X-ray-induced colouring kinetics of (Ba2+, Eu2+)-doubly doped KCl single crystals: influence of the different aggregation–precipitation states of the doping ions

Abstract

ABSTRACTThe effects of different aggregation–precipitation states of the doping Ba2+-ion on the X-ray-induced generation of F-centres in KCl:Ba2+(675 ppm):Eu2+(27 ppm) single crystals was investigated by optical absorption spectrophotometry. Such states were induced to occur in a controlled manner by subjecting the crystals to suitable thermal treatments. During these treatments, the aggregation–precipitation kinetics of the doping Ba2+-ions was monitored, by optical fluorescence spectrophotometry, utilising the doping Eu2+-ion as an optical probe. As dissolved into the host matrix as forming part of impurity–vacancy (I–V) dipole complexes and small aggregates, divalent barium increases the initial F-colouring rate and F-centre solubility limit by impressive factors of 2 ± 1 and 3 ± 1, respectively, in relation to those of a non-doped crystal. As a constituent of incipient metastable BaCl2-type second-phase precipitates, divalent barium reduces the initial F-colouring rate and F-centre concentration at which the second colouring stage begins by factors of 0.6 ± 0.1 and 0.8 ± 0.1, respectively, in relation to those of the crystal containing barium I–V dipole complexes and small aggregates. As forming part of stable BaCl2 second-phase precipitates and large metastable BaCl2-type second-phase precipitates, divalent barium modifies such a rate and concentration by factors of 1.4 ± 0.1 and 0.7 ± 0.1, respectively, in relation to those of the crystal containing incipient metastable BaCl2-type second-phase precipitates. Also, the doping with divalent barium disrupts the equilibrium between the generation and extinction of F-centres that exists characteristically during the F-curve second colouring stage in a non-doped KCl crystal so that such a rate increases as the aggregation–precipitation phenomena occur within the host matrix. This rate is 0.08 ± (0.08, 0.07), 0.10 ± 0.06 and 0.22 ± (0.05, 0.08) cm–1/h for crystals containing the divalent barium in the form of I–V dipole complexes and small aggregates, incipient precipitates of the metastable BaCl2-type second phase, and large precipitates of both the metastable BaCl2-type second phase and the stable BaCl2 second phase, respectively. The structural mechanisms that are behind the observed effects are discussed.