Dynamics of Radiation Damage in a Body-Centered Cubic Lattice

Radiation damage has been studied by numerically integrating the equations of motion of a large set of atoms on a high-speed computer. In this paper the method is applied to a model of $\alpha$ iron. Low energy events have been extensively investigated. The primary knock-on atom is found to initiate an extended sequence of correlated replacements, producing an interstitial at some distance and a vacancy on its original site. The interstitial is found to have a split configuration, as was found earlier in copper, but its axis lies along $〈110〉$. Collision chains are found to be prominent in $〈111〉$ and $〈100〉$, and attenuation rates and focusing parameters for these chains are determined. The threshold energy for displacing an atom is found to be highly dependent on the direction of the knock-on. The lowest threshold is found to be 17 eV, for knock-ons directed near $〈100〉$, and to be about 34 eV and 38 eV for those directed near $〈110〉$ and $〈111〉$, respectively. The probability of displacement for a randomly directed knock-on of energy $E$ is determined for $E$ between 0 and 60 eV. The results are in approximate agreement with experiments of Lucasson and Walker, although more structure is found in the calculated curve than could be tested by the experiments. Pronounced directional effects in low energy electron bombardments of $\alpha$ iron single crystals are predicted.