Influence of radiation intensity and lead concentration in the room-temperature coloring of KBr

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Abstract
The first stage of F-center coloring in quenched samples of lead-doped KBr has been investigated at room temperature as a function of impurity concentration and radiation intensity using optical absorption, photoluminescence, and ionic-thermocurrent techniques. For the range of lead concentration employed in this work (7 up to 250 ppm), the coloring increases monotonically with concentration when the samples are subjected to a radiation intensity of 10 R/min. Moreover, the amount of first-stage coloration was found to be proportional to the square root of impurity concentration, in agreement with the theoretical model recently proposed to account for F-center production in alkali halides doped with divalent cation impurities that change their valence state by irradiation. A quite different result was found to occur when the samples were irradiated at a higher radiation intensity (50 R/min). In this case, the coloring increases with lead concentration only at the very beginning of irradiation. As the irradiation time increases, however, the coloring curve for a heavily doped crystal crosses that for a slightly doped crystal in such a way that more coloring is produced during stage II of the F-center growth curve for the lower concentration. Moreover, as lead concentration increases, the coloring saturates more efficiently the higher the concentration. It was ascertained that the exposure of the crystals to the highest radiation intensity employed in this work also induces lead precipitation in the irradiated specimens as revealed by analyses of the emission and excitation spectra. This effect is produced by the ionizing radiation and not by the room-temperature annealing of the samples during the time in which the irradiations were performed. The influence of impurity precipitation occurring during stage I of the F-center growth curve on the kinetics of color-center formation is analyzed. It is proposed that the phenomenon of impurity precipitation induced by irradiation may be responsible for the experimental observation that more coloring is sometimes produced in slightly doped samples than in heavily doped samples.