The scattering of long wavelength neutrons by irradiated beryllium oxide

Abstract

The scattering of long wavelength neutrons by polycrystalline beryllium oxide irradiated to integrated fission neutron doses between 2·5 x 10¹⁹ and 6·8 x 10²⁰ at temperatures of 75–100°c has been investigated. A considerable amount of extra scattering is introduced by irradiation and this scattering has a strong wavelength dependence. The extra scattering is interpreted in terms of scattering from defects. A model in which half the defects are present in the form of clusters containing a minimum of 10–20 interstitials or vacancies and the other half are randomly distributed, describes very closely the experimentally observed wavelength dependence of the scattering. Based on this model, calculations have been made of the total number of defects present in the irradiated material and it is shown that each primary knock-on results in about 10–20 displaced atoms. The effect on the proposed model of a dilatation around the defects is discussed and it is shown that provided a reasonable figure is chosen for the dilatation, the results are not significantly affected. Annealing studies show that the defects anneal in the temperature range 800–1400°c.