Increased flux pinning upon thermal-neutron irradiation of uranium-doped ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

To assess fission-fragment damage as flux-pinning centers, sintered compacts of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ were prepared with uranium additions of 150 and 380 atomic ppm and exposed to thermal neutron fluences of 4.3×${10}^{17}$, 1.2×${10}^{18}$, and 4.0×${10}^{18}$/${\mathrm{cm}}^2$. Magnetic hysteresis measurements were made as functions of temperature at fields up to 2.5 T. The hysteresis at 1 T for the sample containing 150 ppm uranium increased upon an irradiation of 1.2×${10}^{18}$/${\mathrm{cm}}^2$ by 3.7 times at 4.5 K, 20 times at 63 K, and 8.3 times at 77 K. Much smaller increases in magnetic hysteresis were observed in undoped samples exposed to thermal neutrons. Critical-current densities were calculated from the hysteresis observed at 1 T using the critical-state model, assuming the currents to be restricted to the grains (≊5 μm radius). For the 150 ppm, 1.2×${10}^{18}$/${\mathrm{cm}}^2$ sample, the 1-T intragranular critical-current densities obtained are 1.5×${10}^7$, 1.0×${10}^6$, and 1.4×${10}^5$ A/${\mathrm{cm}}^2$ at temperatures of 4.5, 63, and 77 K, respectively. The critical temperatures by an ac susceptibility technique showed that the 4×${10}^{18}$/${\mathrm{cm}}^2$ irradiation lowered the onset critical temperatures only slightly, from 91 to 90 K and from 91.5 to 89 K for the 150 and 380 ppm samples, respectively.