Study of the Energy Levels ofTl205Using the (γ,γ′) Reaction

Both elastic and inelastic scatterings of monochromatic photons were used for studying the energy levels of ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$; the photons were produced by thermal-neutron capture in iron, and the resonantly scattered spectrum was measured using Ge(Li) detectors. The scattering isotope was identified by using an isotopically enriched thallium target. The energy of the resonance level in ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$ was found to be 7.646 MeV. The spectral shape of the scattered radiation was found to have a strong intensity bump above about 4.5 MeV. Energies and intensities of several $\gamma$ lines were measured in the region 1.0-7.7 MeV. Assuming the high-energy lines to be primary transitions deexciting the resonance level, 25 energy levels were found from the ground state up to 3.4 MeV, 8 of which may be identified with recently reported levels. By measuring the angular distribution of the scattered radiation, the spin of the scattering level was found to be ½. The width of the resonance level was determined and was found to be $\Gamma =0.98\mathrm{}\pm 0.09$ eV, and $\frac{{\Gamma }_0}{\Gamma }=0.58\mathrm{}$. The E1 character of the scattered radiation was inferred from the systematics of the radiation widths of highly excited levels, and the resonance level was interpreted as being associated with the tail of the giant electric-dipole resonance. The ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$ levels are discussed in terms of the predictions of the shell model. The anomalous intensity bump in the $\gamma$ spectrum is discussed in the light of similar bumps obtained in the ($n,\gamma$) reaction on nuclei in the same mass region.