The electron is one of the most common bits of matter around us—every complete atom in the known universe has at least one. But the electron has far rarer and shadier counterparts, one of them being the muon. We may not think much about muons, but they’re constantly hailing down on Earth’s surface from the edge of the atmosphere

Muons can pass through vast spans of bedrock that electrons can’t cross. That’s good luck for scientists, who can collect the more elusive particles to paint images of objects as if they were X-rays. In the last several decades, they’ve used muons to pierce the veils of erupting volcanoes and peer into ancient tombs, but only in two dimensions. The few three-dimensional images have been limited to small objects.

That’s changing. In a paper published in the journal Science Advances today, researchers have created a fully 3D muon image of a nuclear reactor the size of a large building. The achievement could give experts new, safer ways of inspecting old reactors or checking in on nuclear waste.

“I think, for such large objects, it’s the first time that it’s purely muon imaging in 3D,” says Sébastien Procureur, a nuclear physicist at the Université Paris-Saclay in France and one of the study authors.

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Muon imaging is only possible with the help of cosmic rays. Despite their sunny name, most cosmic rays are the nuclei of hydrogen or helium atoms, descended to Earth from distant galaxies. When they strike our atmosphere, they burst into an incessant rainstorm of radiation and subatomic particles.

Inside the rain is a muon shower. Muons are heavier—about 206 times more massive—than their electron siblings. They’re also highly unstable: On average, each muon lasts for about a millionth of a second. That’s still long enough for around 10,000 of the particles to strike every square meter of Earth per minute.

Because muons are heavier than electrons, they’re also more energetic. They can penetrate the seemingly impenetrable, such as rock more than half a mile deep. Scientists can catch those muons with specially designed detectors and count them. More muons striking from a certain direction might indicate a hollow space lying that way. 

In doing so, they can gather data on spaces where humans cannot tread. In 2017, for instance, researchers discovered a hidden hollow deep inside Khufu’s Great Pyramid in Giza, Egypt. After a tsunami ravaged the Fukushima Daiichi nuclear power station in 2011, muons allowed scientists to gauge the damage from a safe distance. Physicists have also used muons to check nuclear waste casks without risking leakage while opening them up.

However, taking a muon image comes with some downsides. For one, physicists have no control over how many muons drizzle down from the sky, and the millions that hit Earth each day aren’t actually very many in the grand scheme of things. “It can take several days to get a single image in muography,” says Procureur. “You have to wait until you have enough.”

Typically, muon imagers take their snapshots with a detector that counts how many muons are striking it from what directions. But with a single machine, you can only tell that a hollow space exists—not how far away it lies. This limitation leaves most muon images trapped in two dimensions. That means if you scan of a building’s facade, you might see the individual rooms, but not the layout. If you want to explore a space in great detail, the lack of a third dimension is a major hurdle.

In theory, by taking muon images from different perspectives, you can stitch them together into a 3D reconstruction. This is what radiologists do with X-rays. But while it’s easy to take hundreds of X-ray images from different angles, it’s far more tedious and time-consuming to do so with muons. 

Muon detectors around G2 nuclear reactor in France. Two facility photos and four diagrams.
The 3D muon images of the G2 nuclear reactor. Procureur et al., Sci. Adv. 9, eabq8431 (2023)

Still, Procureur and his colleagues gave it a go. The site in question was an old reactor at Marcoule, a nuclear power plant and research facility in the south of France. G2, as it’s called, was built in the 1950s. In 1980, the reactor shut down for good; since then, French nuclear authorities have slowly removed components from the building. Now, preparing to terminally decommission G2, they wanted to conduct another safety check of the structures inside. “So they contacted us,” says Procureur.

Scientists had taken 3D muon images of small objects like tanks before, but G2—located inside a concrete cylinder the size of a small submarine and fitted inside a metal-walled building the size of an aircraft hangar—required penetrating a lot more layers and area.

Fortunately, this cylinder left enough space for Procureur and his colleagues to set up four gas-filled detectors at strategic points around and below the reactor. Moving the detectors around, they were able to essentially snap a total of 27 long-exposure muon images, each one taking days on end to capture.

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But the tricky part, Procureur says, wasn’t actually setting up the muon detectors or even letting them run: It was piecing together the image afterward. To get the process started, the team adapted an algorithm used for stitching together anatomical images in a medical clinic. Though the process was painstaking, they succeeded. In their final images, they could pluck out objects as small as cooling pipes about two-and-a-half feet in diameter.

“What’s significant is they did it,” says Alan Bross, a physicist at Fermilab in suburban Chicago, who wasn’t involved with this research. “They built the detectors, they went to the site, and they took the data … which is really involved.”

The effort, Procureur says, was only a proof of concept. Now that they know what can be accomplished, they’ve decided to move onto a new challenge: imaging nuclear containers at other locations. “The accuracy will be significantly better,” Procureur notes.

Even larger targets may soon be on the horizon. Back in Giza, Bross and some of his colleagues are working to scan the Great Pyramid in three dimensions. “We’re basically doing the same technique,” he explains, but on a far more spectacular scale.