Formation ofFCenters at Low and Room Temperatures

It has been found that the x-ray production of $F$ centers at liquid helium temperature is primarily dependent on the type of alkali halide and not on the defect structure of the particular specimen in contrast to observations at room temperature. This result indicates that the low-temperature production of $F$ centers is a bulk property of the alkali halide and occurs as a result of the creation of halide ion vacancies by the ejection of halide ions from their normal lattice sites. The formation of $F$ and $H$ centers in pairs at liquid helium temperature is in accord with this model and is supported by the fact that the energy required to produce an $F$ center at liquid helium temperature in a variety of alkali halides depends on the interstitial space available for accepting the halide atom. A comparison of the energy required to form $F$ centers under various conditions at low temperature and at room temperature gives additional evidence for a two stage coloration process at room temperature.