If we can’t drill our way to the centre of the Earth, how do we know what’s there? Scientists investigate phenomena that can be observed from the surface: earthquakes, erupting volcanoes, and lab experiments that put minerals under extreme pressure are a few examples. Now, for the first time in Canada, a new hint into the planet’s interior is being explored: geoneutrinos, small subatomic particles that came from the Earth’s very core.

“Geoneutrinos are a type of antineutrino produced by radioactive decays happening inside the Earth. By measuring geoneutrinos, we can measure the chemical composition of the planet,” explains Alex Wright, an associate professor in the Department of Physics, Engineering Physics, and Astronomy.

The exciting results come from a Queen’s-led research team working at the SNO+ neutrino detector located in Sudbury, Ontario. Buried under a two-kilometre thick layer of rock, SNO+ is shielded from radiation and other noise. The equipment includes a giant sphere (the size of a four-story building) filled with liquid scintillator, a substance that converts radiation into visible light. The whole sphere is surrounded by light detectors ready to capture the faintest signal of a particle.

In the past, the detector was famously able to catch glimpses of neutrinos from the Sun – a groundbreaking result recognized with the 2015 Nobel Prize in Physics – and antineutrinos produced by reactions inside of Ontario’s nuclear plants. While neutrinos coming from the Sun are the result of the fusion of light atoms (like hydrogen), antineutrinos from nuclear reactors and geoneutrinos coming from the Earth are formed from heavier ones, such as uranium and thorium. The antineutrinos have a unique signature that helps scientists identify them among other particles captured by SNO+.

The new findings make SNO+ the third neutrino detector in the world to detect geoneutrinos, following KamLAND, in Japan, and Borexino, in Italy. Studying geoneutrinos in different spots around the globe is important because scientists are looking at understanding how different geological formations – like the Canadian Shield – impact the type of geoneutrinos detected. That will give them an idea of which neutrinos are coming from the Earth’s outermost layer (the crust) and which ones are coming from inner layers such as the mantle.

A century-old idea, with exciting new developments

While the idea of using neutrinos to study the composition of the Earth goes back almost a century, our capacity to detect and identify these particles has increased exponentially over the past few decades.

In the 1930s, Wolfgang Pauli postulated that neutrinos existed, and after that Austrian physicist Gernot Eder and Hungarian physicist George Marx suggested that the Earth’s radioactivity would produce geoneutrinos. That meant that, if we could ever detect these geoneutrinos, we would be able to peek inside the Earth.
At that time, we had no instruments capable of capturing these elusive particles. But the idea was revived in the 1990s, when a new generation of liquid scintillator detectors was being built.  

“The Physics community has been anticipating doing this for a long time,” says SNO+ lead scientist Mark Chen, a professor at the Department of Physics, Engineering Physics, and Astronomy and Gordon and Patricia Gray Chair in Particle Astrophysics. “It’s very satisfying to finally see these results, which will advance a novel way to study the Earth.”

Working with geologists, the particle physics team is investigating what these results mean for our understanding of the deep Earth. For example, they could help explain how much of Earth's heat comes from internal radioactivity and how much is left over from the planet's formation or clarify whether the mantle is a homogeneous or heterogeneous layer, with compositional hotspots.

A history of excellence

SNO+ is the evolution of the Sudbury Neutrino Observatory (SNO) experiment, built in the 1990s and that became famous following its trailblazing solar neutrinos results. The nickel mine in Sudbury that hosted SNO now hosts SNOLAB, a full laboratory that has housed more than 15 high profile astroparticle physics experiments since its creation.

During the original SNO experiment in the 2000’s, the detector’s giant sphere was filled with heavy water. Following this initial phase, the experiment was upgraded into SNO+, which started collecting data in 2022.

One of the main changes to the detector has been switching the heavy water for liquid scintillator. The scientific results of this second phase are now emerging, including the geoneutrinos findings and the first-ever observation of neutrinos interacting with carbon atoms and turning them into nitrogen.

As a next step, scientists plan to mix in a new chemical element to the liquid scintillator at SNO+: tellurium, a rare semi-metal used in solar panels and thermoelectric devices. Starting 2027, the new phase of SNO+ will attempt to accomplish an unprecedented milestone: observe neutrinoless double beta decay, a rare phenomenon that, if detected, would prove that neutrinos are their own antiparticles.