low-background steel

plural low-background steels

1. countableuncountableSteel with low inherent radiation levels, suitable for instruments and experiments highly sensitive to radiation.
2. countableuncountableSteel created before 1945, and not contaminated by radioisotopes originating from nuclear bomb tests and explosions, and other nuclear technology radioactivity releases.

   The high alloy steel that was recovered from the Tirpitz was valuable owing to the fact that it was one of the few sources of low-background steel—steel produced prior to the detonation of the atomic bombs.

   2018, Daniel Knowles, Tirpitz: The Life and Death of Germany's Last Great Battleship, Fonthill Media

3. countableuncountableUsed other than figuratively or idiomatically: see low-background, steel.