1. G. H. SIGEL Jr, in ?Treatise on Materials Science and Technology?, Vol. 12, Glass 1: Interaction with Electromagnetic Radiation, edited by M. Tomazowa and R. H. Doremus (Academic, New York, 1977) pp. 5?89 and relevant references therein.
2. C. S. SUNANDANA and R. SINGH,Nucl. Phys. Solid State Phys (India) 24C (1981) 85.
3. Idem, Phys. Status Solidi (a) 75 (1983) K91.
4. A. S. MARFUNIN, ?Physics of Minerals and Inorganic Materials? (Springer-Verlag, Berlin, 1979) p. 217.