Vortex pinning by fission tracks in 235U-doped Ag/Bi2223 tapes

Systematic study of the flux pinning at 77 K in 235U-oxide-doped Ag-sheathed Bi2223 tapes irradiated by thermal neutrons has been performed. Within the explored range of 235U contents (c≤0.75 at.%) and neutron fluences (φ≤6×1019 m−2), pinning capacity increases linearly with fission track density (proportional to cφ). In particular, field B∗ for the maximum volume pinning force (Fp max=JcB∗) and the irreversibility field Birr increase linearly with cφ both for fields parallel and perpendicular to the tape plane. Since Jc decreases with c in tapes doped with 235U-oxide, Birr(77 K, perpendicular field) seems limited to about 4 T. However, more subtle addition of 235U-atoms can produce tapes with flux pinning exceeding that in YBCO type of materials.