Various embodiments provide a non-collimated 3D localization technique to detect an unknown radioactive source in a medium material using a plurality of detectors. 3D position information (x, y, z), and strength of the unknown radioactive source(s) can be determined by a comparison or a data fit between the measured detector responses with a mapping of detector responses performed with known calibration radioactive source(s). The non-collimated 3D localization technique can be used to extract lateral and depth position of contaminations in soil, concrete, or metal, to aid in monitoring and localizing radiation for nonproliferation and prevent smuggling of nuclear materials, and/or to detect and localize radioactive source(s) in medical or non-medical purposes.