Mixtures of 30% Bi 2 O 3 and 70% GeO 2 (in molar percents) were prepared followed by the solid state reaction at 700 o C for 30 min. or 24 h using an Al 2 O 3 crucible. After melting at 1050 o C for 10 min. the melt was poured into a preheated graphite plate and then naturally cooled to room temperature (RT). X-ray diffraction measurements (XRD) on the resulting BGO glasses have been performed, and optical absorption spectra recorded at room temperature on the samples obtained using various preparation conditions.

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After direct electrolytical colouring or electron irradiation followed by illumination in the F centres absorption band of the NaCl:(Ag + +Ca 2+ ) crystal samples, a new centre has been observed denoted by us with Z(Ag -). The 300nm absorption band of this centre is close to the well known Ag a -absorption band from 275nm. The photoluminescence band of the Z(Ag -) centre is observed at 600 nm compared to the 440nm photoluminescence band of Ag a -. The photoluminescence band position depends on the nature of the divalent cation impurity peaking at 560 nm for Sr 2+ codoped samples. The obtained results prove that the Z(Ag -) centre is an Ag -ion perturbed by the divalent cation.

Radiation induced colour centres in CeF3 crystals have been studied by spectroscopic techniques using eight crystals produced with various degrees of purity by different manufacturers in slightly different growing conditions. The crystals were irradiated at liquid nitrogen temperature with electrons of 7 MeV at doses of lo4 to 3. lo6 rad. Absorption spectra ranging between 250 and 750 nm recorded at irradiation temperature are quite different from one producer to another, regarding both the values of absorption coefficient (17 cm-' -5 cm-') and the number and form of the absorption bands.Step-by-step annealing up to room temperature produces new absorption bands. In the range of 200-240 K, three well resolved bands appear as in the case of crystals irradiated at room temperature.

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