Muon tomography and its ability to probe the unseeable

The Pyramid of Khufu in Giza, Egypt. (Via dgood40/Pixabay)

By Madeleine O’Keefe
BU News Service

BOSTON – In 1968, American physicist Luis Walter Alvarez set up a detector in a cavity near the center of the base of the Pyramid of Chephren in Giza, Egypt. He endured the relentless heat and blowing sands of Egypt’s Western Desert to probe the inner structure of this enormous ancient tomb. Could more chambers lie unexplored within its thick limestone walls? To find out, without pulling the pyramid apart brick by brick, Alvarez used “muons,” laying the groundwork for decades of research using muon technology.

Muons are fundamental particles like electrons, but 200 times more massive. They form naturally in the Earth’s upper atmosphere, the result of cosmic ray particles colliding with oxygen and nitrogen molecules. What makes muons useful for research is they tend not to decay or get absorbed by matter. This means they can penetrate far deeper into a structure such as a pyramid than most other particles.

When muons reach a structure, most will pass through unimpeded. But some will be absorbed: the denser the material, the more muons it absorbs. If a detector deep within a structure senses more muons coming from a particular direction, that means there must be less material—or even an empty space.

Although Alvarez detected nothing of interest in the pyramid, his experiment with muons proved that scientists could use these particles to scan the interiors of large structures, just as doctors use X-rays to noninvasively image bones.

Nearly 50 years after Alvarez’s experiment, an international group called ScanPyramids arrived in Giza, ready to pick up where Alvarez left off. ScanPyramids researchers planted a variety of detectors in and around the Pyramid of Khufu, just 200 yards from where Alvarez, who used only a single detector, did his work. In the summer of 2016, ScanPyramids turned on their detectors—and waited.

After a few months, researchers began analyzing their data. Nothing. So they tried again in January 2017. Still nothing. Finally, after another round of data collection in the spring of 2017, three independent analyses from three different detectors confirmed the presence of a void above the pyramid’s Grand Gallery that Egyptologists and archaeologists didn’t know existed. Muon tomography had opened the pyramid like a geode without making a crack.

More than 7,000 miles away from the Great Pyramids, nestled in four mesas at the Pajarito Plateau in New Mexico, is Los Alamos National Laboratory, a hub for muon tomography research. It was here that physicist Christopher Morris and his team in 2003 invented a new technique for muon tomography known as “muon scattering” tomography. Morris realized that a muon will travel in a straight line until it encounters an atom with a heavy nucleus—a “high-Z” atom—at which point, the muon’s path is deflected.

The minuscule particle catapults around the heavier nucleus just as a tiny asteroid slingshots around a much more massive planet. Scientists can therefore glean information on an otherwise impenetrable object by comparing muons’ trajectories before it enters and after it exits a material.

Los Alamos scientist Elena Guardincerri is employing Morris’s muon scattering technique to help save a beloved monument in her home country of Italy. The Cathedral of Santa Maria del Fiore in Florence has been an iconic silhouette in the Florentine skyline since it was built in the 15th century. But slow-moving cracks in its famous dome are threatening its long-term stability. Architect Filippo Brunelleschi left no documents detailing the dome’s design, leaving preservationists to figure out how to stabilize it.

The Cathedral of Santa Maria del Fiore in Florence, Italy. (Via Maatkare/Pixabay)

The dome’s walls are double layered and the inner wall is so thick that previous attempts to image its structure with metal detectors and ground-penetrating radar have been ineffective. Guardincerri plans to use muon scattering tomography to map the structure of the dome’s inner wall and find out how (if at all) it is reinforced. Most experts guess that an iron hoop stabilizes the dome, like the metal hoop around a barrel, Guardincerri said.

“There are documents dating back to when the dome was built,” Guardincerri said. “These documents demonstrate that thousands of pounds of iron were purchased, but nobody really knows where this iron is.”

Guardincerri designed two four-by-four-foot muon trackers that can be disassembled into smaller 10-pound pieces and carried up the narrow staircase leading to the inner wall of the dome. There, the detectors will be reassembled and placed on either side of the wall to track muons for about one month. Then, Guardincerri’s collaborators at the University of Florence will slide the trackers up four feet to the next position.

“The objective is to image a vertical slice of the dome up to a certain height and look for reinforcement structure,” Guardincerri said. That vertical slice should be enough to tell engineers whether the monument is reinforced so they can include that image in their models of the dome’s structure and figure out how to stabilize it, she said.

Muons can also be used to address an entirely separate issue: the threat of nuclear weapons. Within the next two to three years, Finland will be placing their spent nuclear fuel stored in specially designed casks in underground repositories, where they will remain forever, without safeguards. The International Atomic Energy Agency (IAEA), an organization that works to promote the safe handling of nuclear materials, needs to verify the contents of Finland’s sealed and shielded storage casks before they go underground; spent fuel contains plutonium, which in the wrong hands can be used to manufacture nuclear bombs.  

Every two years, the IAEA publishes a report that lists research and development goals. Their 2016 report listed “ ‘being able to verify what’s inside a sealed, shielded storage container’ as one of their high priorities,” said Matt Durham, a nuclear physicist at Los Alamos.

“This is something they’re (IAEA) very worried about, and everyone knows that they don’t know how to do this,” Durham said.

Researchers have shown that muon scattering tomography provides a potential solution for the IAEA. Plutonium is a high-Z material, so muons’ trajectories should change as they pass through the spent nuclear fuel casks. To test this, Durham and his team of Los Alamos researchers used two four-by-one-foot panel detectors to probe the contents of a Westinghouse MC-10 spent-fuel cask at Idaho National Laboratory. It worked—the team successfully used muons to determine whether spent fuel was missing from the cask without opening it. Durham and his team are now developing panel muon detectors that will be compatible with the Finnish casks’ unique design.

What’s next for muon tomography research? Physicist Morris said he predicts a rise of handheld muon detectors, perhaps similar to those being developed by doctoral student Spencer Axani at the Massachusetts Institute of Technology. Those can be used to detect nuclear contraband at city or country borders in the event of a high-level threat.

Muon tomography has finally made the invisible visible. From pyramids and buried ancient cities to volcanoes and underground caves, some of Earth’s secrets can be explored using only these particles from the cosmos.

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