noun
radioactive indicator
Technical term used in chemistry, biology, or medicine for a radioactive substance used to trace or measure processes.
放射性指示薬を用いて、化学反応の経路を追跡した。
We traced the pathway of the chemical reaction using a radioactive indicator.
Compound of 放射性 (radioactivity) + 指示薬 (indicator). The exact derivation is straightforward from the components.