The threshold energy for atom displacement in irradiated copper studied by high-voltage electron microscopy

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Abstract
The dependence of atom displacement on irradiation direction and temperature has been studied in copper at low electron energies in a high-voltage electron microscope. The displacement was monitored by observing the nucleation and growth of interstitial dislocation loops. At 550 K a pronounced dependence of the displacement rate on irradiation direction was found. From this it can be concluded that the threshold energy surface exhibits ring-shaped ranges of minimum threshold energy around the 〈110〉 directions. The angular separation of them ranges from the respective 〈 110〉 directions is about 6° and the angular width about, 4°. The minimum threshold energy increases from 16.5 eV at 10 K to 17.5 CV at 70 K and then decreases continuously to 10.5 eV at 550 K. As the result of a sub-threshold process at suitably oriented stacking faults, displacement occurs already at energies that are about 1 eV lower. The observed dependence of atom displacement on knock-on direction and temperature can be explained by the existence of Frenkel pair configurations which under suitable conditions can be created at very low energies but whose stability decreases from higher to lower temperatures.