F-Center Production Efficiencies at Liquid-Helium Temperature

Abstract

The efficiencies for $F$-center production ($\frac{\mathrm{eV}}{F}$ center) have been measured at liquid-helium temperature for KBr, KCl, LiF, and NaCl irradiated with 2.0-MeV electrons from a Van de Graaff generator and with x rays from a tungsten-target beryllium-window x-ray tube. Special dosimetric techniques were developed to measure the energy absorbed by the samples. The $F$-center production efficiencies for easily colored alkali halides are typically of the order of 5000 $\frac{\mathrm{eV}}{F}$ center for Van de Graaff irradiation, decrease to about 1800 $\frac{\mathrm{eV}}{F}$ center for 50-kV x-ray irradiation and then rise again to about 5000 $\frac{\mathrm{eV}}{F}$ center for 10-kV x-ray irradiation. NaCl follows the same general trend but is about six times as hard to color. Irradiations of KBr with x rays whose energies are just above or just below the $K$ absorption edge of bromine indicate that the $K$ edge does not play a dominant role in the coloration process. These results are discussed in relation to proposed mechanisms for the coloration process, and various quantitative and qualitative arguments are presented which indicate that inner shell ionization does not adequately account for either the magnitude of the observed vacancy production or the direction of its energy dependence. A correlation is made between the coloration efficiency and the ionization density along the tracks of the electrons generated by irradiation. It is concluded that mechanisms involving the single ionization of two adjacent halide ions are a more probable cause of the coloration than mechanisms involving either the single ionization of one halide ion or the multiple ionization of one halide ion. A calculation of the yield to be expected for this group of mechanisms is in reasonable agreement with experiment.