The effects of fast neutron irradiation on oxygen atoms in Czochralski silicon(CZ-Si) are investigated systemically by using Fourier transform infrared(FTIR) spectrometer and positron annihilation technique(PAT).Through isochronal annealing,it is found that the trend of variation in interstitial oxygen concentration([Oi]) in fast neutrons irradiated CZ-Si fluctuates largely with temperature increasing,especially between 500 and 700 C.After the CZ-Si is annealed at 600 C,the V4 appearing as three-dimensional vacancy clusters causes the formation of the molecule-like oxygen clusters,and more importantly these dimers with small binding energies(0.1-1.0eV) can diffuse into the Si lattices more easily than single oxygen atoms,thereby leading to the strong oxygen agglomerations.When the CZ-Si is annealed at temperature increasing up to 700 C,three-dimensional vacancy clusters disappear and the oxygen agglomerations decompose into single oxygen atoms(O) at interstitial sites.Results from FTIR spectrometer and PAT provide an insight into the nature of the [Oi] at temperatures between 500 and 700 C.It turns out that the large fluctuation of [Oi] after short-time annealing from 500 to 700 C results from the transformation of fast neutron irradiation defects. The effects of fast neutron irradiation on oxygen atoms in Czochralski silicon (CZ-Si) are investigated systemically by using Fourier transform infrared (FTIR) spectrometer and positron annihilation technique (PAT) .Through isochronal annealing, it is found that the trend of variation in interstitial oxygen concentration ([Oi]) in fast neutrons irradiated CZ-Si fluctuates largely with temperature increasing, especially between 500 and 700 C. After the CZ-Si is annealed at 600 C, the V4 appearing as three-dimensional vacancy clusters causes the formation of the molecule-like oxygen clusters, and more importantly these dimers with small binding energies (0.1-1.0 eV) can diffuse into the Si lattices more easily than single oxygen atoms, thus leading to the strong oxygen agglomerations. is annealed at temperature increasing up to 700 C, three-dimensional vacancy clusters disappear and the oxygen agglomerations decompose into single oxygen atoms (O) at interstitial sites. Results from FTIR spectrometer and PAT provide insight into the nature of the [Oi] at temperatures between 500 and 700 C. It turns out that the large fluctuation of [Oi] after short-time annealing from 500 to 700 C results from the transformation of fast neutron disadvantage defects.