UV induced afterglow of KCl : Eu, KBr : Eu and NaCl : Eu at low temperature

We propose a model for the production of defects by non-ionising radiation in impurity-doped alkali halides based on the formation of impurity-trapped excitons by excitation of the impurity, instead of ionisation of the impur ity as has been proposed before. The dissociation of the trapped excitons i nto F-z and H centres in our model is similar to the case of the formation of F and H centres induced by ionising radiation in pure alkali halides. To gather information about the mechanism of defect creation by non-ionisin g radiation, we investigate the afterglow (AG) emission induced by UV light at low temperature (20 K) in three different alkali halides KCl, KBr and N aCl doped with Eu impurity. We conclude that the AG emission can be split i nto two components; one of them has a fast decay and the other one remains for several minutes, decaying according to t(-1/2). This second component h as a close relation with the thermoluminescence, emission. We show that the decay of the second component can be explained within the framework of our model if we assume that the H centres migrate along lines. We suggest that these lines are dislocation lines. (C) 2001 Elsevier Science Ltd. All righ ts reserved.