An ionizing radiation detector employs optical fibers as the medium for sensing ionizing radiation emitted by a radioactive source. Light in the infrared region is pumped continuously through an optical fiber located in an area or region where the unintentional discharge of ionizing radiation may be expected, so that such emission is detected the moment it occurs. The source of optical light emits a constant output within a specific wavelength band which changes only when irradiation of the fibers by ionizing radiation causes their internal color centers to change. The output of the fibers is optically coupled to a photomultiplier via a light pipe. A single light source, detector, and associated electronics complete the system. A hand-held unit unique for remote sensing may house these components. Due to safety conditions, these components are located at a point remote from the position liable to become under the influence of the radiation exposure field. Annealing of that portion of the optical fiber influenced by irradiation field restores the exposed portion of the optical fiber to substantially its previous level of sensitivity thereby reversibly establishing the fiber optic for any subsequent exposure. In response to particle bombardment from a cobalt-60 source, the detector reacts to the energy remaining after passage of the beam through a shielding enclosure, thereby giving a reliable indication of the shielding effectiveness of the enclosure.