By Nicola Nosengo
Around four tons of ancient Roman lead was yesterday transferred from a museum on the Italian island of Sardinia to the country's national particle physics laboratory at Gran Sasso on the mainland. Once destined to become water pipes, coins or ammunition for Roman soldiers's slingshots, the metal will instead form part of a cutting-edge experiment to nail down the mass of neutrinos.
The 120 lead ingots, each weighing about 33 kilograms, come from a larger load recovered 20 years ago from a Roman shipwreck, the remains of a vessel that sank between 80 B.C. and 50 B.C. off the coast of Sardinia. As a testimony to the extent of ancient Rome's manufacturing and trading capacities, the ingots are of great value to archaeologists, who have been preserving and studying them at the National Archaeological Museum in Cagliari, southern Sardinia. What makes the ingots equally valuable to physicists is the fact that over the past 2,000 years their lead has almost completely lost its natural radioactivity. It is therefore the perfect material with which to shield the CUORE (Cryogenic Underground Observatory for Rare Events) detector, which Italy's National Institute of Nuclear Physics (INFN) is building at the Gran Sasso laboratory.
CUORE, which will begin operations next year, will investigate neutrinos: fundamental particles with no electronic charge and long thought to have no mass. Researchers have confirmed that neutrinos do have a mass, but have been unable to pin down a figure for it. The aim is to use the detector to try to observe a theoretical atomic event called neutrinoless double-beta decay--a radioactive process whereby an atomic nucleus releases two electrons and no neutrinos. "Standard" double-beta decay is accompanied by the release of two neutrinos. By observing this predicted but so far unseen event, physicists hope to estimate the neutrino's mass and to establish whether neutrinos and their antimatter counterparts, antineutrinos, are different particles. Some believe the two to be one and the same.
CUORE scientists will wait for neutrinoless double-beta decay to happen in a 750-kilogram cube of tellurium dioxide placed under 1,400 meters of rock at the Gran Sasso laboratory. But to successfully observe this rare event, they will need to shield their experiment from external radioactivity.
This is where the shipwrecked lead comes into the picture. Lead is, in principle, a shield against radiation, but freshly mined lead is itself slightly radioactive because it contains an unstable isotope, lead-210. "We could never use it for our experiment, which is exactly about keeping background radioactivity to a minimum," says Ettore Fiorini, a physicist at the University of Milan-Bicocca and coordinator of the CUORE experiment. After it is extracted from the ground, however, lead-210 decays into more stable isotopes, with the concentration of the radioactive isotope halving every 22 years. The lead in the Roman ingots has now lost almost all traces of its radioactivity.
The ingots arrived at Gran Sasso thanks to an agreement dating back to 1991. In 1988, a scuba diver discovered the ship's remains at a depth of 28 meters, a mile and a half from Oristano, just over 1 mile from the Sardinian coast. Fiorini recalls reading about the finding in a newspaper, and immediately foreseeing its value to physicists.
"It is not unusual for particle physicists to go hunting for low-radioactivity lead," he says. "Metal extracted from roofs in antique churches or from keels of wrecked ships has often been used in experiments." But the Sardinian finding was unprecedented, both in terms of the age and the abundance of the material.
The ship was in fact a navis oneraria magna , a specialized cargo vessel often used to transport heavy loads such as lead or other metals. It carried more than 1,000 ingots, or 33 tons of metal. Given that civil war was raging in Rome at the time it sank and that the ship was loaded with slingshot ammunition, archaeologists believe that much of the ship's lead may have been destined to end up as shot. They also think that the ship was deliberately sunk on the orders of its captain to prevent it from being seized by enemy forces: it was still anchored, and close enough to the coast for the crew to swim to shore.
When Fiorini learned that the Archeological Superintendence, a government office that oversees heritage projects, in Cagliari did not have enough funds to retrieve all the ingots, he convinced INFN managers to contribute 300 million lira (US$210,000) to the operation, which was completed in 1991. In exchange, a proportion of the recovered lead would become available for physicists. Some ingots were used in experiments during the 1990s, but Fiorini says that CUORE is what he had in mind when he first proposed the deal.
At Gran Sasso, the ingots will be melted into a 3-centimeter-thick lead lining that will surround the cubic CUORE detector. Before the ingots are melted down, the inscriptions on each one will be removed and sent back to Cagliari for preservation. "They are trademarks, bearing the names of various firms that extracted and traded lead," explains Donatella Salvi, an archaeologist at the Cagliari museum.
Salvi says that parting with the ingots has been "painful." The ones given to INFN are the worst-preserved, but are still of exceptional historical value. However, she says she is happy with the collaboration, because physicists are performing important analyses on the lead. For example, Fiorini's team has helped archaeologists to settle a debate about the ship's route. It had first been proposed that its lead could come from Sardinian ores, but Salvi was skeptical. "Romans at that time preferred to preserve Italian ores, which they considered strategic, and instead extracted most of their lead from Northern Africa, Spain and Britain," she says. By studying the particular mix of isotopes in the lead--a signature of its origin--INFN physicists have confirmed that Salvi was right. The ingots came from Sierra de Cartagena, in southern Spain.