Our universe is filled with countless cosmic rays, streaking through space at near-light speeds. These rays can be emitted by the Sun or formed by supernova explosions in distant galaxies. They consist of trillions of subatomic particles. Every second, Earth is bombarded by cosmic rays. When cosmic rays collide with Earth's atmosphere, some are deflected by the planet's magnetic field, while others reach us without causing harm. As cosmic rays traverse the atmosphere, they undergo a series of reactions that generate vast quantities of new subatomic particles. Among these are muons.

March 12, 2025

Artem Vlasov, Office of News and Public Information, International Atomic Energy Agency

Muon imaging has been used to scan the ancient city walls of Xi'an, China. (Photo credit: Adobe Stock)

Our universe is filled with countless cosmic rays, streaking through space at near-light speeds. These rays can be emitted by the Sun or formed by supernova explosions in distant galaxies. They consist of trillions of subatomic particles. Every second, Earth is bombarded by cosmic rays. When cosmic rays collide with Earth's atmosphere, some are deflected by the planet's magnetic field, while others reach us without causing harm. As cosmic rays traverse the atmosphere, they undergo a series of reactions that generate vast quantities of new subatomic particles. Among these are muons.

Cosmic rays entering Earth's atmosphere generate new particle streams, including muons. (Image credit: A. Vlasov/IAEA)

Muons are perplexing because some of their properties show subtle yet significant deviations from the predictions of particle physics' primary theory—the so-called Standard Model. However, scientists have discovered a method akin to traditional radiography that leverages this mysterious particle to peer deep inside large objects inaccessible to physical contact, such as ancient structures, volcanoes, and even nuclear reactors.

"Though invisible to us, muons are ubiquitous on Earth: they constantly pass through us and the objects around us at near-light speeds from every direction," explains Ian Swainson, a nuclear physicist at the International Atomic Energy Agency. "Completely harmless to humans, muons can penetrate hundreds of meters of rock, offering a universal means to understand the composition and dimensions of materials invisible to us."

"The principle of muon imaging is in some ways similar to X-ray or gamma-ray radiography, which is used medically to scan the body and industrially to assess the integrity and safety of structures and components," added Andrea Giammanco, a particle physicist and co-author of the new publication Muon Imaging. However, X-ray and gamma-ray radiography rely on strong artificial radiation sources generated by particle accelerators or radioactive sources, whereas muon radiography is based on natural cosmic rays originating from outer space."

Muon imaging generally falls into two categories: muon radiography and muon scattering tomography.

Muon radiography involves placing a detector beneath or to the side of a structure to capture muons passing through it. The greater the material density, the more muons are absorbed. Some particles that successfully traverse the structure are captured by a detector on the opposite side. In the resulting image, voids through which muons easily pass appear as bright areas, while denser materials appear as dark areas.

While muon radiography relies on material absorption, muon scattering tomography is based on how muons scatter. For instance, by placing two detectors on opposite sides of a vehicle or shipping container, experts can track how particles deflect from high-density materials with greater proton content. This allows inspection of the vehicle or container's interior without physical examination.

Muon Radiography. (Image credit: A. Vlasov/IAEA)

Since the first experiments conducted in the 1950s, muon imaging has been used worldwide to scan a vast array of objects. Currently, muon radiography is being employed to assess the internal structure of Mount Vesuvius near Naples, Italy—the volcano that tragically destroyed the ancient Roman city of Pompeii and several other settlements in 79 AD. Researchers are working to visualize internal processes within Mount Vesuvius using muon detectors, thereby improving modeling critical for predicting any potential eruptions and their progression, and developing strategies to mitigate risks to local residents. The volcano has remained dormant since its last eruption in 1944.

Similarly, muon imaging has been employed to scan the Pyramid of the Sun near Mexico City (the world's third-largest pyramid), cyclones passing over Japan, glaciers in the Alps, and most recently, France's decommissioned nuclear reactors.

The IAEA plans to host a workshop next year titled "Muon Tomography: From Fundamentals to Practical Use and Applications." Participants will discuss various practical application models of the technology, characteristics of detectors used, algorithmic reconstruction of muon trajectories, and data analysis and image reconstruction.

The IAEA's new publication Muon Imaging provides a detailed description of the main techniques of muon imaging and different types of associated detectors. The publication also covers a wide range of applications, from inspecting modern and ancient buildings, volcanoes, and industrial structures to enhancing nuclear security and safeguards. "This publication provides a comprehensive overview of the field of muon imaging, serving as a valuable resource for readers in industry and academia to deepen their understanding of this evolving field," said Swainson. Click here to access the publication Muon Imaging.