Uranium fission fragment pinning centers in melt-textured YBCO

Abstract

In the U/n process YBCO powders are doped with uranium, textured, and then irradiated with thermal neutrons. During texturing almost spherical deposits of (U 0.6Pt 0.4)YBa 2O 6 (called Y-5), 300nm in diameter, are formed. Prior to irradiation Y-5 deposits increase J c about 10% 0.1% M U, where M U is the percentage by weight of admixed uranium which contains an intimate mixture of both the 235U and the 238U isotopes. We find the number of Y-5 deposits ∝ M U. Hence, inter-deposit spacing, S ∝ M U −1 3 . Subsequent irradiation with thermal neutrons produces fission fragment defects that also act as pinning centers. The effective length for pinning, δ, of these pinning centers was estimated theoretically to be 2μm to 4μm. The effect of the chemical and irradiation pinning centers is multiplicative and increase J c, depending on applied field and temperature, by factors of 14 to 40. The value of F n at which J c peaks depends only on the product of F n and M U(235), where F n is the numberof neutrons per square centimeter. We find that optimum F n is inversely proportional to M U(235). M U was varied from 0.0375% to 1.0%, with M U(235) held constant at 0.025%, thus varying S by a factor of 3. The enhancement of trapped field, R M, is 500% at 0.15%<M U<0.30% and drops to 400% for o.375% M U, indicating δ ∼ 2.7 μm. Anisotropy of (3mm) 3 samples decreases monotonically from 3 to 2 with increasing F n. Residual radioactivity six months after irradiation is <200 nCi/g or <7400 Bq/g for samples with 0.025% M U(235) at optimum F n.