Fundamental Studies of Scintillation Phenomena in NaI

Work on pure NaI reported at the Fifth Scintillation Counter Symposium (1956) has been extended using crystals of higher purity at temperatures down to 4°K. Decay of the light pulse from unactivated NaI is not a simple exponential but exhibits a rising component superimposed on an exponential decay. In other measurements, the absolute conversion efficiency (emitted useful light divided by exciting particle energy) was found to be as high as 25 per cent for unactivated NaI at liquid nitrogen temperature (compared with 13 per cent for NaI(T1) at room temperature. Theoretical and practical implications of the results are discussed.