Abstract
Abstract F centres formed within, and not at external surfaces of KCl crystals have been observed. The rate of creation by x-rays has been found to be of the form dN/dt=A − BN, where A and B are temperature dependent constants. The constant A has been found to be close to the rate of liberation of electrons multiplied by an energy factor exp (− 0·05 ev/kT). It is suggested the F centres are formed from electrons trapped at incipient negative ion vacancies. Optical bleaching shows that the thermal activation process from the excited state to the conduction band is similar to that in additively coloured crystals. The quantum efficiency of bleaching indicates that less than 1015 per cm3 of frozen-in vacancies take part in colour centre formation and that at 300°K the created vacancies are only stable when occupied by electrons.
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