In the depths of a gold mine on the outskirts of the small Victorian town of Stowell, located a few hours drive northwest of Melbourne, a laboratory is being built to search for one of the most elusive substances in the universe - dark matter.
A kilometer underground, the lab is now more like a tennis court-sized cave than a multi-million dollar enterprise. This is because the lab - a partnership between the University of Melbourne, ANSTO, Swinburne and others - is still under development. But if successful, it can help solve one of the greatest mysteries of astrophysics.
"This is a critical time for us," said Phillip Urquiho, an associate professor at the University of Melbourne, a particle physicist and technical coordinator for a dark matter experiment called SABER - Sodium Iodide with Active Background Rejection Experiment.
"The laboratory itself should be completed by December. We hope that by November we will be able to bring some of our experimental equipment."
Dark matter, thought to make up 85% of the matter in the universe, is not easy to find. It is not visible at any of the wavelengths commonly used to detect space objects such as gas and dust. Moreover, it seems that it does not interact with electromagnetic forces at all - which means that it does not absorb, reflect or emit light.
Scientists know about its existence only because stars, galaxies and galaxy clusters exert too strong gravitational attraction without any additional explanation, for example, that somewhere there is a pile of dark matter hiding.
“If we can find it, it’s a guaranteed Nobel Prize,” says ANSTO senior adviser for strategic projects, Dr. Richard Garrett. "It's like [gravitational waves. This is another thing that was looking for 30-40 years, until finally these huge experiments (in particular, the Laser Interferometric Gravitational Wave Observatory) did not find it."
But the search for dark matter has so far been unsuccessful. Still.
Under our noses
Researchers are trying to detect dark matter on Earth in a variety of ways.
The first way is to catch dark matter decaying into something we can detect, such as gamma rays or particle-antiparticle pairs. Unfortunately, dark matter is not the only astronomical process that produces them, which adds another layer of complexity to the process.
There are detectors like SABER that try to detect the recoil of hypothetical dark matter particles called weakly interacting massive particles, or WIMPS, from sources deep underground.
But every detector built so far has only detected signals that could be attributed to a different cause. Dark matter remains incomprehensible.
With one exception. For the past 25 years, a detector called DAMA / LIBRA at the Laboratori Nazionali del Gran Sasso near L'Aquila in northeastern Italy has recorded an annual pattern in the number of recorded signals. Called the "annual modulation effect," it could be caused by the Earth moving in and out of our galaxy's dark matter halo.
“Over these 25 years, [DAMA / LIBRA] data has shown that it has this annual modulation effect with an extremely, extremely high level of confidence,” says Urquijo."Through their research and independent reviews of their research, they have not been able to rule out the dark matter hypothesis to explain it."
The Italian laboratory was something of a "white crow" in the world of detectors, as no other detector could replicate their results. One of the reasons for this is that the DAMA / LIBRA team used special crystals of sodium iodide. They were the most radio-cleaned - that is, with very low levels of radioactivity - ever created, and the team still holds this record.
For the production of crystals, sodium iodide powder "astrograd" is used - a compound with low radioactivity, but not yet a crystal. When researchers grow a crystal from a powder, it is common for radioactive contaminants from the environment to become entangled in the crystals, so very specific equipment is required to grow and purify the crystals while maintaining low radioactivity.
"It's actually a very complex and time-consuming R&D process that is very, very niche," Urquijo says of crystals.
But DAMA / LIBRA skeptics do not believe that this is due to the radioactive purity of the crystals. Since this pattern is detected annually, they assume that the detector measures this signal change only in conjunction with the changing seasons.
This is where the fact that we are on the other side of the world with opposite seasons comes in handy.
“If we see the same effect as they do, we will know that this is not a seasonal effect, but something external,” says Urquiho. "Basically, we'll both be seeing dark matter."
Even if it's not dark matter, it would still be something external to Earth that scientists don't yet know about, which would be almost as exciting as finding dark matter. But first they need to finish the detector.
So far, they've made the sodium iodide crystals even more radio-pure than those used in the DAMA / LIBRA experiment - a feat that has required a long process of research and development between institutes around the world.
ANSTO has already set up equipment to test the smallest levels of radiation, and the team is testing all of their materials for radioactivity, making sure everything is as low as possible. Small levels of radiation exist around us - even bananas and humans, for example, are slightly radioactive. Therefore, the team must limit this "normal" radioactivity so that it does not interfere with the detector's operation.
“We have measured all types of sand, gravel and cement powder from all over Australia trying to find the best concrete mix for construction,” says Garrett.
"We are looking deep underground for very, very weak signals, but there is no point in doing this if the concrete we are using is radioactive."
Then - the location. There are many positive aspects to working in an active gold mine. The mining company takes care of all ventilation and safety management. In addition, mine workers can transport scientists in specially equipped mine cars through long, winding tunnels all the way to the laboratory.
But this method has its drawbacks and problems. The construction of the laboratory was delayed for almost three years when the mine changed owners and closed for a while. In addition, the cave must be emptied every eight hours so that the miners can blast for gold.
In the diagrams, the SABER looks like a chandelier inside a vat, enclosed in a metal vault. The detector itself is a chandelier hanging from the top of the vat and filled with 50kg of crystals of radio-purified sodium iodide to detect any tiny hints of radiation.
The chan, which the team calls Veto, is littered with photomultipliers (incredibly sensitive light detectors) and will contain linear alkylbenzene, a liquid commonly used to make detergents, but in this case used as a "liquid scintillator" that will flash with light when it hits him radiation. And there is also a four-meter vault, which even Urquiho can say is a little too much: The SABER will have about 100 tons of steel, which protects the experiment from the radiation of stray particles that can spoil any potential measurements.
“We had real paranoia about the background radiation,” explains Urquijo. "The region of the lowest radioactivity that can be found anywhere in the Southern Hemisphere is right in the center of these crystals."
But now the parts of the detector have not yet been transported to the mine - instead, some of the equipment for searching for dark matter is in the parking lot.
“The University of Melbourne doesn't have much storage space for equipment, so we are using our ANSTO connection to simply place the liquid scintillator in the car park,” says Urquijo.
When the detector is finally installed, all that remains is to sit on the surface and wait for the results. But until then, the team has something to do.
"Every material comes to us, and we measure its radioactivity to see if it's good enough," Garrett says.
"It's a race against time."
