The principal purposes of this study were to observe the effect of an x irradiation of extended duration on the production of color centers in KCl and KBr at low temperatures and to examine the implications with regard to possible color-center models. Growth curves were determined at 5°K for the $F$, $K$, $M$, and $H$ bands in KCl and KBr and for a new band at 245 mμ in KCl and at 278 mμ in KBr, called the $H^{\prime }$ band. Growth curves were determined at 80°K for the $F$, $K$, and $M$ bands in KCl and KBr. The growth of the $V_4$ band in KBr and of a band having its absorption maximum at 240 mμ in KCl, designated as the $V\left(240\right)\mathrm{}$ band, was also followed at 80°K. The ratio of the absorptions at the peak of the $F$ and $K$ bands was found to remain constant indicating that both of these bands arise from transitions in the same center. The concentration of $M$ centers was found to depend quadratically on the $F$-center concentration. This result gives conclusive support to the model proposed by van Doorn and Haven that the $M$ center consists of a pair of associated $F$ centers. The $F-\mathrm{t}\mathrm{o}-H$ ratio was observed to be constant for short x-ray exposures, supporting the viewpoint that the two centers are produced as complementary defects; however, for prolonged exposures, this ratio is no longer constant. The number of $F$ centers is in constant ratio to the combined concentrations of $H$ and $H^{\prime }$ centers provided that the oscillator strength of the $H^{\prime }$ band is suitably chosen. The linear dependence of the generation of both $V_4$ centers in KBr and $V\left(240\right)\mathrm{}$ centers in KCl on $F$-center concentration suggests that the $V_4$ and $V\left(240\right)\mathrm{}$ centers, rather than $V_1$ centers, are fundamentally related to the mechanism of formation of $F$ centers near liquid-nitrogen temperature for a prolonged irradiation.

• Received 5 November 1964

DOI:https://doi.org/10.1103/PhysRev.138.A893