Role of tunnelling recombination in radiation-induced F-centre creation in alkali halide crystals at liquid helium temperatures

Computer simulations of the F-centre accumulation at liquid helium temperatures (LHT) in alkali halides, taking into account the spatial distribution and the tunnelling recombination of the localized F- and H-centres have been undertaken. The presented model easily explains known experimental data at these temperatures, that is: (i) the saturation and possible subsequent decay of the F-centre production rate (ii) the existence of an abnormally high fraction of the aggregate (F₂,H₂,...) centers (in contrast to random distribution), (iii) the formation of large loose aggregates (iv) the existence of F-centre concentrations exceeding 10¹⁹ cm^-3 and (v) the dependence of the production curve on the excitation intensity. A mechanism for loose-aggregates formation is proposed. The applicability of the present model at room temperature is discussed.