Single crystals of silicon grown in an argon atmosphere were irradiated with three different neutron doses; 6*10¹⁶, 3.6*10¹⁷ and 1.2*10¹⁸ cm^-2. Positron annihilation spectroscopy indicates that there are two distinct annealing stages of V-type defects in the three neutron-irradiated silicon samples: one at about 200 degrees C and another at about 500 degrees C. The former is due to the annealing out of P-V and (V₂-O)^- complexes; the latter is due to the annealing out of (V₂-O)⁰ complexes. For samples irradiated with two different higher neutron doses, the intensity due to the radiation-induced monovacancy-type defects is above 58% and disappears at 550 degrees C; the intensity due to the radiation-induced divacancies is above 23%, increases above 200 degrees C and disappears at 600-650 degrees C. For the sample irradiated with a lower neutron dose the intensity due to the divacancies is only 7.3%, it disappears below 200 degrees C, and no secondary divacancies appear. The intensity due to the secondary divacancies is highly dependent on the neutron dose. There are two annealing stages of V₂-type defects at 150-200 degrees C and about 600 degrees C. The former is due to the approach of interstitial silicon atoms to divacancies localized in the cores of neutron-radiation-disordered regions, and the latter is due to annealing out of divacancies in the 'undisturbed' matrix of the silicon crystal and V₃-O complexes. For samples irradiated with two different higher neutron doses, monovacancy-type defects with higher intensities appear again at 600-650 degrees C, which indicates the nature of the '600-650 degrees C acceptor'. Above 700 degrees C, many dislocations with 231 ps<or= tau <or=239 ps (sometimes both dislocations and monovacancy-type defects (240 ps<or= tau <or=266 ps)), are formed, especially for samples irradiated with higher neutron doses.