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A detailed study of the anisotropy of the threshold energy for Frenkel-pair production in copper was carried out experimentally, using in-situ electrical-resistivity measurements in the high-voltage electron microscope. These electrical-resistivity measurements, which are sensitive to small changes in point-defect concentration, were used to determine the damage or defect production rate. Damage-rate measurements in copper single crystals were carried out for approx.40 incident electron-beam directions and six electron energies from 0.4 to 1.1 MeV. The total cross section for Frenkel-pair production is proportional to the measured damage rate and can be theoretically calculated if the form of the threshold-energy surface is known. Trial threshold-energy surfaces were systematically altered until a ''best fit'' of the calculated to the measured total cross sections for Frenkel-pair production was obtained. The average threshold energy of this surface is 28.5 eV. The minimum threshold energy is 18 +- 2 eV and is located near . A ring of very high threshold energy (>50 eV) surrounds the direction. A damage function for single-defect production was derived from this surface and was applied to defect-production calculations at higher recoil energies. This function rises rather sharply from a value of zero at 17 eV to 0.8 at 42more » eV. It has the value of 0.5 at 24.5 eV. Above 30 eV the slope of the curve begins to decrease, reflecting the presence of the high-energy regions of the threshold-energy surface. Both topographical and quantitative comparisons of the present surface with those in the literature were presented. Based on a chi/sup 2/ goodness-of-fit test, the present surface was found to predict the experimentally observed total cross sections for Frenkel-pair production significantly better than the other available surfaces. Also, the goodness of fit varied substantially less with energy and direction for the present surface.« less

Computer simulations were performed using the molecular-dynamics technique to determine the directional dependence of the threshold energy for production of stable Frenkel pairs in copper. Sharp peaks were observed in the simulated threshold energy surface in between the low-index directions. Threshold energies ranged from \ensuremath{\sim}25 eV for directions near $〈100〉$ or $〈110〉$ to 180 eV at the position of the peak between $〈110〉$ and $〈111〉$. The general topographical features of the simulated threshold-energy surface are in good agreement with those determined from an analysis of recent experiments by King et al. on the basis of a Frenkel-pair resistivity ${\ensuremath{\rho}}_{F}=2.85\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ \ensuremath{\Omega} cm. Evidence is presented in favor of this number as opposed to the usually assumed value, ${\ensuremath{\rho}}_{F}=2.00\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ \ensuremath{\Omega} cm. The energy dependence of defect production in a number of directions was investigated to determine the importance of nonproductive events above threshold.

The role of thermal spikes in energetic displacement cascades has been investigated by molecular-dynamics computer simulation. For cascade energies of 3 and 5 keV in Cu, which are the highest energies (in reduced units) yet treated by fully dynamical simulations, it is found that local melting occurs and persists for several picoseconds. The implications of this behavior for atomic mixing, Frenkel-pair production, and point-defect clustering are discussed.