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 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 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 and facing the one of four substantially identical scintillation crystal bars . 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 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 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 and facing the one of four substantially identical scintillation crystal bars . <EOS>