Photoexcitation of low-lying dipole transitions in 236U.

Abstract

Nuclear resonance fluorescence experiments have been performed on the deformed actinide nucleus $^{236}\mathrm{U}$. Bremsstrahlung of 3.9 MeV endpoint energy has been used as the photon source. The scattered photons were detected by three high resolution Ge-\ensuremath{\gamma}-spectrometers installed at scattering angles of 92\ifmmode^\circ\else\textdegree\fi{}, 128\ifmmode^\circ\else\textdegree\fi{}, and 150\ifmmode^\circ\else\textdegree\fi{}, respectively. Precise excitation energies, decay branching ratios, and ground state decay widths of numerous previously unknown spin 1 states in the excitation energy range 1.8--3.2 MeV have been extracted. The dipole strength has been found to be concentrated in the energy range 2.1--2.5 MeV. The systematics of the so-called scissors mode observed as a result of the previous (\ensuremath{\gamma},\ensuremath{\gamma}') and (e,e') experiments on $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$ and, in particular, their combined analysis suggests likewise to attribute these new dipole excitations in $^{236}\mathrm{U}$ to the orbital M1 scissors mode.