Florence Cathedral (Cattedrale di Santa Maria del Fiore). Source: Jonathan Körner on Unsplash.

Muons: How tiny particles could save a cathedral dome

Florence Cathedral is an iconic landmark in a city famous for its architecture. Towering over the surrounding buildings in the city’s historic centre, it seems unfathomable that its dome could one day be at risk of collapse, but this is the concerning suggestion made by engineers analysing cracks in the nave walls.

These unexplained cracks first appeared almost immediately after the dome’s construction was completed in 1436, and have since been widening. Several possible reasons for the fissures have been proposed, including seasonal temperature change, seismic activity and gravitational settling. Some engineers have even suggested that the cracks function more like expansion joints, and help to regulate the gravitational settling of the structure, but nobody knows for certain.

Subatomic Particles from Outer Space

This is where a tiny elementary particle, called a muon, comes in. Muons are unstable, highly energetic particles which are generated in the upper layers of Earth’s atmosphere. The production of muons occurs when high-energy particles from outer space, known as cosmic rays, collide with the nuclei of atoms in the upper atmosphere. These collisions mainly result in the creation of new particles called pions. These pions decay quickly to form muons. Muons have a lifetime of only microseconds, but their near-light-speed velocity means that they can reach the Earth’s surface, where they can pass through concrete, rock and steel. However, their capacity to pass through a structure decreases as the density of the material increases.

The production of muons occurs when cosmic rays collide with the nuclei of atoms in the upper atmosphere.

These properties of muons have enabled physicists and engineers to develop the science of muon tomography, or muography. This involves tracking the number of muons which pass through a target volume to produce a density map. This data can then be used to generate three-dimensional images of the internal structure of buildings, bridges, and even volcanoes.

Saving Lives by Imaging Concrete Structures

One of the reasons that muography is so useful is that muons can penetrate through concrete. In the modern world, we are often surrounded by concrete structures, and concrete is one of the most widely used materials on the planet. Seventy percent of humans live in structures made of reinforced concrete, so it is vital that these buildings are engineered to stand the test of time. The proliferation of concrete in modern cities means that the ability to detect structural weaknesses in concrete could save thousands of lives.

Seventy percent of humans live in structures made of reinforced concrete.

Unfortunately, there have been several recent high-profile cases of concrete structures collapsing, causing substantial loss of life. An example is the high-rise apartment block in Florida which collapsed in June 2021, killing almost 100 people. Engineers analysing the accident have since suggested that some of the concrete pillars used in the building may not have contained enough steel to support the structure.

An experimental proof of concept for the muography of a concrete block, showing the internal steel rebar used for reinforcement, compared to the results obtained using radar imaging. Compared to radar, muography can be used to make images at much greater depths beneath the surface.

Source: E. Niederleithinger, S. Gardner, T. Kind, et al., “Muon tomography of the interior of a reinforced concrete block: First experimental proof of concept,” Journal of Nondestructive Evaluation, vol. 40, pp. 1–14, 3 Sep. 2021.

Why is muography so useful?

Images of structures can already be produced using X-rays, ultrasound and radar, but each of these techniques has its own drawbacks. Radar can only penetrate to a depth of around two metres, while ultrasound measurements can contain a lot of noise due to self-interference and echoes. X-rays are ideal in that they quickly produce detailed images, but they are also a harmful form of radiation and can pose health risks to residents and scientists. In contrast to the limitations of conventional methods, muography can be used to safely image almost any structure on Earth. In 2017, muography even helped to reveal the existence of a previously-unknown secret chamber in the Great Pyramid of Giza.

Muography can be used to safely image almost any structure on Earth.

Adapted from an image in Fanelli G. 2004. Brunelleschi’s Cupola: Past and Present of an Architectural Masterpiece. Florence, Italy: Mandragora.

Imaging Florence Cathedral Using Muography

But what can muography tell us about the cracks appearing in Florence Cathedral? Well, in 2019, a team of physicists at the University of Florence published a paper in which they described how muography could be used to image the dome, based on demonstration measurements that they had already carried out on a mock-up version of the walls. They proposed that similar measurements could be made on the real dome to determine its structural stability and identify any weaknesses. Although muons are extremely tiny particles, they could be responsible for saving the largest premodern dome in the world.

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