A radiation detector array comprising: a plurality of scintillation crystals, optionally cesium doped lutetium yttrium orthosilicate (LYSO), configured to emit bursts of light in response to radiation; a plurality of photodetectors, optionally silicon photomultipliers (SiPM), each photodetector having a light sensitive region optically coupled with at least one optical coupling element to a light emitting face of at least one scintillation crystal wherein the light emitting faces have a larger area than the light sensitive regions and wherein the at least one optical coupling element includes a plurality of light concentrator elements that optically couple the light emitting faces of the scintillation crystals and the light sensitive regions of the photodetectors; a first reflective layer, optionally one of polytetrafluoroethelene, fluorinated ethylene propylene, and perfluoroalkoxy polymer resin, at least partially enclosing each scintillation crystal, the first reflective layer being configured to diffusely reflect a first portion of the light emitted by the scintillation crystal and incident upon the first reflective layer and to pass a second portion of the emitted light; characterised by: a second reflective layer, optionally a specular reflector on a dielectric support, e.g. aluminum sputtered on Mylar, or a specular reflector made of a polished metal sheet, adjacent the first reflective layer for reflecting light that has passed through the first reflective layer back towards the first reflective layer and the scintillation crystal; wherein the light concentrator elements have indices of refraction greater than indices of refraction of the scintillation crystals and the indices of refraction of the concentrator elements are less than indices of refraction of the light sensitive regions. a plurality of scintillation crystals, optionally cesium doped lutetium yttrium orthosilicate (LYSO), configured to emit bursts of light in response to radiation; a plurality of photodetectors, optionally silicon photomultipliers (SiPM), each photodetector having a light sensitive region optically coupled with at least one optical coupling element to a light emitting face of at least one scintillation crystal wherein the light emitting faces have a larger area than the light sensitive regions and wherein the at least one optical coupling element includes a plurality of light concentrator elements that optically couple the light emitting faces of the scintillation crystals and the light sensitive regions of the photodetectors; a first reflective layer, optionally one of polytetrafluoroethelene, fluorinated ethylene propylene, and perfluoroalkoxy polymer resin, at least partially enclosing each scintillation crystal, the first reflective layer being configured to diffusely reflect a first portion of the light emitted by the scintillation crystal and incident upon the first reflective layer and to pass a second portion of the emitted light; characterised by: a second reflective layer, optionally a specular reflector on a dielectric support, e.g. aluminum sputtered on Mylar, or a specular reflector made of a polished metal sheet, adjacent the first reflective layer for reflecting light that has passed through the first reflective layer back towards the first reflective layer and the scintillation crystal; wherein the light concentrator elements have indices of refraction greater than indices of refraction of the scintillation crystals and the indices of refraction of the concentrator elements are less than indices of refraction of the light sensitive regions. a second reflective layer, optionally a specular reflector on a dielectric support, e.g. aluminum sputtered on Mylar, or a specular reflector made of a polished metal sheet, adjacent the first reflective layer for reflecting light that has passed through the first reflective layer back towards the first reflective layer and the scintillation crystal; wherein the light concentrator elements have indices of refraction greater than indices of refraction of the scintillation crystals and the indices of refraction of the concentrator elements are less than indices of refraction of the light sensitive regions.