Archaeologists and physicists are at loggerheads over ancient Roman lead—a substance highly prized by both camps for sharply diverging reasons. Very old lead is pure, dense and much less radioactive than the newly mined metal, so it is ideal for shielding sensitive experiments that hunt for dark matter and other rare particles. But it is also has historical significance, and many archaeologists object to melting down 2,000-year-old Roman ingots that are powerful windows on ancient history.
"Are these experiments important enough to destroy parts of our past, to discover something about our future?" says Elena Perez-Alvaro, an archaeology graduate student at the University of Birmingham in England, who wrote a paper on the dilemmas involved in Rosetta (pdf), an archeological journal published by the University of Birmingham. Some physicists argue that getting hold of the metal is worth fighting for. "These experiments can reveal some of the most fundamental properties of the universe, and answer questions such as what are we and where we come from," says physicist M. Fernando Gonzalez-Zalba of the University of Cambridge, who collaborated with Perez-Alvaro's investigation. "I think it's worth it."
Ancient Roman lead has been used in the Cryogenic Dark Matter Search (CDMS), an experiment in Minnesota that aims to detect the particles that make up the invisible dark matter thought to contribute much of the universe's mass. The same metal has also been used in the CUORE (Cryogenic Underground Observatory for Rare Events) project in Italy, which will soon begin searching for a theorized particle decay process called neutrinoless double beta decay, which, if found, could explain why matter dominates antimatter in the universe. These experiments and others require extreme shielding to block out any extraneous particles that might be mistaken for the rare signals they hunt.
The lead in question once went into the making of coins, pipes, construction materials and weapons in the ancient Roman civilization. It is most commonly found now at shipwreck sites, where private companies harvest it and melt down the Roman ingots into standard bricks before passing them on to customers—many of whom are physicists. "None of us take it casually—you don’t want historical artifacts to be destroyed unnecessarily," says physicist Blas Cabrera of Stanford University, who leads the CDMS project. Nevertheless, ancient lead is the best material available for shielding dark matter detectors, he says, because it releases so little radiation, or background particles. "The kind of background levels that you're achieving with ancient lead are roughly 1,000 times below that of commercially available lead."
All lead mined on Earth naturally contains some amount of the radioactive element uranium 235, which decays, over time, into another radioactive element, a version of lead called lead 210. When lead ore is first processed, it is purified and most of the uranium is removed. Whatever lead 210 is already present begins to break down, with half of it decaying on average every 22 years. In Roman lead almost all of the lead 210 has already decayed, whereas in lead mined today, it is just beginning to decay. (Of course, many lead 210 atoms have already decayed in this ore, too, but the supply is constantly replenished by uranium in unprocessed lead). "The longer since it was originally processed, the lower its intrinsic radioactivity," Gonzalez-Zalba says.
The Romans were not the first lead brick makers—the ancient Greeks were also manufacturing the building material about 200 years earlier. Whereas this lead probably also finds its way into some physics experiments, it is scarcer. And the supply of Roman lead is not exactly plentiful, either. "We may lose all ancient Roman lead—and therefore the information about ancient technology, shipping, trade, etcetera it can offer—if its use for this kind of purpose becomes widespread," says archaeologist John Carman, Perez-Alvaro's advisor at the University of Birmingham. By preserving the ingots, archaeologists hope to learn more about the technology, industry and culture of the Romans. Future technology may be able to pry more secrets out of the artifacts than present studies can do, so leaving the objects undisturbed, preferably at the shipwreck site, is ideal.
The law surrounding this dispute is murky. The 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage (pdf) prohibits commercial exploitation of historical shipwreck artifacts. Whether that applies to physics experiments is unclear. "Because the final use of the lead is for knowledge, not actually for the marketplace, this lies somewhere in between," Gonzalez-Zalba says. "This is where the regulation is not 100 percent clear about it." Both archaeologists and physicists say better guidelines are necessary. "We need the sort of deep analysis of the issues involved that Elena is undertaking, followed by a serious debate involving all interested parties, including international bodies such as UNESCO, to construct a set of clear guidelines that will hopefully protect the interests of the scientific community, including that of archaeology," Carman says.
Ultimately, all parties seek a compromise that will preserve history yet enable cutting-edge physics. After all, the Romans were famous innovators, and would probably smile to know how their lost ingots were being used today.