In A Nutshell
It can take a lot of effort to observe certain subatomic particles, such as neutrinos. Huge, extremely sensitive detectors are needed. Preventing this equipment from being overwhelmed by background radiation requires shielding, and lead is the best material for the job. Unfortunately, fresh lead is itself slightly radioactive. Lead minted by the Romans and lost in shipwrecks 2,000 years ago, though, has just about lost its radiation and is perfect. This has put physicists at odds with archaeologists, as they disagree on whether we should preserve our cultural heritage or allow the material to go toward improving our scientific knowledge.
The Whole Bushel
It’s one of particle physics’ most counterintuitive qualities—to study the tiniest and least significant of particles we require the largest machines ever conceived and built. Marvels such as the Large Hadron Collider are an example of this. Yet Higgs’ tricky boson isn’t the only pesky particle that we can’t spot easily. Neutrinos are a type of particle that have almost no mass, no charge, and that tend to just fly through everything without interacting. Millions of them will pass through your body in the time it takes you to read this.
Observing these particles requires extreme measures. That’s why scientists have built detectors at the South Pole, in deep mines, and even inside mountains. In the case of the latter two, the idea is to use the Earth itself as a shield from radiation from space. For some purposes, though, the best sources of shielding is low-radioactivity lead. Since fresh lead has a half life of 22 years, a batch that’s been hanging around for hundreds or thousands of years is ideal. Commercially available lead is around 1,000 times more radioactive than the old stuff.
One of the best sources of lead ever found came from a Roman shipwreck discovered in 1988. The wreckage contained 33 tonnes of lead made up of 1,000 ingots. They sunk to the seafloor some time between 80 B.C. and 50 B.C. near Sardinia. There was obvious value to local archaeologists, but they didn’t have the money to raise the metal themselves. A physicist that heard about the find arranged to assist the archaeologists financially, in return for a portion of the metal.
Four tonnes of the lead is being used to create a 3-centimeter-thick (1 in) shield around a neutrino detector at Italy’s National Institute of Nuclear Physics. Lead from an 18th-century French shipwreck has been used in an experiment in Minnesota designed to look for dark matter, which makes up most of the mass of the universe and is one of science’s biggest mysteries.
Yet using the lead requires melting it down, which makes many archaeologists uneasy. They see it as a destruction of cultural heritage. To offset this, only the least well-preserved lead bricks tend to be used, and their inscriptions are cut off and saved. The practice falls into a grey area on UNESCO rules, which prevents archaeological sites being exploited for commercial purposes. Yet physics experiments enrich human knowledge, so it may be fine—both sides want clearer guidelines.
Ultimately it’s a matter of compromise. How much of the past should we cling to, and how much of it should we bulldoze through as we move into the future?