  We report on the vortex dimensionality of uranium-doped Ag/Bi2223 tapes,
before and after irradiation to a thermal-neutron fluence. The effective
activation energies as a function of current density and applied field were
calculated from dynamic magnetisation relaxation measurements. A dimensional
crossover from a 3D elastic creep regime to 2D plastic creep was observed in
the nonirradiated tape at an applied magnetic field Hcr = 0.37 T, with an
associated change in the flux hop velocity and temperature dependence of the
activation energy. After introduction of the fission-fragment damage by
irradiation, a shift in the crossover to Hcr = 0.65 T was observed. These
results indicate an enhancement of the c-axis vortex coherence by the
introduced splayed columnar defects, explaining the greater pinning efficiency
of the uranium-fission method in Bi2223 rather than the less anisotropic Y123.
Conflicting results obtained for the irradiated tape in the absence of any
temperature scaling of the activation energies demonstrate the importance of
the inclusion of scaling in the magnetisation relaxation analysis.
