The mighty muon

The Earth’s atmosphere is continually bombarded with particles (such as protons) from outer space. When these particles collide with the atmosphere, they produce other particles, including a type of subatomic particle called a muon, that rain down on the planet. 

By using a detector to register how many muons are absorbed by an object, it is possible to infer the object’s density and structure. Physicist Luis Alvarez famously used muography, as this technique is called, in the 1960s to look for hidden rooms in the pyramids at Giza, Egypt. (Muons are capable of passing through hundreds of feet of solid rock.)

In the early 2000s, Chris Morris, a physicist at Los Alamos National Laboratory, developed a more sophisticated muography technique called muon scattering tomography, in which muons arriving at the Earth’s surface from the atmosphere travel through a detector, through an object, and then through a second detector. These paired detectors can track how muons change course when they interact with an object’s atoms—data that can be used to make a 3D model of the object’s structure.

Muon scattering tomography is especially useful for imaging materials with a high atomic number, such as uranium or plutonium, and for looking inside objects that are too dense to be penetrated by x-rays or gamma rays (which are used in other kinds of imaging). In 2006, Los Alamos licensed its muon scattering tomography technique to Decision Sciences International, which developed a detector system that is now used in the Bahamas, Singapore, and on the U.S.-Mexico border to search for drugs and other illicit substances hidden inside cargo.

More recently, researchers at Los Alamos have been exploring the use of muon scattering tomography to help ensure that radioactive waste created by nuclear reactors isn’t diverted for nefarious purposes. After uranium is irradiated inside a nuclear reactor, the spent fuel—which contains plutonium produced as a byproduct—is submerged in a pool of water for around a decade before being sealed inside a cask and dispositioned for long-term storage. To prevent any radioactivity or nuclear material from escaping, the casks, which are large (weighing up to 250,000 pounds when full) and heavily shielded with steel and lead, are welded shut. 

However, after a cask has been sealed, verifying the contents can be difficult for International Atomic Energy Agency safeguards inspectors, who are sometimes called upon to ensure that none of the material inside a cask has been stolen or diverted. “There isn’t any good way to look inside these containers,” says Matt Durham, a researcher at Los Alamos. “You can’t image them with neutrons or gamma rays because of all the shielding. The only way to see inside a container is to open it up again.”

With colleagues from the University of New Mexico and the Colorado School of Mines, in the summer of 2025, Durham and other Laboratory researchers will bring a state-of-the art detector developed by Decision Sciences to Idaho National Laboratory, where they will attempt to use muons to image the contents of spent-fuel casks. Previous research and modeling have demonstrated the experiment’s feasibility. “We have much larger detectors now, so we can look at the casks from multiple sides,” Durham says. “That will give us a huge amount of data relative to what we were able to capture before.”

If all goes well, the experiment could support the development of tools that safeguards inspectors could use to verify spent-fuel casks’ contents. The research could also open the door to other applications of muon scattering tomography.

For example, muon scattering tomography could conceivably be used to verify the contents of small modular reactors (a kind of advanced nuclear power reactor that researchers around the world are working to develop). Like spent-fuel casks, these reactors are heavily shielded, which could make verifying their fuel content challenging. Someday, muons—a few of which have likely passed through you as you’ve read this article—might help ensure that these reactors don’t pose proliferation risks, supporting nuclear security into the future. ★

Article by Jake Bartman, National Security Science magazine writer