5. Conclusions
Two main emission centers (I and II types) are identified in NaI:Eu. The structure and behavior of thefirst type of centers (center I) are similar to the usual (Eu2þ vc ) centers observed earlier for some other alkali halides. The luminescence efciency and spec- troscopic characteristics of the second type of centers (center II) are quite different. These centers II are responsible for the efficientscintillation in NaI:Eu. A signifibe suggested. These contaminations are always present in hygro- scopic NaI. The proposed composition of the center II is the sim- plest, considering the various effects presented in this paper. Unlike the aggregation of activator centers, the model of oxygen containing radicals is in agreement with the observed increase of the activator emission yield with the activator content (see Fig. 4). It also explains the appearance of E and F TSL peaks and increase ofTSL intensity for highly doped NaI:Eu crystals (Fig. 5). Suggested model allows to take into account the key role of hygroscopicity in spectroscopic properties of emission centers. It may therefore be applicable also to other related hygroscopic crystalline systems.
Acknowledgments
The authors are grateful to V. Vistovskyy, S. Tkachenko, A. Mitichkin, D. Sofronov and E. Galenin for experimental help.
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