Frontiers Physics World  December 2017

Muons reveal large void in Great Pyramid

Inside job Members of the ScanPyramids collaboration at Khufu’s Pyramid. (ScanPyramids Mission)

A large void hidden deep within Khufu’s Pyramid at Giza in Egypt has been discovered by detecting the muons that shower the Earth. The measurements were made by the ScanPyramids collaboration, which includes researchers from Egypt, Japan and France. They used three separate muon-imaging techniques to study the pyramid, which was built in about 2500 BC and is also known as the Great Pyramid and the Pyramid of Cheops.

The idea of using muons for imaging was pioneered in the 1960s by the future Nobel-prize-winning physicist Luis Alvarez, who placed a muon detector in a chamber in the nearby Pyramid of Khafre. Known as muography, the technique relies on the fact that muons travel relatively unhindered through the air but get absorbed by stone. So if more muons than expected reach a detector in the pyramid, they must have passed through an air-filled void.

In 2016 chemical-emulsion muon detectors developed at Nagoya University in Japan were deployed in the Queen’s Chamber – the lowest known chamber in the pyramid. Much like photographic film, the emulsion reacts chemically when exposed to muons, leaving 3D tracks that indicate the directions from which the particles came. As well as detecting known voids, these detectors provided the first evidence for a previously-unknown large void about 30 m long.

To verify its existence, scientists from the KEK particle-physics lab in Japan installed instruments comprising layers of plastic scintillator, which measure muon trajectories, at a separate location within the Queen’s Chamber. Outside the pyramid, meanwhile, physicists from France’s nuclear-research agency CEA monitored the muon flux through the pyramid using gas-filled “micromegas” detectors.

As a result of the three different 2D images taken from three different angles, the team could locate the void in 3D. A computer reconstruction based on the data suggests it is similar to the Grand Gallery of the pyramid – an inclined passage about 47 m long. The new void is, however, 50–70 m above ground level, putting it above the Grand Gallery. It is not clear, though, if the void is a single chamber or multiple chambers, or whether it is horizontal or inclined.

Hamish Johnston