A gamma ray detector module that includes at least one crystal element arranged in a plane, a plurality of light sensors arranged to cover the at least one crystal element and to receive light emitted from the at least one crystal element, and a light guide arranged between the at least one crystal element and the light sensors, the light guide being optically connected to the at least one crystal element. Further, the light guide includes a narrow portion that positions at least one light sensor of the plurality of light sensors closer to the at least one crystal element than other light sensors of the plurality of light sensors. In addition, the light guide may include an angled recessed portion that positions another light sensor at an oblique tilt angle with respect to the plane of the at least one crystal element.