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

The effects of exposure to tritium BETA particles at 77 deg 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 He/sup 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 mu into materials for which the BETA range is about 2 mu . 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. (auth)