The search for dark matter finds gold

A method for locating seams of gold and other heavy metals is the un­likely spin-off of Swinburne University’s in­volvement in a huge experiment to detect dark matter down a mine in Stawell, Victoria.

Associate Professor Alan Duffy, from Swinburne’s Centre for Astrophysics and Super­computing and a member of the Sodium iodide with Active Background REjection (SABRE) project, said cosmic radiation was effectively creating an X-ray of the Earth between the under­ground detector and the surface.

In the mine, the SABRE experiment seeks to detect particles of dark matter, some­thing no-one has conclusively achieved yet. Any signal from dark matter would be minis­cule, and so the SABRE team created a phenomenally sensitive detector, which, it turns out, is also sensitive to a host of cosmic particles that can help us to locate gold.

Detecting particles that aren’t dark matter is unwant­ed noise to SABRE — which is why they located the experiment one kilometre down a mineshaft, where the rock above was thought thick enough to absorb any cosmic radiation.

Nature’s own penetrating scanner

However, the team found some radiation still penetrated — not ideal for isolating rare dark matter events, but creating a powerful source of informa­tion. “Nature has given us the most powerful penetrating scan­ner you can create, and there’s no licence required,” said Duffy.

These particles that make it to the Stawell Underground Physics Laboratory are muons: short-lived particles similar to electrons, but 200 times heavier. Muons are preferen­tially scattered by atoms with high atomic numbers and so deposits of heavy metals, such as gold, whose atomic number is six times greater than that of carbon, create shadows similar to bones in a medical x-ray image.

The idea’s not entirely new, but Duffy noted that the technology “had come of age”. The team’s redesigned muon detector prototype is a far cry from its 1960s predecessor, a box of bulky high-voltage elec­tronics that needed two people to lift it. Miniaturisation of electronic components driven by smartphone technology contributed to Duffy’s device, which he likens in size to “a fashionable paperweight.”

“The first one we built was a cylindrical piece of scintillating plastic in a paint tin. You’ve nev­er seen anything that looked so rough and ready, but it just gave the most beautiful detections.”

Small and cheap enough to set and forget

The size is perfect for lowering into mineral explo­ration boreholes, and because the technology is cheap and can connect via optical fibre, Duffy envisages “deploying half a dozen of them and walking away.” Weeks later a picture of the minerals in the surrounding rock could be reconstructed from the data, which will re­quire processing similar to that used in astrophysics.

Duffy is excited by plans to build a company around the device, and the team already has interest from potential partners. But, the first step is to turn their prototype into an instrument robust enough for an active mine site.

“Dark matter is ethereal,” Duffy added. “It’s fundamental to physics, but it’s hard to think of practical uses. Yet I see real commercial outcomes in this spin-off.”