[en] Influence of cascade atoms on the preferential emission of isotopes during sputtering has been studied by a static Monte Carlo simulation program, for zero-fluence and low-energy (below 1 keV) Xe ion bombardment of a ⁶³Cu_0.5⁶⁵Cu_0.5 target. Calculations show that the probabilities of the sputtered cascade atoms of ⁶³Cu and ⁶⁵Cu are over 80%. The calculations also show that towards the surface, the momentum flux of the cascade atoms of ⁶³Cu is much more than that of ⁶⁵Cu. The momentum asymmetry in the collision cascade plays a dominating role in preferential emission of the lighter ⁶³Cu atoms in the near-normal direction during low-bombarding-energy ⁶³Cu-⁶⁵Cu sputtering