Low-background steel

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.

World anthropogenic background radiation, caused by atmospheric nuclear testing, peaked at a level 0.11 mSv/yr (4%) above the natural 2.40 mSv/yr. It began to fall in 1963, when the Partial Nuclear Test Ban Treaty was enacted, and by 2008 it had decreased to only 0.005 mSv/yr above natural levels. This has made special low-background steel no longer necessary for most radiation-sensitive uses, as new steel now has a low enough radioactive signature. Some demand remains for the most radiation-sensitive uses, such as Geiger counters and sensing equipment aboard spacecraft. For the most sensitive equipment, even low-background steel can be too radioactive, and other materials like high-purity copper may be used.

At the end of World War I, the German Navy scuttled around 50 ships in Scapa Flow instead of turning them over to the British Royal Navy. These ships have historically been one source of low-background steel. 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. However, some–including Andrew Brockman, a maritime crime researcher and archaeologist–argue that conventional salvage is more likely.

Other low background materials

Some specialized particle detectors, including CUORE, XENON100, and Canfranc Underground Laboratory, have used low-background lead for shielding. This is sourced from ancient Roman lead, some of which is sourced from ancient Roman shipwrecks. Ancient lead has lower radiation levels than freshly refined lead because lead ore is naturally radioactive, and after processing, it still contains lead-210, a radioactive isotope of lead which has a half-life of 22 years. Ancient lead has had enough time for the lead-210 to decay to the point where it is "totally absent", so the lead can be used in highly sensitive equipment.

Cultural impact

In 2025, technology experts such as Rajiv Pant and John Graham-Cumming began to use low-background steel as a metaphor to describe the practice of archiving web content that existed before 2022 in an effort to preserve information that was produced by humans without "contamination" from large language models such as ChatGPT.

References

References

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