Silicon samples, irradiated with swift Kr (245 MeV) and Bi (710 MeV) ions at room temperature, were investigated using a continuous and a pulsed positron beam and conventional Doppler broadening and lifetime spectroscopy. In the fluence and depth ranges studied, creation of large voids and amorphization was not observed. The dominant defects were found to be divacancies, present from the near surface region all along the ion tracks. We found that the formation of divacancies from ion-induced vacancies as predicted by Monte-Carlo-calculations is higher in the case of the heavier Bi ion.