Observation of Lattice Defects in Fission Fragment-irradiated Graphite

Abstract

Some electron microscopic observations of large defects in graphite induced by fission fragment irradiation are shown. The observations were done on refined natural graphite powder mixed with uranium oxide powder in the ratio 10:1. The material was exposed to thermal neutrons in a reactor to total doses of the order of 10/sup 1//sup 6/ n/cm/sup 2/. Tracks of fission fragments from U/sup 2// sup 3//sup 5/ are sometimes observed as straight lines about 100 A wide accompanied by small dots. By tilting the specimen, their contrasts turn from black to white as the matrix changes from white to black. This indicates that the tracks are essentially the diffraction contrasts caused by variations in the Bragg conditions in the crystal and that these narrow linear regions have a crystallographic nature or orientation slightly different from the matrix, possibly exhibiting the thermal spike effect of fission fragments passing through the crystal. Dislocation lines are also partly due to absorbtion of interstitial atoms produced by the irradiation. (N.W.R.)