Radiation Equilibrium ofFandMCenters in KCl

A study of $F$- and $M$-center coloration by electron-irradiating single crystals of KCl is made as a function of radiation intensity, electron energy, and crystal preparation. The quadratic relation between $M$-center and $F$-center concentration in any particular sample, previously observed for both x-ray and gamma irradiation, is verified for electrons if a constant current is used throughout the irradiation. However, the ratio of $M$-center concentration to the square of the $F$-center concentration is dependent upon electron current and sample preparation. Moreover, if a specimen is irradiated at one radiation intensity for a long period and then the intensity is changed for a short time, the relative $M$- and $F$-center concentrations are characteristic of the last irradiation intensity. It is concluded from this result that a dynamic equilibrium exists between $F$ centers and $M$ centers during irradiation. Interpretation of the results is proposed in terms of four simultaneous processes producing and destroying $F$ centers and $M$ centers, respectively. The processes for destruction of $F$ centers and creation of $M$ centers are thought to involve a mobile imperfection created by the irradiation. Means by which small concentrations of impurities can affect some of these processes to change the $F-M$ equilibrium and $F$-center coloring curves are discussed.