Abstract: A position sensitive radiation spectrometer for identifying, quantifying, and locating a radioactive source in a target. The position sensitive radiation spectrometer includes an array of gamma-ray detectors arranged along one or more dimensions of a container to be assayed. A known source placed at various calibration positions is used to establish counting efficiencies at various energy levels. From the counting efficiency and the actual count rate, the activity level of an unknown source relative to the calibration positions is calculated. The relative activity levels correlate to the position of unknown source and provide useful information about the actual distribution of radioactive material.