Determination of the relaxation mechanisms for polarizedB12implanted in single-crystal aluminum

Polarized ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ was produced in the reaction ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}(d, p){}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and recoiled into single-crystal samples of aluminum. The relaxation of the polarization was measured and found to be exponential throughout the temperature range ($68\le T\le 751$ K) investigated. The relaxation time as a function of temperature could be described in terms of four mechanisms: conduction-electron effects, diffusion, trapping, and radiation damage. A spin-relaxation model was developed in which each of these mechanisms was quantitatively treated in order to give a seven-parameter fit to the data. The Korringa relaxation rate was determined. The diffusion kinetics of ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ in aluminum were described by an Arrhenius relation with an activation energy 0.65 eV and a preexponential factor 6 × ${10}^{-14\mathrm{}}$ sec. The density of traps in aluminum for ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ is ≈ 1 × ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$; the nature of the trap was not identified in a definitive manner. The radiation damage observed below room temperature gave rise to fluctuating-electric-field gradients at the ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ nucleus; the temperature dependence could be described by an Arrhenius relation whose parameters indicated that the observed radiation damage was in the form of dislocation loops. The diffusion kinetics of boron in aluminum determined in this experiment were used to account for anomalous results obtained from channeling experiments on boron-implanted aluminum.