1. A method for creating a look-up table for determining a crystal of interaction in a gamma ray detector, the method comprising: arranging a mask configured to cover a subset of crystals of a plurality of crystals in a scintillation array so as to block exposure of the subset of crystals to gamma rays, the mask being aligned on the scintillation array in a first position to cover a first set of crystals; exposing the scintillation array to gamma rays having a predetermined energy; collecting a first set of data from at least one photosensor positioned to receive light generated by the scintillation array; realigning the mask in a second position on the scintillation array to cover a second subset of crystals of the plurality of crystals, the second subset of crystals being different than the first subset of crystals; repeating the exposing step with the mask aligned on the scintillation array in the second position; collecting a second set of data from the at least one photosensor with the mask aligned on the scintillation array in the second position; creating a first flood histogram from the first set of collected data and creating a second flood histogram from the second set of collected data; superimposing the first flood histogram with the second flood histogram to create a superimposed flood histogram; and generating a look-up table by assigning regions of the superimposed flood histogram to corresponding crystals of the plurality of crystals.