A device in which a radiation sensitive crystal is placed in a plastic housing and worn on the body when entering areas with radiation, usually around nuclear reactors.
The gamma rays emitted by the radiation source energize an electron in the crystal lattice. To be read, the crystal is placed in a machine which runs a charge through the crystal and measures the amound of light given off when the electrons emit the energy gained from the gamma rays and return to their normal orbital. Very accurate.