Some drivers crossing the Jõgisoo Bridge this week may have wondered about the strange box placed on the structure. For those still curious, here's the answer — this was the world's first test of using cosmic radiation, specifically muons, to assess the condition of a bridge open to traffic.

"These particles are born in the atmosphere, exist for just 2.2 milliseconds and travel at nearly the speed of light. They are charged particles that pass through everything, and as they pass through materials, they either lose some energy or scatter. When they go through this bridge, some scattering and energy loss occur. We have four boxes here, each detecting about 20,000 particles per minute," explained Sander Sein, product manager at GScan.

By analyzing the trajectories of these particles, it is possible to determine what materials were used in different parts of the bridge and assess its condition — for example, whether the steel reinforcement has started to rust. While this technology has been tested in the UK, Jõgisoo is the first place in the world where it is being used to evaluate the structural integrity of a bridge that remains open to traffic.

"So far, bridge assessments have largely relied on expert evaluations, which has been a challenge. In most cases, the decision has been to demolish the existing bridge and build a new one," said Tõnis Tagger, environmental coordinator for road infrastructure at the Estonian Transport Administration.

Since constructing a new bridge is extremely costly, the administration hopes that this new technology will help identify more efficient solutions — allowing as much of the existing structure as possible to be preserved and repaired instead of being entirely replaced. A few years ago, muons were used to get a picture of what lies inside the former nuclear reactor in Paldiski and the same technology has also been tested at airports.

"It worked, but it was a bit slow because the cosmic radiation reaching Earth is not very intense — there aren't enough particles. If we need two minutes to determine the material composition, that's too slow for an airport setting," Sein explained.

According to Sein, muon tomography has many potential applications and could even be used as a future alternative to X-ray imaging.

However, if anyone is now thinking of standing under the bridge to get their body scanned, they shouldn't bother. First, they'd have to stand still for an hour, and second, the security patrol would be there within minutes.

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