A controlled beam of muons has revealed the silhouette of a lead object after passing through thick concrete shielding.
Muons are particles similar to electrons, but about 200 times heavier. They therefore pass through rock, metal, or concrete much more easily. Natural muons are constantly created when cosmic rays strike the upper atmosphere. Their passage can be used to look inside objects that X-rays have difficulty penetrating.
Diagram of the experimental setup used to produce and measure the muon beam. T32 and SiRO, positioned 12 m from the target, were used to control and characterize the beam. The mobile μ36 detector was moved to different distances and positions to measure its profile and produce images. The lead object used for muography is not shown in this diagram.
This technique, known as muography, already works with muons arriving from the sky. Its main problem is their low number. Measurements sometimes have to be accumulated for a very long time to obtain a usable image. Producing these particles on demand could therefore greatly accelerate observations.
The researchers used a very powerful laser to accelerate electrons in a gas. These electrons then struck a solid target and generated highly energetic radiation. This radiation ultimately produced pairs of muons. The collision also produced many gamma photons and stray neutrons, whose signal had to be strongly attenuated.
The team installed several layers of absorbing materials, followed by a 2 m-thick concrete wall. The most energetic muons continued on their path. A mobile detector, moved up to 42 m from the source, then measured the beam. In particular, it recorded the beam passing behind a stack of lead bricks.
The resulting image is still simple, but the shape and position of the stack are recognizable. Simulations show that the signal used to form the image was dominated by muons. Their energy reached a few billion electronvolts, enough to pass through the thick shielding placed in their path.
The main advantage of such a beam lies in its controllability. With cosmic muons, one must wait for nature to provide the particles. An artificial source can operate on demand.
The setup therefore functions as a two-part system: a muon source on one side and a detector on the other. The object being examined must be located between them, so that the muons pass through it before being measured. Such a configuration could be used to probe the interior of thick structures, such as buildings, concrete infrastructure, or certain rock formations.
The method is still far from being a ready-to-use device, however. The resulting image still has limited resolution, and the laser installation used is imposing. Progress is needed in muon production and detector technology before more detailed objects can be examined.
The work is currently available as a preprint on arXiv. It has therefore not yet undergone the full peer-review process of a scientific journal. The next experiments will mainly need to show whether this source can provide more muons while remaining stable enough for repeated measurements.