Nuclear-Resonance Fluorescence inTb159

Levels at 363 and 580 keV in ${\mathrm{Tb}}^{159}$ populated in the decay of ${\mathrm{Gd}}^{159}$ have been investigated by the resonance-fluorescence method using the centrifuge technique. Powdered terbium oxide (${\mathrm{Tb}}_4$ ${\mathrm{O}}_7$) was used as the resonance scatterer. From the resonance scattering cross sections and known branching ratios, the mean lives of the levels were computed to be: $\tau (363\mathrm{}-\mathrm{k}\mathrm{e}\mathrm{V}\mathrm{l}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{l})$: (2.2±0.15)×${10}^{-10\mathrm{}}$ sec; $\tau (580\mathrm{}-\mathrm{k}\mathrm{e}\mathrm{V}\mathrm{l}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{l})$: (1.1±0.15)×${10}^{-12\mathrm{}}$ sec. Angular-distribution studies revealed that the 363-keV $\gamma$ ray previously identified as $E1\mathrm{}$ was predominantly dipole in character, with an $\frac{〈M2\mathrm{}〉}{〈E1\mathrm{}〉}$ amplitude ratio $\delta =-\left({6_{-2\mathrm{}}}^{+1\mathrm{}}\right)$%. These studies also showed that attenuations due to extranuclear fields in the oxide scatterer were very small. In the case of the 580-keV $\gamma$ ray, the measured angular distribution confirmed the earlier assignment of spin ½ for this level. Estimates of the $B\mathrm{}\left(E2\mathrm{}\right)\mathrm{}$ of this $\gamma$ ray derived from Coulomb excitation, combined with the above value of the mean life, gave the $\frac{〈E2\mathrm{}〉}{〈M1\mathrm{}〉}$ amplitude ratio of the 580-keV $\gamma$ ray as $\delta =\pm (3\mathrm{}\pm 1.5)$%.