Australia turns gold mine into physics lab to study dark matter

Half a mile-deep lab is shielded with 100 tons of steel.
Ameya Paleja
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Dark Matter Lab

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A gold mine located over half a mile (one km) underground in Victoria, Australia, has been converted into the Stawell Underground Physics Laboratory to study dark matter, a press release from Australia's Nuclear Science and Technology Organization (ANSTO) said.

Scientists believe that dark matter, the invisible substance largely unknown to mankind, makes up 85 percent of our universe's mass. To know more about it, scientists have been building dark matter detectors, and one of the "most sensitive" detectors delivered some significant results last month.

As with all things in science, one does not just stop with one data record. It needs to be repeatable as well as verifiable — to do so, other detectors also need to be built. One such detector is the SABRE Dark Matter Detector which is housed in the recently inaugurated underground lab.

SABRE Dark Matter Detector

SABRE stands for Sodium iodide with Active Background Rejection. The detector consists of a vessel that measures 8.5 feet x 10.1 feet (2.6 m X 3.1 m) and is made of pure steel that is free from radioactivity. Contained inside are 13.22 tons (12 tonnes) of a liquid scintillator — a material that can convert high-energy radiation into visible light. The scintillator is based on an organic solvent, Linear Alkyl Benzene (LAB), and is mixed with fluorescent chemicals.

Immersed inside the liquid scintillator are ultra-pure sodium iodide crystals, which are instrumental in detecting dark matter interactions. To make this detection possible, photomultiplier tubes (PMTs) — instruments that can even detect a single photon of light, have been coupled to the sodium iodide crystals as well as scintillator liquid.

Researchers know that dark matter interactions with normal matter are rare. So, any detection of light that happens in both the crystals and the scintillator liquid will be rejected. Additionally, the scintillation liquid will also act as an active veto as it eliminates background radiation, the press release said. This is a major improvement over SABRE's predecessor DAMA/LIBRA lab in Italy, which is also tasked with detecting dark matter.

Protections around the dark matter detector

In addition to the features of the detector, the researchers have added multiple layers of protection to prevent radiation from other sources from reaching the detector. One among them is the underground location of the lab.

Housed in a site that was once a gold mine, the detector cannot be disturbed by cosmic radiation. In addition, the setup is located in a research hall which is 108 feet (33 m) long, 32 feet (10 m) wide, and 40 feet (12.3 m) high. Around 4,700 cubic meters of rock were excavated from the site during the construction of the laboratory, the press release stated.

The detector itself is heavily shielded with around 110 tons (100 tonnes) of steel and polymer shielding.

"For too long, our understanding of dark matter has been in the dark. Our elimination of background radiation will give the chief investigators confidence that any particles they detect are not something else," said Richard Garrett, Senior Advisor at ANSTO, in the press release.

Apart from understanding dark matter, ANSTO plans to use the facility for sensitive measurement of environmental samples as well as to investigate the development of biological systems in the absence of background radiation.

We have only begun to unravel what dark matter is, and knowing more about its behavior may even be in conflict with our best theories of the universe.

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