Additional evidence regarding the role of imperfections in the gamma-ray coloring and optical bleaching of the alkali halides has been obtained by comparing crystals with and without $U$ centers. Parameters varied were gamma-ray exposure rate, crystal temperature, hydride concentration, $F$-band illumination intensity, and thermal history of the crystal. The coloring efficiency increased with temperature, trap depth and vacancy or $U$-center concentration. Under $F$-band illumination, both pure and $U$-centered crystals exhibited an initial period of rapid bleaching, followed by a long slow-bleaching period. Interrupting the bleaching in $U$-centered crystals by storage in the dark restored the higher bleaching rate. The slow bleaching was temperature-dependent, increased slightly with $F$-band illumination intensity, and was essentially independent of the $U$-center concentration. The results support the hypothesis that coloring and bleaching rates are primarily controlled by the availability of suitable holes and vacancies. Crystals containing the hydride color more efficiently than pure crystals because all constituents of the $F$ center are present in the parent $U$ center; whereas pure crystals bleach faster because of the accessibility of the holes.

• Received 7 July 1954

DOI:https://doi.org/10.1103/PhysRev.97.277