A photon detector, comprising: a sensor array of optical sensors disposed in a plane, each optical sensor configured to sense luminescence; four substantially identical scintillation crystal bars, each of the four scintillator crystal bars being a rectangular prism with four side surfaces (62, 64, 72, 74) and first and second end surfaces, each scintillation bar has two side surfaces which each face a side surface of another scintillation bar, and each scintillation crystal bar being configured to generate a light scintillation in response to interacting with a received gamma photon; a first layer disposed in a first plane disposed between and adjacent facing side surfaces of the four substantially identical scintillation crystal bars with a light sharing portion adjacent the first end surface and a reflective portion adjacent the second end surface; a second layer disposed in a second plane orthogonal to the first plane and disposed between and adjacent facing side surfaces of the four substantially identical scintillation crystal bars with a light sharing portion adjacent the second end surface and a reflective portion adjacent the first end surface; and the photon detector being characterized by further comprising a signal processing unit connected to the sensor array configured to estimate a depth of interaction estimate of one of the four substantially identical scintillation crystal bars (26, 60) from a detected event based on a ratio of sensed luminescence of the two of the four substantially identical scintillation crystal bars diagonal to each other (26, 68, 70) and facing the one of four substantially identical scintillation crystal bars (26, 60). a sensor array of optical sensors disposed in a plane, each optical sensor configured to sense luminescence; four substantially identical scintillation crystal bars, each of the four scintillator crystal bars being a rectangular prism with four side surfaces (62, 64, 72, 74) and first and second end surfaces, each scintillation bar has two side surfaces which each face a side surface of another scintillation bar, and each scintillation crystal bar being configured to generate a light scintillation in response to interacting with a received gamma photon; a first layer disposed in a first plane disposed between and adjacent facing side surfaces of the four substantially identical scintillation crystal bars with a light sharing portion adjacent the first end surface and a reflective portion adjacent the second end surface; a second layer disposed in a second plane orthogonal to the first plane and disposed between and adjacent facing side surfaces of the four substantially identical scintillation crystal bars with a light sharing portion adjacent the second end surface and a reflective portion adjacent the first end surface; and the photon detector being characterized by further comprising a signal processing unit connected to the sensor array configured to estimate a depth of interaction estimate of one of the four substantially identical scintillation crystal bars (26, 60) from a detected event based on a ratio of sensed luminescence of the two of the four substantially identical scintillation crystal bars diagonal to each other (26, 68, 70) and facing the one of four substantially identical scintillation crystal bars (26, 60).