A system and method for determining a probability of the location of an illicit radiation source within an environment based on directional detectors. An embodiment includes a plurality of directional radiation detectors distributed about the environment and integrated with a processing unit adapted to determine the probability of the source location based on the radiation count data received from the plurality of detectors. The processing unit is further adapted to output information indicative of the location of the radiation source within the environment.
 The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and the UChicago Argonne, LLC, representing Argonne National Laboratory.