Low-background steel

Low-background steel, also known as pre-war steel and pre-atomic steel, is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s. Typically sourced from ships (either as part of regular scrapping or shipwrecks) and other steel artifacts of this era, it is often used for modern particle detectors because more modern steel is contaminated with traces of nuclear fallout.

Steel
Since the end of atmospheric nuclear testing, background radiation has decreased to very near natural levels, making special low-background steel no longer necessary for most radiation-sensitive uses, as brand-new steel now has a low enough radioactive signature that it can generally be used. Some demand remains for the most radiation-sensitive uses, such as Geiger counters and sensing equipment aboard spacecraft. For the most demanding items even low-background steel can be too radioactive and other materials like high-purity copper may be used.

In cases where World War II–era shipwrecks in and near the relatively shallow Java Sea and western South China Sea have been illegally scavenged, it has been suggested that the target is low-background steel. Andrew Brockman, a maritime crime researcher and archaeologist, argues that it is more likely to be conventional salvage.

Radionuclide contamination
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 in welding rods.

World anthropogenic background radiation levels peaked at 0.11 mSv/yr above natural levels in 1963, the year that the Partial Nuclear Test Ban Treaty was enacted. Since then, by about 2008, anthropogenic background radiation has decreased to 0.005 mSv/yr above natural levels.