γ-ray induced coloration of KCl:Eu2+ crystals

The formation of color centres at RT in γ-irradiated KCl crystals doped with Eu2+ ions in a wide concentration range is studied by optical absorption, thermoluminescence, and the ITC method. In contrast to the ordinary F-band behaviour a decrease of the V2-type band with increasing dopant concentration is observed. The thermoluminescence spectra reveal the presence of two glow peaks positioned at 363 and 453 K independently of the europium concentration. On the basis of the ITC measurements it is found that the number of F-centres formed does not correlate with the number of the europium dipoles destroyed by irradiation for all samples studied. Moreover, optical and thermal bleaching of color centres F- and V2-type is not accompanied by the recovery of europium dipoles. The results obtained are consistent with the previous model ascribing the decay of europium dipoles to a radiation-induced aggregation process. It is proposed that the disappearance of the V2-like band with dopant concentration is caused by the strong interaction of the europium clusters formed during irradiation with the halogen interstitials, drastically lowering the pertinent oscillator strength. (author)