Float-zone (FZ) Si irradiated with 1 MeV neutrons was investigated by means of positron annihilation lifetime spectroscopy and photoluminescence. Three types of defects were observed: di-vacancies, small vacancy clusters and an unknown defect with the defect-related lifetime of (285 ? 5) ps that contribute to positron trapping only at low temperatures. Two annealing stages were observed: one at 350 ?C and another at 500 ?C. While the former is due to the annealing of divacancies, the latter is caused by the annealing of vacancy clusters and the unknown defect. ? 2004 Kluwer Academic Publishers