radiation indicator
nuclear detector
nuclear radiation detector

You see, my wristwatch strap happened to be a radiation indicator.'

Among his patents were those for a radiation indicator, electron emitters, an air-particle meter and a radiation dosimeter.

The vaporized plastic and metal that had been the ship had cooled and scattered into invisibility, and only the deafening rattle of static on the pod frequency was left-that and the radiation indicator, which was rapidly slipping back to normal.