Getting the Lead Out – Particle Shielding and History

If the Superman comics teach us anything accurate about physics, it’s that the Man of Steel’s X-ray vision is powerless to see through a shield of lead. What the comics don’t reveal is that not all lead is created equal when it comes to shielding objects from radiation. The older the lead, the better the protection, and this situation is creating an unexpected conflict between two groups of researchers who would otherwise keep to themselves: particle physicists and Roman archeologists.

Superman and Batman are stymied by the lead-masked Composite Superman! Courtesy of Dial B for Blog.

This difference arises from a curious fact: although lead’s density and very positive nucleus make it an excellent absorber of X-rays and gamma radiation, all samples of lead are themselves slightly radioactive. Lead ore contains small amounts of radioactive uranium 235, which decays over time by emitting particles in the form of alpha and beta radiation. The decay chain of uranium 235 includes radioactive lead 210; this isotope gives off beta radiation with a half-life of 22 years. When lead ore is processed to extract lead, the uranium is removed from the resulting metal. The lead 210 in the metal then continues to decay without being replaced by decaying uranium, so the overall radioactivity of the sample decreases over time.

While freshly mined lead is just starting its process of decay, ancient Roman lead has had about 2,000 years, or nearly 91 lead 210 half-lives, for its radioactivity to dissipate. This means that for every octillion (that’s a 1 followed by 27 zeroes) lead 210 particles present in a Roman ingot when it was purified, roughly one remains to the present day. For most purposes, this difference is insignificant, but for scientists working with particle detectors, it’s absolutely crucial: the background radiation given off by new lead creates “noise” that can drown out the very rare “signal” events sought by particle physicists.

Large quantities of ancient lead can be found in shipwrecks. One recent expedition off the coast of the Italian island of Sardinia uncovered over 33 tons of the metal, most of it believed to be originally destined for slingshot ammunition in the Roman civil war resulting from the Catiline Conspiracy. Most of the lead will still end up in Italy, but for an entirely different purpose: shielding the detector of the CUORE (Cryogenic Underground Observatory for Rare Events) at the Italian National Institute of Nuclear Physics.

Archeologists, however, are worried about the irretrievable loss of history that comes from melting down these ancient ingots. Elena Perez-Alvaro, an English archeology graduate student, raised the issues in a recent paper, arguing that “the study of sunken vessels is essential to history because entire continents have been discovered, colonised, invaded and defended by sea.” The lead is often inscribed with Latin phrases that describe its origins and the ancient companies that mined and shipped it, providing invaluable information to historians. If the companies that sell the lead to physicists fail to record the writing before smelting the metal down, the data can never be replaced.

It’s possible to reach a compromise that benefits both camps of researchers: for the CUORE lead, the inscriptions are being carefully removed from each block and sent to archeologists before melting. Without this level of care, however, science may be guilty of stripping the past to investigate the future.

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