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 measuremtens 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.