The scattering of long wavelength neutrons by defects in neutron-Irradiated graphite

The additional elastic scattering of monochromatic neutrons, wavelength 7·5 Å, produced by irradiating a sample of AGXP graphite to a dose of 6 × 1019 fast neutrons/cm2 at 30°C has been measured as a function of the scattering angle. If we make the reasonable assumptions that equal concentrations of vacancies and interstitials are present and also that the vacancies scatter isotropically an estimate of the state of agglomeration of the interstitials may be made, provided that allowance is made for the relaxation of the neighbouring atoms. The result is 4·2 interstitials per cluster; also the values of the interstitial and vacancy concentrations are each about 2%. The radiation produced scattering disappears almost completely after an anneal at 1100°C for one hour, while at intermediate temperatures the shape of the scattering curve is identical to that obtained in the as irradiated condition. This suggests that the predominant annealing mechanism in this temperature range is not the free migration of particular sizes of small interstitial clusters.