DECAY TIME MEASUREMENTS ON PURE CSI SCINTILLATORS PREPARED BY DIFFERENT METHODS

The discovery of the fast decay time of pure CsI and the various results of the measured samples led us to investigate the decay time of CsI crystals prepared by different methods. Carefully grown or prepared pure CsI behaves as fast (ns range) scintillators with a strongly or totally suppressed slow (mu-s range) decay component. The presence or the absence of the slow component is related to the preparation method, and to the remaining built-in contamination of the samples. Pure CsI crystals prepared by the zone melting technique show a fast luminescence at 310 nm with an intensity of 5% relative to NaI(Tl) for electrons. The slow luminescence is strongly suppressed, practically absent. This fast luminescence consists of two components: an ultrafast component and a fast one with decay times of 1.8 ns and 12.9 ns respectively; the intensity ratio of the two components is 0.32 for gamma radiation. It means that the relative intensity of the ultrafast component is 1.2%, that of the fast one is 3.8% relative to NaI(Tl) for electrons. Pure CsI prepared by the cold press technique has an ultrafast component of 1.0 ns and a fast one of 9.6 ns with an intensity ratio of 0.65 for electrons; the slow component is totally absent. The relative light output of this luminescence is 4%; i.e., the relative intensity of the ultrafast component is 1.6%, that of the fast one is 2.4% relative to NaI(Tl) for electrons. The relative intensity of the ultrafast component of BaF2 (220 nm) is 4%, that of the slow (310 nm) one is 16% relative to NaI(Tl) for electrons. Pure CsI crystals prepared by the conventional growing method have a slow luminescence (decay time: 3-mu-s) with a relative intensity of 26% to NaI(Tl) for electrons. They have a fast luminescence of 8% relative to CsI(Tl) for alphas if the surface was polished by ethanol, but a strong slow luminescence of 58% relative to the CsI(Tl) if it was polished by water or ethylene glycol.