Iron-Impurity-ControlledF-to-MConversion in X-Irradiated NaF

Comparative studies of x-irradiated NaF samples with (type I) and without (type II) iron impurity have been made near room temperature. The results of optical-absorption studies indicate that (1) during x-irradiation type-II samples color more readily than type-I samples, (2) optical bleaching with $F$ light is more efficient in type-I samples, and (3) the optical bleaching process exhibits very pronounced "aging" effects in type-I samples. These results are accounted for in terms of a proposed iron-impurity-controlled mechanism of optical bleaching. This mechanism is based on the formation of cation-anion vacancy pairs, which are attributed to the excess cation vacancies associated with the iron impurity.