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

Systematic study of the flux pinning at 77 K in $^{235}$U-oxide doped Ag-sheathed Bi2223 tapes irradiated by thermal neutrons has been performed. Within the explored range of $^{235}$U contents ($c\leq 0.75$ at\%) and neutron fluences ($\phi \leq 6\cdot 10^{19}$ m$^{-2}$) pinning capacity increases linearly with fission track density (proportional to $c\phi $). In particular, field $B^{\ast }$ for the maximum volume pinning force ($% F_{p\max }=J_{c}B^{\ast }$) and the irreversiblity field $B_{irr}$ increase linearly with $c\phi $ both for fields parallel and perpendicular to the tape plane. Since $J_{c}$ decreases with $c$ in tapes doped with $^{235}$% U-oxide, $B_{irr}$(77 K, perpendicular field) seems limited to about 4 T. However, more subtle addition of $^{235}$U atoms can produce tapes with flux pinning exceeding that in YBCO type of materials.