Interstitial Defects in LiH and NaCl Irradiated at Low Temperatures

Expansions of the lattice parameters of both LiH and NaCl exposed to $\beta$ radiation at 77° and 245°K have been observed using a vacuum x-ray diffractometer stage operated at 77°K. The source of the $\beta$ radiation was 5.56% LiT contained in a LiH crystal which was ground and mixed with NaCl in the samples. The LiH lattice parameter increased 0.0020±0.0007 A in three samples. This result is consistent with density measurements and with the formation of interstitial ${\mathrm{He}}^3$ atoms in the LiH lattice. The NaCl lattice parameter increased 0.0061±0.0009 A even though the average $\beta$ does for the NaCl was less than 1% of that in the Li(H,T) and tritium $\beta$ particles with maximum energy cannot produce direct displacements in NaCl. A mechanism is proposed for the production of ${\mathrm{Cl}}^{-}$ interstitials and vacancies (Frenkel defects) in NaCl which is similar to one proposed by Klick for the production of $F$ and $H$ centers. This mechanism is related to observations of the production of $\alpha$ centers in alkali halides irradiated at low temperatures.