Search for isomers in {\boldmath $^{199-203}$}Tl

The $^{203}$Tl($n,xn\gamma$) reactions were used to study excited states in $^{199-203}$Tl isotopes. The data were taken using the GEANIE spectrometer comprised of 26 high-purity Ge detectors. The pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided neutrons in the energy range from 0.6 to 250~MeV. The time-of-flight technique was used to determine the incident neutron energies. Partial $\gamma$-ray cross sections were measured from the beam-on data while half-lives of isomers were determined from the beam-off data. A candidate for the unknown 9/2$^{-}$ isomer in $^{203}$Tl has been identified. The half-life of this state is less than the shortest half-life that could be determined in the present experiment (typically, the half-lifes that can be currently measured with GEANIE vary between a few $\mu$s to a few ms). The candidate state is located at 1484-keV excitation energy, in excellent agreement with the theoretical prediction for the excitation energy of the 9/2$^{-}$ state in Ref.~[1]. In $^{202}$Tl, for the previously known 7$^{+}$ isomer at 950-keV excitation energy a 592(4)~$\mu$s half-life was obtained in the present work, which differs by $\sim 4 \%$ from the value adopted in the literature [572(7)~$\mu$s]. In the lighter Tl isotopes, the life-time measurements yielded results in agreement with previously known values reported in the literature. This work was supported by the U.S. Department of Energy under Contracts No. DE-AC52-06NA25396 (LANL) and W-7405-ENG-48 (LLNL). \\ \noindent [1] G. E. Arenas Peris and P. Federman, Phys. Rev. {\bf C 38}, 493 (1988).