Temperature dependence of the threshold energy for atom displacement in irradiated molybdenum

The effective threshold energy for atom displacement is measured in molybdenum single crystals in a high-voltage electron microscope. Displacement is monitored at 70 K ≦ T ≦ 300 K by observing the nucleation of interstitial dislocation loops, and at 500 K ≦ T ≦ 800 K by measuring the growth rate of previously induced loops. By use of reference irradiations the results of both methods may be calibrated to allow comparison over the whole temperature range covered. At 70 K ≦ T ≦ 150 K the effective threshold energy for irradiation along a 〈100〉-direction is found to have a constant value of (34.5 ± 0.5) eV. Above this range the threshold energy decreases continuously with temperature to (26.5 ± 0.5) eV at 800 K. The role of subthreshold processes is discussed. It is found that such processes contribute little to the displacement cross section. The observed temperature dependence of the displacement threshold energy is explained in terms of the effect of thermal vibrations on the production of stable Frenkel pairs.