Fast-neutron sputtering of niobium

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Abstract
Neutron sputtering yield measurements of niobium are reported for 14.8‐MeV (d,t) neutrons and for fast neutrons in the forward direction produced by 40‐MeV deuteron bombardment of a thick Be target. Six irradiations were performed with 23 niobium targets with a range of 1.7×1015 to 6.1×1016 (d,t) neutrons. One irradiation was performed with five niobium targets irradiated with a range of 4.7×1016 to 4.5×1017 (d,Be) neutrons. Sputtering targets included single crystals and annealed and cold‐worked material with various surface preparations. Target preparation overlapped or was identical, in several cases, to that used by another research group which has reported the observation of fast‐neutron‐induced ’’chunk’’ or micrometer particle emission. Several cm2 of collector which faced Nb targets irradiated to fluences of (0.4–1.1) ×1016 (d,t) neutrons/cm2 were carefully examined by scanning electron microscopy, but no evidence for neutron‐produced chunk emission ?0.5 μm was found. Thermal neutron activation analysis was used to determine absolute yield on most of the silicon collectors which faced the front and back surfaces of the Nb targets. These determinations indicate that the yield in atoms/neutron for a wide variety of Nb target preparations, including those which are reputed to be high‐probability chunk emitters, are likely to be less than several times 10−5. The experimental procedures are presented in detail. The present results for fast‐neutron sputtering yields are discussed in relation to other recent results for niobium and theoretical estimates. Limitations in our experiments and others are also discussed. The present results, as well as other recent results for niobium, indicate that neutron sputtering will not be a significant factor in the erosion of the first wall or in plasma contamination in future fusion reactors.