INSTIOUMUNTATION A gamma-ray photon is detectable only by means of the enersy it loses in an interaction or interactions with matter. processes: Gamma-ray photons lose energy by three main photoelectric effect, Compton effect, and positron-negatron pair production. The cross-section for each effect is dependent on the energy of the gamma-ray photon and the atomic number of the detecting material. The enersy given up by the gamma-ray photon under-~ going any of these processes ultimately manifests itself in the molecular excitation of the detectinzs material. Many organic crystels(such as anthracene, stilbene, anda diphenylbutadiene Jand many inorsanic crystals(such as thallium-activated sodium iodide, indium- or thallium- activated lithiumiodide, and potassium todide) relieve this excitation in the form of useful scintillations of light whose predominant color denends on the crystal. The intensity of the light flashes produced in these materials is dependent (in some cases, linearly) on the energy of the incident photon absorbed). (provided it is totally These light vulses are detected by a photo- multiniier tube which converts them into electrical pulses. 3