Laser-Made Muons Produce First Image of Hidden Object in Romania

Physicists in Romania have used a beam of artificially produced muons to image a structure for the first time, recording the shadow of a pile of lead bricks placed in front of a van at the Extreme Light Infrastructure Nuclear Physics (ELI-NP) facility. The proof-of-concept result, described in an arXiv preprint, follows the 2025 demonstration of the first laser-based muon source and suggests muon imaging could move beyond its dependence on cosmic rays.

Muons are heavier cousins of electrons, and their larger mass lets them penetrate further into thick, dense objects than conventional electron tomography. Muon tomography, or muography, is therefore used to image large structures in archaeology, geosciences and civil engineering, and in non-invasive scenarios such as the core of a burning blast furnace or a nuclear waste storage facility.

Why an artificial source matters

Until now, muography has relied on muons produced naturally when cosmic rays collide with atoms 15–20 km above the Earth's surface. That cosmic muon flux is just 1 particle/cm²/s, and it arrives from only one direction: straight up. Low flux plus fixed directionality means accumulating enough data for tomography can take months.

In 2025, physicists led by Davide Terzani in the Accelerator Technology and Applied Physics division at Lawrence Berkeley National Laboratory built a compact muon source based on laser plasma acceleration (LPA), a technique that generates electrons with energies in the 10 GeV range. When these electrons strike a solid target, they produce a shower of secondary particles — electrons, positrons, gamma rays, neutrons and, critically, muons.

How the Romanian team filtered the beam

Teams led by Alexandra Saftoiu and Domenico Doria at the Horia Hulubei National Institute for Physics and Nuclear Engineering used ELI-NP's 10 petawatt-class laser to produce a similar beam, then set out to optimize it.

"The main challenge here," Saftoiu said, "was to achieve an optimal balance between suppressing the background particles, unavoidably coming from the same process that creates muons, to a level that allows us to unambiguously detect muons, while keeping the muon beam high in flux and energy."

The researchers ran extensive Monte Carlo simulations on the GEANT4 platform to model how different particles pass through matter. The simulations pointed to a three-layer shield:

• A first layer of polyethylene sheet stacks and bricks, 100 cm thick along the beam axis.
• A second layer of paraffin bricks, 60 cm thick along the beam axis and 40 cm beside and above it, encapsulating the entire polyethylene structure.
• A third layer of a lead beam dump and a two-metre reinforced concrete wall.

With the shield in place, three scintillator-based detectors monitored the beam. Their measurements show 90% of the particles in the beam were muons. The team then used this beam to create the low-resolution image of the lead bricks in front of the van.

What could it be used for?

Doria said an artificial muon source would find applications in border security, scanning containers for trafficked weapons or radioactive materials, and in nuclear safety, for tasks such as inspecting waste-storage casks or reactors themselves. "In principle, it could also be employed in resource management (scanning for ores with a single borehole to probe distances of kilometres rather than just metres), archaeological searches for hidden spaces and structures, safety assessment (for example, scanning dams, tunnels and mine galleries) and even natural hazard assessment (monitoring volcano and glacier activity, for instance)," he said.

Terzani, who was not involved in the ELI-NP work, said the study shows progress in developing filters that isolate muon signals from background. "Without removing secondary particles, detectors would be swamped by noise that is difficult to subtract, resulting in essentially unusable images," he said. "This is like trying to photograph a partial solar eclipse without a filter on your camera: even if the Sun is 99% covered, the remaining light will still overwhelm the camera's sensors."

What stands between lab and field?

Saftoiu and Doria emphasize that real-world deployment remains distant. "First, the laser technology employed in this context needs to be made substantially more compact and be able to operate at higher repetition rates," they said. Muography also requires detectors with higher spatial resolution, capable of measuring intense muon bunches rather than relying on single-particle counting. They add that still more effective shielding strategies would help further minimize the background generated alongside the muons.

The next measurable milestone for this line of research will be a compact, high-repetition-rate laser source paired with bunch-capable detectors.