Irradiation of Si at room temperature produces mobile vacancies and interstitials. The former trap Oi yielding A-centres, or VO complexes . These are the primary oxygen-related defect in radiation damaged Cz-Si, and form irrespective of the irradiation method (electrons, protons, neutrons, -particles, -rays, etc.) The structure of the negatively charged centre has been established by EPR , whereas uniaxial stress studies on the local vibrational mode (LVM) at 836 cm-1 (4.2K) has shown that the neutral defect has a similar structure . This consists of an O atom bridging a pair of Si neighbours of the vacancy (Figure 5.1). The defect has associated vibrational modes at 836 and 885 cm-1 for the neutral and negatively charged species respectively. The defect has an acceptor level at Ec-0.17 eV, possibly due to the strained Si-Si bond between the other pair of Si atoms bordering the vacancy. The lines are, however, broad and no 29Si isotope shifts are seen . The reorientation energy of the centre is surprisingly low, at only 0.38 eV . Their barrier to migration is 1.460.29 eV  (also given as 1.86 eV [80,31]), allowing them to migrate at lower temperatures than Oi.