Anelastic relaxation peaks in graphites after neutron irradiation at low temperature

Internal friction experiments (torsion at 1 Hz and flexion at 300 Hz) have been performed on pyrolitic and nuclear graphite after neutron irradiation at liquid nitrogen temperature. In the data at 1 Hz, two small peaks are observed at 112 K (P 1 '; 0·24 eV) and at 135 K (P 1 ''; 0·28 eV), and a further sharp peak appears at 375 K (P 3 ; 0-8 eV). The frequency factors are close to 10 11 Hz. These peaks are described in terms of relaxation by point defects. Analysis of the effect of varying the neutron dose and of annealing on the P 1 ' and P 3 peaks shows that both peaks are due to the same defect species. At low temperature these defects have triclinic symmetry and give rise to P 1 ' peak, but at higher temperatures their symmetry becomes monoclinic, as a result of thermal activation, and they then give rise to the P 3 peak. Various types of defect structures are discussed in relation to the effects of annealing on a number of physical properties.