The Characterization of Radiation Damage with Electron Backscatter Diffraction (EBSD): An Examination of Microlite from the Harding Pegmatite, Taos County, New Mexico, USA

The formation of Kikuchi patterns associated with electron backscatter diffraction (EBSD) reflect the interaction of the primary electron beam with a crystalline substance. It is expected that radiation damage in a crystalline substance will inhibit the formation of Kikuchi patterns analogous to the decrease in x-ray intensity and the broadening of peaks observed in XRD analyses. To test this hypothesis, naturally occurring microlite crystals from the Harding Pegmatite were examined.

The analysed microlite crystals vary in color and several grains show distinct light honey-yellow rims with dark brown-to-black cores; other grains show more discontinuous or patchy domains of light and dark, and some grains are entirely black. Kikuchi pattern image quality (IQ) measurements from spot analyses, line profiles, and beam-raster maps across multiple microlite crystals show significant variation and are interpreted to reflect variation in the degree of crystallinity within individual crystals. Kikuchi pattern formation does not occur in the black domains consistent with a lack of crystallinity due to radiation damage. These darker colored domains are associated with UO₂ contents greater than about 1 wt. %, and significant chemical alteration and cracking is observed in the these domains indicative of radiation damage. Kikuchi patterns were observed within lighter colored domains (UO₂ < 1 wt. %) and IQ gradients were measured. IQ values generally decrease with increasing U content consistent with gradients in radiation damage across the crystals.

The IQ parameter associated with EBSD analyses appears to provide a qualitative measure of crystallinity in naturally occurring microlites and may be a useful parameter to characterize the spatial distribution and the relative amount of radiation damage in crystalline substances at the mm-scale.