F‐Centre saturation phenomena in KCl during low‐energy electron irradiation

The absorption levels in the F‐ and M‐bands in KCl have been monitored during controlled high‐intensity irradiation with 5 keV electrons at room temperature, and novel saturation effects have been observed. The F‐centre concentration rises to a maximum of between 4 and 7 × 1019 cm−3 (depending on the dose rate) and then decays slowly to an equilibrium level. The M‐band absorption rises steadily except for a “plateau” coinciding with the F‐centre maximum. After irradiation F‐to‐M‐centre conversion takes place, but this is reversed on re‐irradiation. These effects can be explained using a model in which F‐centres produced less than about 25 Å apart aggregate spontaneously; in addition, M‐centres appear to be split up into F‐centres by the radiation itself. The relatively large amount of energy (45 keV) apparently expended in forming each vacancy suggests that there is also a high rate of recombination with interstitial defects during irradiation.