X-Ray Study of Interstitial Defects in Radiation-Damaged Alkali Halides and LiH

The effects of exposure to tritium $\beta$ particles at 77°K on the lattice parameters of Si, LiH, LiF, NaCl, KCl, and KBr were examined by x-ray diffraction studies by employing a sample stage cooled by liquid nitrogen. The Si and LiH lattice parameters remained constant during $\beta$ irradiation from an external source, but all of the alkali halides showed large increases in lattice parameter. Maximum unit cell volume expansions observed were 0.2% in LiF, 0.6% in NaCl and KCl, and 0.3% in KBr. Measurements of unit cell expansion and bulk crystal expansion of LiH crystals containing LiT were also continued to determine more accurately the expansion per interstitial ${\mathrm{He}}^3$ atom. The results are interpreted on the basis of the formation of Frenkel pairs (interstitial anion plus anion vacancy) in the alkali halides by a moderately efficient process. The process involves the transport of interstitial defects through 50 μ into materials for which the $\beta$ range is about 2 μ. It is proposed that this may occur either directly, by an interstitialcy mechanism, or indirectly, by exciton interactions within the crystals. Comparisons between the results of low-temperature studies of lattice expansion and color centers in alkali halides are made, and possible interrelations between the various defects formed by radiation damage are proposed.