Vacancy-Production Efficiencies in KBr at Low Temperatures

The growth of the prominent absorption bands (alpha, $F$, $H$, 2300 Å) 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 $\frac{\alpha }{F}$ 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×${10}^{16}$ ($F$ centers)/ ${\mathrm{cm}}^3$. Continued irradiation causes the $\frac{\alpha }{F}$ ratio to drop to about 4:1 at 3×${10}^{17}$ ($F$ centers)/ ${\mathrm{cm}}^3$. This variation in the $\frac{\alpha }{F}$ 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 $\frac{\alpha }{F}$ 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.