Kinetic non-lattice Monte Carlo atomistic simulations are used to analyze the ion beam induced defect evolution. This is studied in terms of the probabilities of emitted interstitials being recaptured by other defects or in turn, being annihilated at the surface. In this way, the ripening and dissolution of interstitial defects can be explained as a competition between the average distance among defects and their distance to the surface, that acts as an efficient sink. At the beginning of the anneal Si interstitials are mostly exchanged among defects, with a minimal loss of them through diffusion to the surface. This leads to the formation of larger defects at the expense of the smaller ones. As their size increases and the defect dose decreases, the average distance among them increases. Then, the loss of Si interstitials through diffusion to the surface prevails, causing their dissolution. The presence of large and stable defects near the surface is also possible in low energy implants if the dose is high enough.