Studies were made of the effect of a ${\mathrm{Cl}}^{-}$ impurity on the formation of vacancies in KBr by x irradiation at low temperature and on the thermal annealing of the $\alpha$ band produced by x rays. It was found that both the annealing stage of the $\alpha$ band at 21°K and the sharp decrease of the formation efficiency of vacancies with temperature around 21°K are suppressed by the addition of the ${\mathrm{Cl}}^{-}$ impurity. The production of an "interstitial band" (5.36 eV) in pure and doped KBr was also studied. It is suggested that in doped KBr the ${\mathrm{Br}}^{-}$ interstitials are replaced by more stable ${\mathrm{Cl}}^{-}$ interstitials and that this replacement accounts for the higher formation efficiency above 21°K. The distance of the ejection of a ${\mathrm{Br}}^{-}$ interstitial by x ray is estimated to be about 10 atomic spacings.

• Received 10 December 1964

DOI:https://doi.org/10.1103/PhysRev.138.A1766