Roman shipwreck to aid in search for neutrinos

Twenty years ago, divers found a Roman shipwreck off the coast of Sardinia. The ship had gone down some time between 80 and 50 B.C., and it was an unusual find in many ways. For instance, the ship sank straight down vertically and there was no evidence of mooring attempts despite its proximity to shore or any evidence of what caused it to sink, no sign of fire or of it being crushed by waves or shoals. Archaeologists speculate that the captain might have intentionally scuttled the ship to keep it out of enemy hands.

Lead ingot stamped with Pontilieni nameThe star of the show, however, was its cargo. The ship was full of lead. Usually lead lined the hull of Roman ships, but this was a full-on shipment of lead ingots from mines in Cartegena, Spain, with Rome as its ultimate destination. The hull was found to contain approximately 2000 carefully stacked ingots of lead, each weighing 33 kg (73 lb) or 100 Roman pounds, the maximum amount by law that a slave was allowed to carry. All the ingots were stamped with the names of the merchants who extracted and shipped the lead, Caius and Marcus Pontilieni and their servant Pilip claiming the majority.

Lead ingots still stacked on sea floorThe ship was 36 meters and 12 meters wide of a type called navis oneraria magna, a cargo vessel specially designed to carry heavy loads. It would have to be, because that cargo of lead weighed 39 tons. This is the largest lead cargo to have ever been found. It exponentially increases the total amount of ancient lead we have access to.

That’s where the neutrinos come in. Neutrinos are extremely difficult to detect because although billions of them pass through us every day, their signature can easily be obscured by every day elements, like cosmic rays or naturally occurring radiation in rocks. Scientists struggle to find material to shield their neutrino experiments from these kinds of interference. Even lead, which as we all know blocks Superman’s X-ray vision, contains trace amounts of decaying lead-210 isotopes.

But ancient lead, on the other hand, has long since outlasted the half-life of all its radioactive isotopes.

When nuclear physicist Ettore Fiorini at the University of Milan-Bicocca read about the find in a newspaper he went to Cagliari to offer the financial support of the Italian National Institute for Nuclear Physics (INFN) in excavating the vessel and its precious cargo. Accepting the offer, archaeologists in Cagliari at the time gave the INFN 150 ingots in return, and they recently sent off a second batch of 120 ingots, which reached the Gran Sasso laboratory last week. These will now be stripped of their historically interesting manufacturers’ names, cleaned of any incrustations and then melted to provide a shield for the CUORE experiment.

CUORE, which should be ready in about two or three years time, will use 750 kg of tellurium dioxide to try and discover an extremely rare nuclear process predicted by theory and known as neutrinoless double beta decay. Involving the transformation of two neutrons into protons and electrons but no neutrinos, this decay would imply that neutrinos are, uniquely, their own antiparticle. Observing the decay would also provide physicists with a way of directly calculating the mass of the neutrino, something that to date can only be done indirectly.

Scientists have used old lead from shipwrecks before — US researchers used lead from a 450-year-old Spanish galleon for the IGEX experiment — but the sheer quantity of this find, its age and the purity of ingots make this collaboration of ancient and cutting edge particularly exciting.