Muography, the technique that uses particles from space to see inside structures

In 1954, a boat made of cedar wood was found buried in a pit of the Great Pyramid of Giza; it was in 1224 pieces. It took ten years to finally rebuild it, piecing the boat together like an immense puzzle. Then, it was time to come up with some theories. Most likely, it was a funeral boat that at the time symbolized the pharaoh Cheops’s journey to the afterlife. 

More than 4500 years after it was built, the Great Pyramid of Giza still holds endless surprises. Part of the interior of the largest and oldest pyramid in Egypt remains unknown, and it is estimated that a large number of passages and chambers remain as yet undiscovered.

However, a new imaging technique promises to reveal what’s inside the pyramids’ limestone and granite walls without venturing inside: this is muography, a system that uses subatomic particles from space to show us what’s behind a surface. 

This technique, which made it possible to discover the hidden chamber that served as a tomb to the pharaoh Cheops, promises to revolutionize not only the study of ancient history, but also construction technology. 

Muons from space

Muography and muon tomography are imaging techniques that use muons to analyze what’s inside a structure – for example, the pyramids – much like how X-rays offer images of our bodies. But let’s start at the beginning: what are muons?

Muons are subatomic particles that come from the upper layers of the Earth’s atmosphere. There, they are made by collisions between cosmic rays and the nuclei of atoms. They reach the Earth’s surface in large quantities, almost at the speed of light: it is estimated that about 10,000 fall per second on each square meter. 

The Gulf of Mexico seen from space. NASA (Unsplash)

This implies that, as you are reading these lines, millions of muons are falling around you. You cannot see them, and they exist for just a few microseconds. We can go further still: these muons are also passing through your body right now: the particles have the ability to penetrate surfaces, passing through some as dense as steel or concrete. This is precisely what makes them so valuable for science. 

By using complex systems, physicists can calculate which muons have passed through a certain surface, which makes it possible to form images of the interior of infrastructures or materials. According to the International Atomic Energy Agency (IAEA), there are two types of muon-based imaging techniques: muography and muon tomography. 

Both techniques are based on comparing how muons act when they come into contact with different materials. They easily cross through less dense ones and have more difficulty doing so as the density increases. For example, they penetrate the human body much more easily than cement; they pass through cement, in turn, better than steel. 

Detectors for studying pyramids and volcanoes

Years before it was used to analyze the interior of the Great Pyramid of Giza, muography had already been tested on another: the Bent Pyramid of Dahshur, built thousands of years ago by Pharaoh Sneferu in what is now southern Cairo. On this occasion, the aim was not to find hidden ships but to check the accuracy of the technique.

Bent Pyramid of Dahshur. Dmitri Zhodzishkii (Unsplash)

 Dozens of muon-sensitive plates were placed inside the pyramid to confirm that the resulting images showed the presence of the two burial chambers that were already known. When setting up this type of detector in the right places, it is possible to identify where there are empty areas (which the muons pass through with no problem) and where the densest materials are located (which the muons have more difficulty passing through). 

These techniques offer a non-invasive solution, which makes it possible to obtain information from inside the structures without drilling or digging. Its use can be very interesting to analyze the interior of volcanoes, for example, and improve early warning systems of eruptions.

Naples at the foot of Vesuvius. Francesco Baerhard (Unsplash).  

Its main drawback is the time it takes to form an accurate image with muons. In the case of the test done on the Bent Pyramid of Dahshur, 40 days of exposure were needed before collecting the samples. But this is inevitable: it is a physical limitation, not a technological one.

Muography in construction

Muon-based imaging techniques can also have very useful applications in the world of construction. For example, with regard to maintenance. Currently, different lines of research are studying how to use muons to detect faults in concrete surfaces and infrastructures.

Arch of La Défense in Puteaux, France, a concrete building. Pierre Châtel-Innocenti (Unsplash).

The use of muography and muon tomography would make it possible to detect problems that are not noticeable to the naked eye, such as wear on the material or the presence of cracks and cavities. It would also serve to ensure that the reinforcements made of other materials, such as metal or steel, are in good condition.

This would improve building and infrastructure inspections and could prevent landslides; this would have a highly significant social, economic, and environmental impact in practically all regions of the world. 

Until now, muons have been used to find rooms created thousands of years ago and to rewrite history. Maybe, in a few years, they will also help keep infrastructures that aren’t in the headlines, but are equally important, still standing.