Errant particles from everyday radioactive materials are a major obstacle for particle physicists. The solution? Lead from the bottom of the sea.
In 2017, Chamkaur Ghag, a physicist at University College London, got an email from a colleague in Spain with a tempting offer. The year before, an emeritus professor at Princeton University, Frank Calaprice, had learned of old Spanish ships that had sunk off the New Jersey coast 400 or 500 years ago, while carrying a cargo of lead. Calaprice obtained a few samples of this lead and sent it off to Spain, where a lab buried within the Pyrenees tested its radioactivity. It was low: just what Aldo Ianni, the then-director of the Canfranc Underground Laboratory, was hoping for. Now that sunken lead was being offered to any physics laboratory willing to pay 20 euros per kilogram—a fairly high price—for it.
Lead is mined and refined all over the world, but that centuries-old lead, sitting in a shipwreck, has a rare quality. Having sat deep underwater since before the United States of America was born, its natural radioactivity has decayed to a point where it’s no longer spitting out particles. For particle physicists, that makes it exceptionally valuable.
“It’s sort of like gold dust,” Ghag says.
Forget plutonium: Plenty of everyday objects, from ceramics and glass to metals and bananas, are radioactive, to varying degrees. Should the particles from their decay hit the detectors of particle-physics experiments, they could give scientists false positives and dig potholes on the road to scientific discovery. Even the experiments themselves, built from all kinds of metals, have lightly radioactive components.
Just a few inches of lead can shield detectors from all kinds of rogue radiation, and one of the best ways to block sneaky, unwanted particles is to surround them with lead that itself is barely radioactive. The best source of such lead just so happens to be sunken ships, some of which have been corpses near coastal waters for as long as two millennia.
Particle-physics experiments look for the most fundamental building blocks of the cosmos, including dark matter, an as-yet unseen substance that acts like glue within and between galaxies. This ancient lead, then, is helping humanity unlock the secrets of the universe—but obtaining it often presents practical and ethical uncertainties.
Shipwrecked lead belongs to a class of items known as low-background materials, which have very low levels of intrinsic radioactivity. There is no agreed-upon standard for what constitutes a low-background material, but, based on the sensitivity to an experiment’s background radiation, it’s clear what level’s needed, says Alan Duffy, an astrophysicist at the Swinburne University of Technology. “If you’re building a Geiger counter, you need the Geiger counter to not pick up on itself,” he says.
Take steel: It’s an excellent shield from intruding vagabond particles—so much so that Fermilab, a particle-physics and accelerator laboratory in Illinois, has used tons of it in the past few decades to shield its own experiments, says Valerie Higgins, Fermilab’s historian and archivist. That steel frequently came from decommissioned warships, many of which existed around the time of, or served in, the Second World War or the Korean War, including the Astoria, the Roanoke, the Wasp, the Philippine Sea, and the Baltimore........MUCH MORE
