Effects of Self-Ionization ofβ-Active Elements in Nonmetals

Abstract

A new mechanism is proposed for the formation of stable substitutional or interstitial diatomic molecules in nonmetallic crystals in which self-ionization of $\ensuremath{\beta}$-active ions occurs. In particular, the probability of self-ionization of tritium ions in LiT and LiH: LiT crystals due to "shake-off" following $\ensuremath{\beta}$ decay is calculated. It is found that the probability of obtaining a ${\mathrm{He}}^{++}$ ion is 0.05, and for a ${\mathrm{He}}^{+}$ ion 0.32; and that the presence of these ions can lead directly to the formation of hydrogen molecules without diffusion. Using a model of LiH which includes noncentral forces, the energetics of this process, which is essentially temperature independent, and the rates of formation of hydrogen molecules are calculated. If, as experiments suggest, ${\mathrm{H}}^{0}$ and $\mathrm{H}_{2}^{}{}_{}{}^{\ensuremath{-}}$ are indeed absent in irradiated LiH, then the formation of ${\mathrm{H}}_{2}$ molecules (which accounts for the initial stage of swelling of LiH: LiT) is diffusion controlled above 200-300 \ifmmode^\circ\else\textdegree\fi{}K but is due to the "shake-off" mode at lower temperatures.