Anion-vacancy processes occurring after room-temperature F-light bleaching or irradiation of NaCl

The thermally activated processes occurring in the dark after room-temperature F-light bleaching of x- or gamma-irradiated NaCl have been investigated. It has been shown that, near room temperature, the concentration of free-anion vacancies decays exponentially with time. The kinetics of the process has been studied by putting the crystal in the dark for a variable time after bleaching and then subjecting it to a short illumination or irradiation pulse which detects the disappearance of the vacancies. It has been observed that the decay in vacancy concentration is closely associated with a decrease in the F'-band intensity and with increases in the F- and M-band intensities. The experimental information has led to the conclusion that vacancies become trapped at F' centers according to a first-order reaction, giving rise to F and M centers through the mechanisms F'(+) anion vacancy - 2F, and F' + anion vacancy --> M, respectively. The activation energy for the trapping process has been found to be E=0.48 +/- 0.06 eV. These phenomena also occur after an x or gamma irradiation, and are shown to provide a coherent picture of the previously reported anomalous coloring behavior appearing at the start of a new irradiation period.