Radiation Annihilation ofF-Aggregate Centers in KCl

The radiation annihilation of $F$-aggregate centers has been measured in KCl as a function of trace impurity and of temperature. The data have been found to support the idea that $F$-aggregate-center destruction occurs when a radiation-produced interstitial is trapped by an aggregate center. Comparison of the results with the $F$-center-production behavior of the same material has permitted the following conclusions about $F$-center production to be drawn: (i) The impurity dependence of $F$-center production at 80°K is due to a secondary process involving motion of interstitials. (ii) The large rise with temperature of the $F$-center-production efficiency between 120 and 240°K is a reflection of the temperature dependence of the primary defect-production mechanism and may be related to a change of the relative probabilities of Frenkel-pair separation and recombination during the production process. Analysis of the experimental results has also yielded values for the energy necessary to produce a Frenkel pair at various temperatures. Above 200°K this energy is so low [less than 100 eV/(defect pair)] that it is possible to rule out defect-production mechanisms that require multiple ionization.