A group from ELI-NP (Extreme Light Infrastructure Nuclear Physics), a research center located in Romania that studies the frontiers of nuclear physics through the use of extreme intensity lasers, produced a laser beam (considered one of the most powerful in the world) to create a beam of particles and made it pass through a 2-meter concrete wall. The detector placed after the concrete wall detected the image of a shadow produced by a target placed after the wall, made of a block of lead.
The team thus demonstrated that using a very powerful laser it is possible to produce particles that see through very thick objects. The particles in question are muons, elementary particles similar to electrons but about 200 times heavier. To pass through meters of concrete, rock or even metals, muons are considered the ideal particles. The team demonstrated that they can obtain an image of muons starting only from a laser source, without using one of the large traditional circular particle accelerators (such as the Large Hadron Collider (LHC) in Geneva).
This technique muographic (i.e. the possibility of making an x-ray using muons) has enormous advantages compared to techniques with which other particle sources are used. For example, radiation such as
There muography made with muons coming from cosmic rays (streams of very high energy subatomic particles that come from deep space and continuously bombard the Earth) has already been used to explore the inside of pyramids and volcanoes, but the problem is that about one of these natural particles arrives from space per minute per square centimeter, and the generation of detailed images in this way is very slow and low resolution.
An artificial beam of muons, however, would make this technique practicable in areas that are impossible to obtain with natural or traditional muon sources. For example in the context of customs controls or military security, to quickly carry out checks inside containers in naval ports or in structures with walls made of heavy materials that may contain radioactive material. The aim of the research team was in fact to try to produce a beam of muons on demand to make imaging faster and more controllable.
Images of this type had already been made in particle accelerators, but the key factor that the ELI-NP researchers achieved was to obtain an image composed entirely of muons and that the particle beam was fast to generate and potentially transportable.
The countless applications of the muographic technique
Muons are generally produced inside particle accelerators, but these facilities are huge facilities and on the other hand are not used for commercial application purposes. Therefore, if you want to carry out a muography you must bring the object (such as a container or a large concrete structure) to where the accelerator is located. A portable and transportable system would instead allow on-site muographic inspections of critical infrastructures or in places where intervention is required.
Muography with cosmic rays has already given notable results in the past. For example, it revealed a large void inside the Cheops pyramid, more than 30 meters long, then a corridor of about 9 meters on the north face, environments which were impossible to physically access. Another case was that of the Fukushima nuclear power plant after the 2011 accident, where two muon detectors were used. Detectors found no trace of the fuel inside the reactor core, leading to the conclusion that it had melted and settled at the bottom of the structure. This information was obtained without operators having to enter an inaccessible and highly radioactive location such as the inside of a nuclear reactor.
But producing muons artificially would allow us to extend the scope of application to a much more diverse number of cases, such as inspections of materials hidden in shielded containers (for example, to hide radioactive material for military use) or in naval containers. In fact, at ports and borders only a small percentage of containers are thoroughly inspected, and an on-demand beam would speed up scanning and could be carried out in a few minutes, for example to inspect shielded containers without opening them. An artificial muon source would also offer the possibility of controlling the direction through which to observe, producing a 3D image of the target, and would have a much higher observation frequency and resolution than a natural source such as cosmic rays.
The experiment that allows you to see beyond the walls
To create muons you need a lot of energy concentrated in a very short time. In fact, the ELI-NP researchers used a 10 petawatt laser, considered the most powerful in the world. The petawatt (abbreviated PW) corresponds to one million billion watts. But this power is so high because the laser energy is delivered in a very short time, around 23 femtoseconds, or equal to one millionth of a billionth of a second.
For comparison, the energy emitted by the ELI-NP’s laser would correspond to the energy emitted by a 60-watt light bulb turned on for about four seconds. The power, however, is enormous because all that energy is compressed into a very short time, on the order of a femtosecond. So for an instant the laser provides more than 1000 times the power of the world’s electrical grids combined (again for comparison, the average global electrical capacity is about 10 terawatts, or a trillion watts).
To produce muons, the researchers used this laser to hit a gas that releases electrons and accelerates them to very high energies. The electrons collide with a lead target and emit photons, which, colliding with other lead nuclei, finally produce muons.
However, the beam also contains many other particles that must be shielded. To select only the desired source (i.e. the muon beam) and screen other particles that would have compromised and dirtied their muon images, the research team used, after months of simulations and experiments, a large cube of plastic and paraffin.
Finally, in the experiment the filtered beam passes through a 2 meter thick concrete wall and reaches the detectors transported inside a van parked on the other side. Between the wall and the van there was a pile of lead blocks, the shadow of which was obtained. It is not a real shadow, because fewer muons arrive behind the lead, and therefore the detectors record less signal in that area. However, the decrease was such as to guarantee that the image that was formed was consistent with the size and shape of the target used.
The goal of this experiment was to demonstrate that artificial muography could become feasible and fast. However, the goal is still far away, because the ELI-NP laser is still a gigantic and very difficult to transport system, which fires at most once a minute. So the success of this technique will depend on the development of more compact lasers with higher repetition rates.
