A large-area directional radiation detection system may include a large number of slab-shaped detectors stacked side-by-side comprising alternate long and short detectors, where the long detectors are longitudinally longer than the short detectors. The long detectors may collimate or restrict the lateral field of view of the short detectors, so that a particular short detector that is aligned with the source has an unobstructed view of the source. By comparing detection distributions in the long and short detectors, a processor can determine the angular position and distance of a source. The high detection efficiency and large solid angle of the detector array may enable rapid detection of even well-shielded threat sources at substantial distances, while simultaneously determining the positions of any sources detected.