By John von Radowitz, PA Science Correspondent
Sub-atomic particles created by cosmic rays from space are to be used to probe a giant Mexican pyramid and solve one of the world’s greatest archaeological mysteries.
Investigators are to install detectors beneath the Pyramid of the Sun that look for muons – charged particles generated when cosmic rays hit the atmosphere which continuously shower the Earth.
They hope the rate at which muons pass through the pyramid will reveal any hidden burial chambers inside.
The step pyramid, about 30 miles north-east of Mexico city, is 740 feet on each side, 215 feet tall, and thought to date back 2,000 years.
No-one knows who erected the monument, one of the biggest pyramids in the Americas, or why.
The site was abandoned 600 years before the arrival of the Aztecs, who named it The City of the Gods.
There has been speculation that the pyramid was used for burial, as was the nearby Pyramid of the Moon built 200 years later.
But although several tunnels have been cut through the monument by archaeologists, all they uncovered was earth and rock.
The new technique should at last unlock any secrets the pyramid conceals.
Scientists plan to install six box-shaped muon detectors in an ancient tunnel running 26ft below the base of the pyramid.
Experts believed the pyramid was built on top of the tunnel, which runs directly below the middle of the monument.
Each metre-wide detector will act as an “eye” looking upwards in search of muons.
The particles strike the Earth’s surface at a rate of about 10,000 per square metre per minute. They pass through large masses of rock or metal, yet the electrical charge they carry makes them easy to track.
Open spaces inside the pyramid would allow more muons to pass through them than the surrounding solid structure.
This difference can be measured from a number of directions to pinpoint the position of the chambers.
Dr Arturo Menchaca-Rocha, director of the physics institute at the National Autonomous University of Mexico, who is leading the team, said: “Muons are coming from everywhere. This detector covers an angle; it’s like an eye looking up. It covers a large fraction of the volume of the pyramid.
“If more muons than expected are observed in a given direction, that is an indication of less density than assumed in that direction, ie a possible cavity.”
Dr Menchaca-Rocha described the project today at the American Association for the Advancement of Science’s annual meeting in Washington DC.
He said the detectors had been built and were now being tested. Once installed, it would take a year for the work to be completed.
The meeting heard that muons could also be used to spot nuclear weapons or materials being smuggled in trucks or cargo containers.
Efforts to shield nuclear materials with lead or other heavy metals would actually make them easier to detect.
Dr Chris Morris, from the University of California’s Los Alamos Laboratory, has been working on a prototype detector similar to the X-ray booths already used to screen vehicles.
“We believe we’ve worked through all of the major obstacles to building a prototype system for a range of security scenarios,” said Dr Morris.
Dr Kanetada Nagamine, from the KEK Muon Science Laboratory in Japan, told how muons could provide a volcano early warning system.
Detectors can probe the inside of volcanoes and look for interior channels where molten rock may be rising, an early sign of a potential eruption.