Jacinta Bowler
This article first appeared in Cosmos Weekly on 13 August 2021. For more stories like this, subscribe to Cosmos Weekly.
Deep in a gold mine on the outskirts of the small Victorian country town of Stawell, several hours’ drive to the north-west of Melbourne, a lab is being built to find one of the universe’s most elusive substances: dark matter.
The lab, located a kilometre underground, currently looks more like a tennis-court sized cave than a multi-million-dollar operation. That’s because the lab – a partnership between the University of Melbourne, ANSTO, Swinburne and more – is still very much a work in progress. But if successful in its quest it could help solve one of the greatest mysteries of astrophysics.
“It’s crunch time for us,” says University of Melbourne Associate Professor Phillip Urquijo, a particle physicist and a technical coordinator of the dark matter experiment, called SABRE – the Sodium Iodide with Active Background Rejection Experiment.
“The lab itself should be completed by December. We’re hoping by November we can start bringing in some of our experimental equipment.”
For something that is thought to make up 85% of the matter in the universe, dark matter hasn’t been easy to find. It can’t be seen in any of the wavelengths that would normally be used to detect space stuff like gas and dust. In fact, it doesn’t seem to interact with electromagnetic force at all – meaning it doesn’t absorb, reflect or emit any type of light.
For something that is thought to make up 85% of the matter in the universe, dark matter hasn’t been easy to find.
Scientists only know it exists because stars, galaxies and galaxy clusters have way too much gravitational pull without some further explanation, such as a bunch of dark matter hiding somewhere.
“If we manage to find it, that’s a guaranteed Nobel Prize,” says ANSTO strategic projects senior advisor Dr Richard Garrett. “It’s like [gravitational] waves. That’s another thing they were looking for for 30 to 40 years until, finally, these enormous experiments (namely, the Laser Interferometer Gravitational-Wave Observatory) found it.”
But the search to detect dark matter has so far been lacklustre. Until now.
Underneath our noses
There’s a couple of different ways researchers have been trying to detect dark matter on Earth.
The first attempts to catch dark matter decaying into something we can detect, like gamma rays or particle-antiparticle pairs. Unfortunately, dark matter isn’t the only astronomical process that produces these, adding another layer of difficulty to the process.
Then there are detectors like SABRE, which try to detect the recoil of a type of hypothetical dark matter particle – called weakly interacting massive particles, or WIMPS – off targets deep underground.
But every single detector built has so far only been able to find signals that could be attributed to another cause. Dark matter has stayed obscured.
With one exception. For the last 25 years, a detector called DAMA/LIBRA under the Laboratori Nazionali del Gran Sasso, near L’Aquila central-eastern Italy, has been noting a yearly pattern in the number of signals they capture. Called an “annual modulation effect”, the team believe it could be due to Earth moving closer to and further away from our galaxy’s dark matter halo.
“Over those 25 years, the data [at DAMA/LIBRA] showed that it has this annual modulation effect with extremely, extremely high levels of significance,” says Urquijo. “Through their studies, and through independent reviews of their studies, they couldn’t rule out a dark matter hypothesis to explain it.”
The Italian lab has been something of a black sheep of the detector world, as no other detector has been able to replicate their results. One reason for this is due to the particular sodium iodide crystals the DAMA/LIBRA team has used. They were the most radio pure – meaning very low levels of radioactivity – ever made, a record that the team still holds to this day.
No other detector has been able to replicate the Italian lab’s results.
The crystals are made by starting with “astrograde” sodium iodide powder – a compound that’s low in radioactivity but not yet a crystal. When researchers grow the crystal from the powder, normally radioactive contaminants from the environment end up tangled in the crystals, and so very specific machinery is needed to grow and refine it while keeping radioactivity low.
“It’s actually a very difficult and time-consuming R&D process that is very, very niche,” says Urquijo about the crystals.
But sceptics of DAMA/LIBRA don’t think it’s to do with the radio purity of the crystals. Because the pattern is detected annually, they propose that the detector is only measuring this variation in signal due to the changing seasons.
That’s where being on the other side of the world with opposite seasons comes in handy.
“If we see the same effect as theirs, we know it’s not a seasonal effect, it’s something external,” says Urquijo. “We’ll both be seeing dark matter, essentially.”
Trying different rocks
Even if it’s not dark matter, it would still be something external to the Earth that scientists don’t know about yet, which would be almost as fascinating as finding dark matter.
But first they have to finish the detector.
So far, they’ve made sodium iodide crystals even more radio pure than the ones in the DAMA/LIBRA experiment – a feat that took a long research and development process between institutions around the world.
ANSTO already had facilities set up to test minute levels of radiation and the team are testing all their materials for radioactivity, making sure everything is as low as possible. Small levels of radiation exist all around us – even bananas and human beings, for example, are both a little radioactive. So the team must limit this “normal” radioactivity so that it doesn’t interfere with the detector.
“We’ve been measuring all kinds of sands and gravels and cement powders from all over Australia, trying to find the best concrete mix for the construction,” says Garrett.
“We’re [deep underground] looking for very, very weak signals, but there’s no point doing that if the concrete we use is radioactive.”
Then there’s the location. Working in an active gold mine has many positives. The mining company takes care of all the ventilation and safety management. Plus, the mine workers can transport the scientists in specially designed mine cars through the long winding tunnels all the way down to the lab.
But it has its drawbacks and challenges. The construction of the lab was delayed for almost three years when the mine changed owners and shut down for a while. Plus, the cave has to be vacated every eight hours so the miners can blast for gold.
Once the detector is finally set up, there won’t be much to do but sit up on the surface and wait for results.
Diagrams show the SABRE instrument looking a bit like a chandelier inside a vat, encased in a metal vault. The detector itself is the chandelier, hanging down from the top of the vat and filled with 50kg of the radio-pure sodium iodide crystals to detect any tiny hints of radiation.
The vat, which the team call the Veto, is lined with photomultiplier detectors (incredibly sensitive light detectors) dotted throughout, and will be containing linear alkylbenzene – a liquid normally used to make detergent, but in this case used as a “liquid scintillator” that will flash with light when hit with radiation. And then there’s the four-metre-tall vault, which even Urquijo might tell you is a little over the top: SABRE will have something in the region of 100 tonnes of steel shielding the experiment from stray particle radiation that would pollute any potential measurements.
“We were really paranoid about background radiation,” Urquijo explains. “The region of lowest radioactivity you’ll find anywhere in the Southern Hemisphere is right in the middle of those crystals.
“We have to be better because we’re coming second.”
But right now, the detector parts have not yet been moved into the mine – instead, some of this dark matter-finding machinery is sitting in a car park.
“Melbourne Uni doesn’t have a lot of space to store equipment so we’re using our link through ANSTO to just sit [the liquid scintillator] at the car park there,” says Urquijo.
Once the detector is finally set up, there won’t be much to do but sit up on the surface and wait for results. But until then, there’s plenty to keep the team busy.
“Every material comes in and we measure it for radioactivity to see if it’s good enough,” says Garrett.
“It’s a race against time.”
This article first appeared in Cosmos Weekly on 13 August 2021. To see more in-depth stories like this, subscribe today and get access to our weekly e-publication, plus access to all back issues of Cosmos Weekly.