Low-background steel
Low-background steel is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s. With the Trinity test and the nuclear bombings of Hiroshima and Nagasaki in 1945, and then subsequent nuclear weapons testing during the early years of the Cold War, background radiation levels increased across the world. Modern steel is contaminated with radionuclides because its production uses atmospheric air. Low-background steel is so-called because it does not suffer from such nuclear contamination. This steel is used in devices that require the highest sensitivity for detecting radionuclides . One source of low-background steel is ships constructed before the Trinity test, most famously the scuttled German World War I warships in Scapa Flow. Old freight cars are another source. Since the cessation 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 applications, as brand-new steel now has a low enough radioactive signature that it can generally be used in such applications. 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. 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, anthropogenic background radiation has decreased to 0.005 mSv/yr  above natural levels. This is low enough that, in 2010, "Straight Dope" said that "reduced radioactive dust plus sophisticated instrumentation that corrects for background radiation means new steel can now be used in most cases". There have been incidents of radioactive cobalt-60 contamination as it has been recycled through the scrap metal supply chain. This is due to recycling sealed radioactive sources such as radiotherapy machines, not from the continued recycling of steel. Applications. Devices that require low-background steel include: As these devices detect radiation emitted from radioactive materials, they require an extremely low radiation environment for optimal sensitivity . Low-background counting chambers are made from low-background steel with extremely heavy radiation shielding. They are used to detect the most minute nuclear emissions.