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
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.
