radiation from a subject which is received by a scintillation crystal interacts with the scintillation crystal to produce a burst of scintillation light photons . the depth distribution at which the interaction occurs varies in accordance with the energy of the received radiation . low - energy radiation tends to interact at a shallow depth relative to a front face of the scintillation crystal and radiation with a high energy tends to interact relatively deeply into the crystal . a moment processor processes electronic information from photomultiplier tubes which view the scintillation crystal to generate zero - th moment or energy information , first moment or coordinate information and second moment or depth information . the event information is filtered in accordance with depth , e . g ., sorted into acceptable / unacceptable information , information corresponding to each of two or more energies of radiation , and the like .