The absorption coefficient (α) was obtained for different γ-doses. The analysis of the data revealed the existence of optical allowed direct transition from Egd= 5.82 to 6.03 eV for an increase of the radiation doses from 1 M rad to 10 M rad. An absorption band appears at ∼310 nm., shifted towards the short wavelength as the doses increase. This band could be related to electrons or holes trapped at lattice defects and associated with a growth of colour centres.