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

Systematic study of the flux pinning at 77 K in U-235-oxide-doped Ag-sheath ed Bi2223 tapes irradiated by thermal neutrons has been performed. Within t he explored range of U-235 contents (c less than or equal to 0.75 at.%) and neutron fluences (phi less than or equal to 6 x 10(19) m(-2)), pinning cap acity increases linearly with fission track density (proportional to c phi) . In particular, field B-* for the maximum volume pinning force (F-p max = J(c)B(*)) and the irreversibility field B-irr increase linearly with c phi both for fields parallel and perpendicular to the tape plane. Since J(c) de creases with c in tapes doped with U-235-oxide, B-irr(77 K, perpendicular f ield) seems limited to about 4 T. However, more subtle addition of U-235-at oms can produce tapes with flux pinning exceeding that in YBCO type of mate rials. (C) 2001 Elsevier Science Ltd. All rights reserved.