How can shipwrecks help in the search for dark matter?
WHAT have sunken ships got to with neutrino detectors and dark matter? Well, the lead recovered from shipwrecks might turn out to be the most important metal in our attempt to understand neutrinos and dark matter.
Italy’s National Institute of Nuclear Physics (INFN) just reported that it took possession of 120 bricks of old lead from a museum in Sardinia. These bricks had been salvaged 20 years ago from a Roman ship that sank of the coast of Sardinia 2000 years ago. The INFN will be melting this lead and creating a 3 centimetre-thick lining that will surround its Cryogenic Underground Observatory for Rare Events (CUORE) detector, according to a news report in Nature.
CUORE is watching for an extremely rare event called neutrinoless double-beta decay, in which a radioactive element decays, and the decay products include two electrons, but no neutrinos (normal beta decay produces antineutrinos). Neutrinoless double-beta decay is theoretically predicted, but has never been convincingly observed. The experiment is monitoring 750 kilograms of tellurium dioxide for this rare event. The study will reveal more about the mass of neutrinos.
But the experiment can be fouled by ambient radioactivity, which can create a lot of noise in the detector. That’s why you need lead to shield the detector. However, new lead has an isotope called lead-210, which is itself radioactive, and can cause the same problems as ambient radioactivity. What’s a physicist to do?
Here’s where shipwrecks come in. In lead that has been lying on the seabed for a couple of thousand years, the lead-210 – which has a half-life of 22 years – has pretty much all decayed. Using this lead solves the problem.
CUORE, however, is not the first experiment to hit upon this idea. The Cryogenic Dark Matter Search (CDMS) experiment, being performed in the Soudan Mine in Minnesota, has been using lead from shipwrecks for a similar purpose for years now. CDMS is looking for signals from dark matter particles hitting crystals of germanium or silicon (though none have been seen yet).
Here’s a paragraph from The Edge of Physics talking about the use of lead in the CDMS experiment:
One of the biggest sources of false signals, or noise, is ambient radioactivity. Even humans are radioactive enough to be a problem. So the germanium detectors had to be shielded inside layers of copper and lead. But lead can contain lead-210, a radioactive isotope with a half-life of twenty-two years (that is, after twenty-two years, half of a given amount of it will have radioactively decayed). So the team had to find old lead, in which most if not all of the lead-210 had decayed already. An Italian colleague mentioned that he had been using lead taken from two-thousand-year-old Roman ships that had sunk off the Italian coast. The CDMS team located a company that was selling lead salvaged from a ship that had sunk off the coast of France in the eighteenth century. Unaware that they were doing anything illegal, the researchers bought the lead. The company, however, got in trouble with French customs for selling archaeological material.