Efficiency of defect production in cascades

In copper and silver, the defect production by energetic ions has been investigated for projectile masses between 1 and 209 in the energy range between 104 and 106 eV. Thin-film residual-resistivity measurements were used to determine the defect production below 10K. Calculations of the number of Frenkel pairs produced by various projectiles as a function of energy were performed, based on Lindhard's cross sections and a modified Kinchin-Pease expression. A comparison of measured and calculated damage rates shows that the defect-production efficiency decreases with ion mass for light projectiles, whereas at masses >20 the cascade efficiency remains rather constant and assumes a value of xi approximately 0.3 for both Ag and Cu. The decrease in defect efficiency indicates a transition from isolated Frenkel-pair formation to the production of energetic displacement cascades