Research into cosmic rays by scientists at Stellenbosch University (SU) and the University of the Western Cape (UWC) has been energised by the delivery, from France, of a muon detector. The detector is on loan from the French Institute of Physics of the Two Infinities (IP2I), located in the city of Lyons.

Muons are used by ground-based observers as an indirect way to study cosmic rays. These latter are high-energy particles, travelling at close to the speed of light, which come from interstellar space and cannot penetrate far into Earth’s atmosphere. But when these rays encounter the atoms and molecules of the topmost levels of the atmosphere, the resulting interactions produce muons, which “rain down” on the Earth below.

“We are bombarded by about 10 000 muons per square metre per minute,” explained SU Department of Physics experimental physicist Dr JJ van Zyl. “Or, about one muon goes through your outstretched hand every second. Muons also come down in showers that emanate outwards from the point where they were formed in the atmosphere.”

Muons are, in fact, very unstable and decay into three other particles (an electron and two different types of neutrinos) in about two-millionths of a second. But, because muons are also travelling at nearly the speed of light, they can travel long distances before they decay, allowing them to be detected on, and even in some cases under, the Earth’s surface.

The fact that some muons can penetrate significant amounts of rock explains the second key reason that the IP2I has lent the muon detector to SU and UWC. It will also be used to help determine the merits of the Paarl Africa Underground Laboratory (PAUL) project. PAUL is planned to be located in a service tunnel in the Huguenot tunnel complex, which links Paarl to Worcester in the Western Cape province, under the Du Toits Kloof mountain.   

“Like electrons, muons are negatively charged. But they are also 200 times heavier than electrons,” he elucidated. “This means they lose energy when passing through matter like the atmosphere and mountains, but they lose only a small amount of energy for every centimetre of material, depending on its density. A cosmic muon reaching the top of Du Toits Kloof mountain will lose about 5% of its energy for every metre it passes through. After about 20 m of rock, they will be stopped. Only the very large energy muons will make it through the 800-m-thick bedrock that covers the Huguenot tunnel.”

Once set up in the service tunnel, the detector will be used the measure the muon background and flux in the tunnel. It will also be used to image the geology of the rock above and around the tunnel complex. The resulting data will help establish the construction criteria for PAUL, as well as assist with the design and set-up of the laboratory’s detectors.

South Africa’s Department of Science and Innovation recently committed R5-million to fund the scientific and engineering feasibility studies for PAUL. The prime objective for PAUL, like the other underground laboratories around the world, will be to detect, observe and study the rare interactions between neutrinos, as well as the still mysterious dark matter particles, and ordinary matter. But it will also serve other scientific disciplines as well.