Views: 0 Author: Site Editor Publish Time: 2024-10-17 Origin: Site
In 1988, about 10 kilometers off the coast of Sardinia, Italy, divers were diving to find the remains of a shipwreck. When they descended to a depth of 28 meters, they suddenly discovered the outline of a wreck.
Since the beginning of the Age of Sail, the seas have swallowed countless ships. These shipwrecks hold treasures and histories from different eras, and each one found will be of great interest to maritime archaeologists, as it is the best clue to their recreation of the past.
Based on the shape of the ship's clay pots, archaeologists determined that it was an ancient Roman shipwreck. The ancient Roman civilization is more than 2,000 years old, and time and the sea destroyed most of the wooden structures, but some corrosion-resistant stone tools and metal objects are still intact. Although it is not surprising that an ancient Roman shipwreck has been found on the seabed near Italy, this ship is special, it is much larger and stronger than most wrecks.
Archaeologists discovered the reason why the ship was so strong – it was loaded with a large number of metal bars, more than a thousand lead ingots, or about 33 tons of metal, the largest number of shipwreck excavations at the time. For these heavy "treasures", archaeologists are undoubtedly very surprised! Suspiciously, physicists are just as excited.
Each of these ingots is roughly trapezoidal, 45 centimeters long and weighs about 33 kilograms, so they were still neatly stacked when they were found.
Lead was an important metal in ancient Rome and was cast into pipes, coins, weapons, or structures. Although the exact purpose of this large batch of lead ingots is not clear, the discovery of this large number of lead ingots also confirms the strength of ancient Rome's manufacturing capacity and the development of economic trade. The inscription on the lead ingot also allows archaeologists to peek into the technological, industrial and cultural history of lost civilizations.
Most of the ancient lead ingots were salvaged from deep-sea shipwrecks, but some were also buried in the earth. A study published in May in the Journal of Roman Archaeology examined in detail three lead ingots excavated from the 20th-century site of Belmes in Córdoba, Spain.
By analyzing the chemical composition and stable isotopes of the lead ingots, the researchers found that the three lead ingots were produced in the same mining area, and two of the lead ingots with the letters "SS" came from the same mining company, "Societas Sisaponensis", which is headquartered in Córdoba. Tests of the shipwreck's ingots showed that more than half of the ingots were mined from this mine. The latest findings seem to further confirm that Córdoba may have had the most important metallurgical network in the ancient Eastern Mediterranean, reflecting the level of possible industrialization at the time.
Lead ingots can help archaeologists connect the history of the Eastern Mediterranean, and apparently they are eager to leave all the ingots found in their place or send them to museums for further examination and analysis. But the physicists who were "peeping" on the sidelines didn't think so, and what they wanted most was to melt down these Roman lead ingots and use them to explore the mysteries of the universe.
In 1988, after reading in the newspapers about the discovery of this huge cargo ship, Ettore Fiorini immediately foresaw the importance ·of these lead ingots for physicists (or more precisely, particle physicists). Fiorini is a physicist at the University of Milan-Bicocca in Italy and the experimental coordinator of the Cryogenic Underground Observatory for Rare Events (CUORE).
At the time, Italy's National Institute of Nuclear Physics (INFN) was building the CUORE detector underground at the Gran Sasso laboratory. The goal of this experiment is to find a theoretical particle decay event called neutrino-free β decay. A standard β decay releases two neutrinos, but in a neutrino-free β decay event, the nucleus releases only two electrons and not neutrinos.
Even in theory, neutrino-free binary β decay events are rare and we have never observed them, but if they were to be observed, they might measure neutrino mass, answer the question of whether neutrino antimatter is itself (Majorana neutrino), and perhaps reveal the mystery of the asymmetry in the distribution of matter-antimatter in the universe.
To observe this rare decay event, CUORE's scientists needed to build a tellurium dioxide cube weighing about 750 kilograms at a depth of 1,400 meters below the rock formation. Due to the rarity of such events and the very weak signal, this experiment (and similar experiments) had to be strictly insulated from all external radioactive events, keeping background radioactivity to a minimum – and this is where Roman lead comes in.
The entire CUORE is built underground, protected by 1.4 kilometers of mountain rock formations from the background radiation of cosmic neutrinos, but this is not enough. Because the rock formations used to protect the facility are also slightly radioactive, CUORE also needed a "shield" that was strictly shielded from radiation. The nucleus of lead is large and heavy, so it only needs a thin layer to block many tiny particles from penetrating. Ideally, pure lead is suitable for use in radiation barriers.
But the reality is not ideal. All newly mined lead in nature contains a certain amount of the radioactive element uranium-235, which decays over time into the unstable isotope lead-210, while the half-life for it to decay into a more stable isotope is 22 years. Although the process of processing lead ore removes most of the uranium, the lead-210 that is already present still emits weak radiation over the years. Obviously, lead in reality itself is a source of radiation and cannot be used directly as a radiation barrier for particle physics experiments.
However, lead, which has been silent underwater for thousands of years, has almost completely lost its natural radioactivity over a long period of time, making it the perfect material for shielding particle detectors. In 1991, the INFN team and collaborators examined the radioactivity of Roman lead in detail in a paper (Fiorini was a co-author), and various detection methods showed that Roman lead did not contain lead-210 at all, and the background radiation level was only about one thousandth of that of modern lead, making it the best shielding material in the research samples at that time.
In 2019, a study published in The European Physical Journal A further tested the radioactivity purity of lead samples from Rome with the latest cryo-detection technology and reported the lowest ever limit for Pb-210 measurements.
"Particle physicists often look for low-level lead," Fiorini says, "from the roofs of old churches to metals extracted from shipwreck keels that are often used in experiments." "However, the discoveries in Sardinia are unprecedented, both in terms of age and in terms of the richness of the material.
In 1991, Fiorini learned that the archaeological institution in Cagliari did not have enough funds to recover all the lead ingots from the seabed, and he persuaded INFN managers to donate about $210,000 to the operation. In exchange, physicists could use a portion of the recycled Roman lead.
In the 90s of the 20th century, some lead ingots were used in INFN experiments. In 2010, the Grancaso laboratory "stocked" another 4 tons of Roman lead from a museum in Sardinia.
Archaeologists at the Cagliari Museum say the separation from these lead ingots is very painful. Although the lead ingots handed over to INFN are in the worst state of preservation, they are still of extraordinary historical value. Fortunately, the physicist would cut out the inscription and send it back to Cagliari for preservation before melting the lead ingot. The remaining ingots, along with the previous lead, will be melted into a 6-centimeter-thick lead liner that will envelop the CUORE detector.
Many archaeologists have objected to the casting of these historic lead blocks. Elena · Perez-Alvaro, Ph.D. in Cultural and Natural Heritage Management, has questioned: "Are these experiments important enough to destroy parts of the past to discover the future?" M. Fernando · Gonzalez-Zarba, physicist at the University of Cambridge in United Kingdom. Fernando Gonzalez-Zalba said: "I think these experiments can explain some of the most basic properties of the universe, and I think it's worthwhile. ”
Roman lead is not the only material that meets the requirements of sensitive experiments, and ancient Greece also used this building material. Greece lead was rarer, but Roman lead was not in sufficient supply. Archaeologist John · Carman said that if physicists used it widely, archaeologists could lose all the ancient Roman lead and thus all the information it could provide about the technology, culture, and industry of the Romans.
There is no clear legal provision for this dispute. The 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage prohibits the commercial exploitation of historical shipwrecks, but it is not clear whether this applies to physical experiments.
Although the specifics are not known, the dispute was eventually settled by a compromise between the parties – the CUORE team had already begun collecting data from their experimental apparatus in 2017 and published their latest results in 2022. Unfortunately, they found no traces of neutrino β decay.
Currently, INFN is trying to upgrade CUORE to CUORE Upgrade with Particle Identification (CUORE) to add particle recognition capabilities. The best news for archaeologists is that this upgrade does not require additional Roman lead.
Interestingly, CUORE's main scientific goal is to find evidence of Majorana neutrinos, but its ability to identify and measure low-energy events makes it also well-suited to exploring dark matter – astrophysical observations at various scales have shown that 27% of the universe is made up of undiscovered dark matter, but we have yet to unravel the mystery of what exactly dark matter is.
