A radiation detector, such as for a PET scanner, may include an array of scintillator crystals, with each scintillator crystal having a polished end, a roughened end opposite the polished end, and polished sides extending between the polished end and the roughened end. The detector may also include a specular reflector layer between adjacent polished sides of adjacent ones of the array of scintillator crystals, and a diffusive reflector layer adjacent the roughened ends of the array of scintillator crystals. The detector may further include at least one photodetector adjacent the polished ends of the array of scintillator crystals.