INFLUENCE OF THE STATE OF AGGREGATION OF DIVALENT CATION IMPURITIES ON THE F-COLORING KINETICS OF NaCl

Abstract

According to our present views on the mechanisms for irradiation coloring, the first stage of the room-temperature F-growth curve is associated to the capture of halogen interstitials by impurity centers, essentially vacancies and impurity-vacancy dipoles. In this work, a variable concentration of halide vacancies and dipoles has been obtained by irradiating samples at different times after a severe quenching. The influence of these concentrations on the kinetics of coloring has been thoroughly studied for NaCl : Ca++ (100 pprn), NaCl : Mn++ (90 ppm) and NaCl : Pb++ (80-600 ppm). The following techniques have been used to characterize the state of aggregation and charge of the impurity : EPR, UV absorption, photo and thermoluminescence and ionic conductivity. The most important results are : 1) During the first stage of dipole aggregation where the dipole concentration decreases as much as 90 %, the F-coloring efficiency for the three systems is essentially constant. 2) On the contrary, other techniques reveal a number of processes which very strongly depend on the state of aggregation of the impurity, e. g. a) In NaCl : Ca++, a well defined UV band at 200 mµ appears, whose intensity is proportional to the concentration of dipoles at the start of the irradiation. b) In NaCl : Mn++, the thermoluminescence and EPR spectra indicate that the rate of the valence changing reactions Mn++ → Mn+ → Mn0 is strongly dependent on the state of aggregation of the impurity. The same behaviour is observed in NaCl : Pb++, with regard to the Pb++ → Pb+ process. The implications of these data on the primary and secondary processes induced by irradiation are discussed.