Structural behaviou of neutron irradiated quartz

Abstract

X-ray and density studies have been made on single crystals of quartz irradiated with fast pile neutrons at approximately 100°C. The structural effects of the irradiation follow at least two processes with doses up to 1·2 × 10²⁰ neutrons/cm². Crystals irradiated with less than 3 × 10¹⁹ neutrons/cm² expand anisotropically in a manner that is similar to the thermal expansion of unirradiated quartz. In this dosage range the volume increase is the same, as determined by x-ray and hydrostatic methods, and little or no distortion is observed in x-ray diffraction patterns. The model which seems to fit these results, best consists of point defects and slightly disordered regions which result in an elastic distention of the lattice. After doses in excess of 3 × 10¹⁹ neutrona/cm² the damaging prom is more complex since the volume change indicated by x-ray measurements of the host lattice becomes larger than the bulk volume change determined hydrostatically. After irradiation with 8 × 10¹⁹ neutrona/cm² an inhomogeneous shear strain is observed in the extreme skewing of the (22·0) reflection which is interpreted as being due to the crowding of oxygen ion interstitials into the open c-axis channels. Amorphous regions enmeshed in the host lattice are identified in crystals that still possess long range order after irradiation with 7 × l0¹⁹ neutrons/cm². The diffuse x-ray scattering halo associated with the amorphous material changes peak position, shape, and intensity with increasing dosages of irradiation. It is suggested that these changes result from a reorientation of disordered regions which is assisted by the increased expansion of the lattice and the presence of ruptured silicon-oxygen bonds. A second kind of diffuse scattering appears at these high doses which cannot be accounted for on the basis of a thermal origin but is probably related to scattering resulting from inhomogeneous lattice strains. In a complementary study it wm found that coesite, a high density crystalline silica remained stable under neutron irradiation dosages which completely disorder quartz. These results indicate that the rate of damage in silica solids is structure dependent.