Development of defects from displacement cascade damage in low temperature D-T neutron-irradiated metals

Pre-thinned pure metal foils were irradiated with D-T neutrons below 20 K and transferred in a cryostat to an electron microscope for observation of the displacement damage cascades at 130 K. Subsequently, the specimens were annealed isochronally to observe cascade relaxation. Pure gold was examined most extensively in the present experiment. After the very low fluence (6x1019 n/m2), the defects observed at low temperature were interstitial loops while vacancy clusters were not observed. These defects may be retained in an unrelaxed structure. The interstitial loops disappeared partially with an increase of annealing temperature. At the low fluence of 3x1020 n/m2, interstitial loops grew significantly at 220 K to 250 K and shrunk at 300 K to 430 K. Small dotted clusters (vacancy stacking fault tetrahedra) appeared at 300 K near existing interstitial loops and some grew up until 520 K. At the medium fluence of 6x1021 n/m2, many defects observed at 130 K showed the typical triangular contrast of stacking fault tetrahedra. Ninety-five percent of the defects observed at 130 K were found to be of vacancy type by the 2 1/2 D method. Those defects were annealed significantly at room temperature. Visible vacancy clusters seem to be formed only by overlap of cascades during low temperature irradiation. Small interstitial clusters which remain in specimens move to vacancy clusters during the room temperature annealing. At medium fluence, clusters of both vacancy and interstitial type were formed in nickel, while almost all defects were of an interstitial type in copper. (orig.)