2D MONTE-CARLO SIMULATIONS OF RADIATION EFFECTS ON NONEQUILIBRIUM COLLOID GROWTH

The formation of colloids in alkali halides induced by ionizing radiat ion is studied by the Monte Carlo simulation of aggregation of F centr es. The model consists of the following mechanisms. (i) A two-dimensio nal lattice-gas Ising model is defined by a Hamiltonian that describes the thermally driven transition from gas (F centres) into the solid p hase (condensed colloids). In addition two mechanisms specific for the irradiation of alkali halides are introduced. (ii) Highly mobile reco mbining centres (H centres) diffuse to the colloid with a flux approxi mately equal to the flux of F centres. After reaching the surface the H centre and one surface alkali-metal atom of the colloid recombine. ( iii) The lattice is subject to a continuous generation of radiation-in duced lattice excitations. At the surface of the colloid the excitatio ns result in displacements, which distort the surface of the growing c luster. The morphology and growth kinetics are studied. It appears tha t the F centres do not condense into one compact cluster. Instead they are spread over a confined region containing small subclusters. The s pread increases considerably if the annihilation fraction is increased above 80%. The surface displacements prevent the cluster from relaxin g to its equilibrium shape, causing a roughening of the cluster surfac e.