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August 24, 2018

Italy’s famous dome is cracking, and cosmic rays could help save it


The soaring dome atop the Cathedral of St. Mary of the Flower justly dominates the Florence skyline and has stood for centuries, ever since Filippo Brunelleschi designed it in the early 15th century. But scholars aren’t quite sure how this goldsmith with no formal architectural training managed to construct it. Brunelleschi built a wooden and brick model of his plan but deliberately left out crucial details and left no comprehensive blueprints so his rivals could not steal his secrets.

Elena Guardincerri, a physicist at Los Alamos National Laboratory who grew up in a nearby town in Italy, thinks she can help resolve part of the mystery with the aid of a subatomic particle called a muon.

Inverted tension

Brunelleschi found inspiration for his design in the inverted catenary shape of the Pantheon, which is an ideal shape for domes because the innate physical forces can support the structure with no need for buttressing. Robert Hooke phrased it best in the 17th century: “As hangs the flexible chain, so but inverted stands the rigid arch.” A chain suspended between two points will naturally come to rest in a state of pure tension; inverting that catenary shape into an arch reverses it into a shape of pure compression. Standard building materials like masonry and concrete would break fairly easily under tension, but they can withstand large compressive forces.

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The Pantheon’s circular dome has a single concrete shell. Brunelleschi’s design called for an octagonal dome spanning 150 feet and soaring nearly 300 feet in height with no flying buttresses for support. He used two shells: a very thick inner shell and a much thinner outer shell. Historians believe he used three pairs of large stone chains (which are still part of the structure) to act a bit like barrel hoops, applying sufficient pressure to hold the bricks in place while the mortar set.

The final dome is a spectacular achievement. Almost immediately, however, cracks began to appear in the structure, albeit very slow-moving cracks. “Nobody is expecting it to fall down any time soon,” said Guardincerri. But a botched restoration effort in the 1980s exacerbated the problem, adding a greater sense of urgency to the quest to preserve the dome, which is one of Florence’s chief tourist attractions. However, the lack of detailed information about the internal structure remains a stumbling block.

Preservationists have employed many different methods over the years to try to fill in their gaps in knowledge. In 1987, 300 different devices were hooked up to the dome, prompting The New York Times to declare it “the world’s most carefully monitored structure.” But the inner shell is so thick, most conventional methods can’t penetrate it. Specifically, it would be nice to know whether the stone chains used to stabilize the dome were reinforced with iron bars, clamps, or more chains to fortify its structural integrity. That’s where muon imaging should be able to help.

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