A gamma-ray detector for determining the direction to a source of gamma-rays, the detector comprising: a first scintillation body coupled to a first photodetector and a second scintillation body coupled to a second photodetector , wherein the first scintillation body and the second scintillation body are arranged on a pointing axis of the detector wherein the pointing axis passes through both the first and second scintillation bodies; and a processing circuit arranged to receive output signals associated with the first and second photodetectors for a plurality of different orientations of the pointing axis of the detector relative to a reference direction, and wherein the processing circuit is operable to determining a direction to the source of gamma-rays relative to the reference direction based on the output signals associated with the first and second photodetectors for the plurality of different orientations of the pointing axis of the detector relative to the reference direction, wherein the processing circuit is operable to establish a signal parameter for each orientation of the pointing axis of the detector from the respective output signals associated with the first and second photodetectors for the respective orientations, wherein the signal parameter established for each orientation of the pointing axis of the detector is based on a ratio of a difference in output signals associated with the first and second photodetectors to a sum of output signals associated with the first and second photodetectors, and characterized in that the processing circuit is further configured to determine the direction to the source of gamma-rays relative to the reference direction by comparing the signal parameter for the plurality of different orientations of the pointing axis with a predetermined model of the variation of the signal parameter for different orientations of the pointing axis relative to a source of gamma-rays. a first scintillation body coupled to a first photodetector and a second scintillation body coupled to a second photodetector , wherein the first scintillation body and the second scintillation body are arranged on a pointing axis of the detector wherein the pointing axis passes through both the first and second scintillation bodies; and a processing circuit arranged to receive output signals associated with the first and second photodetectors for a plurality of different orientations of the pointing axis of the detector relative to a reference direction, and wherein the processing circuit is operable to determining a direction to the source of gamma-rays relative to the reference direction based on the output signals associated with the first and second photodetectors for the plurality of different orientations of the pointing axis of the detector relative to the reference direction, wherein the processing circuit is operable to establish a signal parameter for each orientation of the pointing axis of the detector from the respective output signals associated with the first and second photodetectors for the respective orientations, wherein the signal parameter established for each orientation of the pointing axis of the detector is based on a ratio of a difference in output signals associated with the first and second photodetectors to a sum of output signals associated with the first and second photodetectors, and characterized in that the processing circuit is further configured to determine the direction to the source of gamma-rays relative to the reference direction by comparing the signal parameter for the plurality of different orientations of the pointing axis with a predetermined model of the variation of the signal parameter for different orientations of the pointing axis relative to a source of gamma-rays. <EOS>