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.