Structure Sensitivity ofF-Center Generation by X Rays at Low Temperatures

Recent internal friction and elastic modulus experiments suggest that $F$ centers are formed at dislocations in the alkali halides during x irradiation even at liquid helium temperature, but that enhanced colorability is suppressed at low temperatures by the absence of diffusion of the generated centers. In the present experiments $F$-center growth curves were obtained for both asreceived and plastically deformed (up to 15%) NaCl and KCl crystals at temperatures ranging from 10° to 325°K. From these data it is found that both the rate of capture of electrons by negative-ion vacancies and the final saturation concentration of $F$ centers achieved are strongly temperature sensitive and that there exists an optimum temperature at which a given color center concentration is attained with a minimum amount of irradiation. The colorability of lightly deformed crystals is found to be enhanced only at temperatures where diffusion of $F$ centers appears to be possible; otherwise a rapid saturation of the coloration is observed. However, the initial $F$-center growth rate in a heavily deformed crystal, relative to an undeformed one, becomes larger as the temperature is decreased, and can be interpreted in terms of the presence of a larger number of $F$ centers formed near dislocations. With further x irradiation, this additional contribution saturates.