A portable nuclear material detector generally includes a scintillating fiber radiation sensor, a light detector, a conditioning circuit, a frequency shift keying (FSK) circuit, a fast Fourier transform (FFT) circuit, an electronic controller, an amplitude spectral addition circuit, and an output device. A high voltage direct current (HVDC) source is provided to excite the light detector, while a separate power supply may be provided to power the remaining components. Portability is facilitated by locating the components of the detector within a handheld-sized housing. When bombarded by gamma particles, the radiation sensor emits light, which is detected by the light detector and converted into electrical signals. These electrical signals are then conditioned and converted to spectral lines. The frequency of a give spectral line is associated with a particular radioactive isotope, while the cumulative amplitude of all spectral lines having a common frequency is indicative of the strength and location of the isotope. All or part of this information (identity, strength, direction, and distance) may be provided on the output device.