Some electron microscopy observations of 14 MeV neutron damage in niobium

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Abstract
Structural damage has been produced in two niobium specimens by irradiating them with 14 MeV neutrons. The first specimen has been irradiated to a fluence of 420 neutrons nm−2 at 300°C and the second to a fluence of 6600 neutrons nn−2 at about 25°C. The structural damage in both cases was similar to that observed in fission neutron-irradiated niobium and consisted of either clusters of interstitials and vacancies or Frank dislocation loops ranging from about 1 to 12 nm in diameter. The size number–density distribution of clusters in the second specimen was compared with those reported for fission neutron-irradiated niobium at comparable fluences and temperatures. It was found that the rate of accumulation of displacement damage during 14 MeV neutron irradiation was substantially greater than that during irradiation with an equivalent flux of fission neutrons (E < 0.1 MeV). Evidence was observed of multiple cluster formation from high-energy displacement cascades.