Cascade structure in low temperature fission and fusion neutron-irradiated metals

With the cryotransfer TEM technique, defects which were formed in fusion- and fission-neutron irradiated metals below 20 K were observed at the early stage of their formation. At a low D-T neutron fluence of 1x1020 n/m2, TEM visible defects were of interstitial type in Au, Cu, Ag and Cu3Au, and no defects were observed in Ni, Fe and Al. This suggests that vacancy defects at isolated displacement damage cascades have sub-microscopic sizes. At a very low D-T neutron fluence of 5x1018 n/m2, few single-isolated clusters of interstitials were observed in Au. In D-T neutron-irradiated Au, Ag and Cu, clusters of interstitials tend to form groups whose size increases with the fluence, while such group formations of point defects were not observed in D-T neutron-irradiated Ni, Fe and Al. Such groups of interstitial point defects may be formed with strain-affected thermal diffusion after the ejection of interstitials from the cascade core. At a medium D-T neutron fluence of 1x1021 n/m2, TEM visible vacancy clusters were observed even in metals which were irradiated below 20 K. The development of visible vacancy clusters is explained due to the cascade energy overlapping. TEM observations showed that sub-microscopic interstitial clusters are also formed together with visible ones in D-T neutron-irradiated metals. During TEM observations, sub-microscopic clusters of both interstitial and vacancy types grew to visible ones by being assisted with momentum transfer from the TEM electrons. Differences are noticed between observed defects in fission- and fusion-neutron irradiated metals. Point defect processes of damage formation in fission- and fusion-neutron irradiated metals are discussed. (orig.)