Vacancy-Production Efficiencies in KBr at Low Temperatures

The growth of the prominent absorption bands (alpha, F, H, 2300 {}) produced by low-energy x-ray and 2.0-MeV electron irradiation has been studied in KBr at temperatures between 5 and 9^∘{}K. The growth curves are not, in general, linear when plotted as a function of absorbed dose. The total vacancy production (F plus alpha centers) is a smooth but nonlinear function of absorbed dose. The αF ratio is not constant as the irradiation progresses. The ratio is initially 4:1 and reaches a maximum 9:1 at an F-center concentration of 5×{}10¹⁶ (F centers)/ cm³. Continued irradiation causes the αF ratio to drop to about 4:1 at 3×{}10¹⁷ (F centers)/ cm³. This variation in the αF ratio is the same for all types or "qualities" of radiation when plotted as a function of the F-center concentration. The intensity dependence of vacancy production was investigated by varying the beam current by a factor of 120 for the 2.0-MeV electron irradiations. More energy is required to produce a vacancy at higher intensities, but the αF ratio is unaffected. The variation of total vacancy production efficiency in KBr with radiation "quality" reported here parallels the F-center efficiencies reported earlier. One is led to the same conclusion reached previously, i.e., the low-temperature coloration process is dominated by mechanisms of the sort proposed by Klick and by Williams which involve the single ionization of two adjacent halide ions.