F- andM-Band Absorption in Heavily Gamma-Irradiated KCl

Measurements of $F$- and $M$-band absorption at room temperature as a function of gamma irradiation are used to study the relation between $F$ centers and $M$ centers in a number of KCl crystals from a variety of sources. Although the $F$-center coloring curves exhibit a variety of forms, presumably due to different trace impurities and imperfections in the crystals, the $M$-center density seems in all cases to be nearly proportional to the square of the $F$-center concentration. The ratio of the $M$-center concentration to the square of the $F$-center concentration appears to be related to trace impurity content. Zone refining decreases the relative $M$-center introduction, as does deliberate doping with calcium; whereas, plastic deformation has no effect on the ratio. Bleaching increases the $M$-center concentration at the expense of $F$ centers; however, short re-irradiation with gamma rays causes the ratio of $M$-center concentration to the square of $F$-center concentration to return to its prebleaching value. The results are thought to indicate that a radiation equilibrium exists between $F$ and $M$ centers which is frozen in when the crystals are removed from the source.