Low-background steel

Low-background steel, also known as pre-war steel, pre-atomic steel, or pre-nuclear steel, is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s. Typically obtained from scrapped ships, salvaged shipwrecks, and other steel artifacts of this era, it is often used as a shielding material for particle detectors and whole-body counting equipment because more modern steel is contaminated with traces of nuclear fallout. Other low-background materials, such as ancient lead, are sometimes used for especially sensitive equipment.

History and rationale

From 1856 until the mid 20th century, steel was produced in the Bessemer process, where air was forced into Bessemer converters converting the pig iron into steel. By the mid-20th century, many steelworks had switched to the BOS process, which uses pure oxygen instead of air. However, as both processes use atmospheric gas, they are susceptible to contamination from airborne particulates. Present-day air carries radionuclides, such as cobalt-60, which are deposited into the steel, giving it a weak radioactive signature. Another source of radioactive contamination was the coating of steel cauldrons in cobalt-60 in order to monitor wear. Steel that would otherwise be expected to be low-background can itself be contaminated due to thorium contained in welding rods, such as those used in gas tungsten arc welding.