There is another kind of wreck used for extreme low background physics experiments. These experiments (put deep underground) need even lower background than me

There is another kind of wreck used for extreme low background physics experiments. These experiments (put deep underground) need even lower background than medical equipment. They use lead for shielding, but any modern lead is contaminated with Pb-210, an isotope with a half life of 22 years. This contamination comes from the radioactive decay of uranium in the lead ore. These experiments have come to use lead that's been underwater, in wrecks, for centuries, so almost all the Pb-210 has decayed away.