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The spectra of the luminescence from unactivated Nal and thallium activated NaI have been measured in the temperature range -190Â°C. to +20Â°C. using alpha particles, gamma rays, and ultraviolet light as sources of excitation. The characteristic emission from NaI ("pure") is in a band centered at approximately 300 mÂ¿. The intensity is weak at room temperature but increases by a factor of ten in cooling the crystal to -190Â°C. By comparison, the characteristic emission from the commonly used NaI(10-3Tl) is in a band centered at approximately 420 mÂ¿ and changes in intensity by a factor of less than two over the above temperature range. The intensities are approximately equal at -190Â°C. Another crystal, NaI(10-6Tl), was weakly luminescent at room temperature, but emitted both bands strongly at the lowest temperature, with a total intensity approximately equal to the intensity of NaI(10-3Tl). Differences in the spectra which result from the different sources of excitation will be discussed. The results, when combined with those of other investigators, have suggested a model for the scintillation phenomenon in Nal which is at least internally consistent. The model in turn leads to a plausible explanation of the high relative luminescent efficiency of NaI(Tl) and unactivated NaI as compared with some others of the alkali halide family. The characteristic decay time of the scintillation light from unactivated NaI was found to vary between 10-8 seconds at 20Â°C. and 3 Ã 10-8 seconds at -190Â°C. The pulse height divided by particle energy at -190Â°C.