Development of NaI survey protocol for induced radioactive materials on clearance

A radiation survey with a survey meter is one of the processes required to declare a component non-radioactive for unrestrictive release. The gamma-rays which can reach the Nal detector were limited by geometry and self-shielding in the material. The region in which gamma-rays are detectable was examined by means of the MCNP code. The detector sensitivity to a Co-60 volume source in iron was obtained as 4378 cpm/(Bq/g) from the detectable region for uniform activity distribution. The minimum detectable activity (MDA) was defined as 4 times of the standard deviation of background count rate. The MDA was found to be lower than 0.1 Bq/g which corresponds to 1/10 of the ANSI-recommended screening level for Co-60. (author)