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.....Wow, I definitely didn't know any of that.
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.
Wednesday, December 18, 2013
An Ancient Battle
Physicists and archaeologists are fighting over ancient Roman lead (yeah, really):