Flux pinning by radiation damage in oxygen-doped niobium

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Abstract
Niobium foils, doped with 10, 250, 480 or 904 wt. p.p.m. of oxygen, were irradiated with fast neutrons (E>0·1 MeV) to a total fluence of 9×1019 cm−2 at ∼50°C. The radiation damage as characterized by transmission electron microscopy consists of dislocation loops and ‘black spot’ damage (probably small, unresolved dislocation loops). The density of dislocation loops increases by about a factor of 20 with increasing oxygen content whereas the root mean square dislocation loop diameter decreases so as to maintain the total area of dislocation loop approximately constant. Despite the great increase in dislocation loop density ρ with oxygen content, the pinning force density F p at low reduced magnetic induction decreases by more than a factor of 20 from the 10 to the 904 p.p.m. oxygen sample. These results show that the decrease in loop size through its effect on the elementary pinning force f p of the loop far outweighs the increase in loop density. Tests of summation models are made using calculated values of the f p(D) for a loop of diameter D and the experimentally determined loop density distribution ρ(D). The linear summation of Dew-Hughes [F p ∝ ρ(D)f p(D)], which neglects the rigidity of the flux line lattice (FLL), is found to be inconsistent with the large decrease observed in F p with oxygen content whereas the quadratic sum of Labusch [F p∝ (D)f p 2(D)] and others who take the FLL rigidity into account is found to produce results in reasonable agreement with the observed decrease.