fl ashes and noting exactly where the muon entered and left the chamber, researchers can calculate its energy and trajectory. Menchaca Rocha thinks the detector will reveal any cavity more than 75 cm tall.

The results could help to answer a question that has stumped archaeologists for decades: what was the purpose of the pyramid?

Linda Manzanilla, an archaeologist with UNAM, studies Teotihuacán, once a bustling centre full of pyramids and temples. “It was a city that attracted many people,” she says, “and it flourished for five centuries.” She believes a volcanic eruption drove people from the north towards the valley where the metropolis rose: “There they built a temple to appease the fire gods.”

Originally devoted to agriculture, the temple became a symbol of the state and its rulers. Could one of those rulers lie in a tomb within the Pyramid of the Sun? Manzanilla doesn’t think so. No tombs have been found there yet, and she doesn’t think that will change.

Without a muon detector, Manzanilla can’t test her predictions. “The Pyramid of the Sun is so wide and high,” she says. “Ground-penetrating radar can see only a small depth and width. We can expand that view using the muon detector.” And, she hopes, put to rest speculation about what is inside.

Scwitters is eager to answer similar questions at La Milpa in Belize, but the design of his detector is slightly different.

It’s a gas-filled cylinder wrapped with strips of material that detect muons as they enter and leave. Another detector, at the bottom of the cylinder, picks up fl ashes of light from muons zipping through the gas.

Rather than putting his detector directly below the pyramid, which would require digging a tunnel, Schwitters plans to place two detectors in shallow shafts on either side and 50 to 60 metres apart. This will eliminate blind spots and make it easier to construct a 3-D image.

Only the most energetic muons will be used for the reconstruction; since they are not as easily deflected, their paths through the site are truer and more direct. Schwitters says it should take about 10 days to record and trace 1,000 muon arrivals.

While waiting for the funding they need to set up a laboratory in Belize, Schwitters and his team have been testing the five-metre-long prototype detector they have built at the University of Texas in Austin.

Big stacks of bricks stand in for the stony bulk of the pyramid; the team moves the bricks up onto the roof and into other difficult positions to see how the detector handles the challenge. From the data, Schwitters and his group can see not only the piles of bricks, but also the shadows of the nearby engineering and physics buildings, massive structures that impede the flow of muons. If the detector can distinguish these dense objects, Schwitters says, it can find cavities as well.

“The technology has really improved since the time of Alvarez,” says Schwitters. “The detectors are simpler and more robust. We’re looking to make the detector more portable and improve our software, and then we can get serious.”

Designing the detector is the first of many challenges for the Schwitters team. Once they get funding and permission, they will have to transport the detectors to Belize and dig holes in which to put them. They’ll also need to find a way to power the lab at the remote site. “It’s a slow process and we’ve got a lot to do here,” Schwitters says, adding that he hopes to move to Belize by the first half of 2009.

La Milpa remains shrouded in mystery. Hammond says, “Some questions we cannot yet answer are: why was the city founded? What was its strategic or economic importance? Why did it collapse? And why was it abandoned in a time of great construction?” Muon detection may answer at least some of these questions and give archaeologists an edge to unearth the next exciting discovery.

Haley Bridger is a science writer at the Broad Institute in Boston. This article originally appeared in Symmetry magazine, Mar/Apr 2008.

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