Properties of low-energy dipole states in ${}^{232}\mathrm{Th}$ have been investigated with the nuclear resonance fluorescence technique. The present work used monoenergetic $\gamma$-ray beams at energies of 2–4 MeV from the high-intensity $\gamma$-ray source at Triangle Universities Nuclear Laboratory. Over 40 transitions corresponding to deexcitation to the ground state and first excited state were observed for the first time. Excitation energies, integrated cross sections, decay widths, branching ratios, and transition strengths for those states in ${}^{232}\mathrm{Th}$ were determined and compared with quasiparticle random-phase-approximation calculations. A large number of $E1$ transitions were observed for the first time in actinide nuclei with summed strength of $3.28(69)\times {10}^{-3}\hspace{0.5em}{e}^2{\mathrm{fm}}^2$. The observed summed $M1$ strength of 4.26(63)${\mu }_N^2$ is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei.

• Received 30 July 2010

DOI:https://doi.org/10.1103/PhysRevC.83.034615