Dipole strength distributions in the stable Ba isotopesBa134–138: A study in the mass region of a nuclear shape transition

Abstract

The low-lying dipole strength distributions in the odd-mass isotopes $^{135,137}\mathrm{Ba}$ were studied in nuclear resonance fluorescence experiments (NRF) performed at the Stuttgart Dynamitron facility using bremsstrahlung beams with end point energies of 4.1, 3.1, and $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Numerous excited states, most of them unknown so far, were observed in the excitation energy range up to $4\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Detailed spectroscopic information has been obtained on excitation energies, decay widths, decay branching ratios, and transition probabilities. The results for $^{137}\mathrm{Ba}$ are compared with calculations in the framework of the Quasiparticle-Phonon Model. The new data for $^{135,137}\mathrm{Ba}$ complete the systematics of low-lying dipole excitations as observed for the even Ba isotopes $^{134,136,138}\mathrm{Ba}$ in previous NRF experiments in Stuttgart. The complete systematics within the Ba isotopic chain, exhibiting a nuclear shape transition, is discussed with respect to $E1$ two-phonon excitations, $M1$ scissors mode excitations, and in regard to the new critical point symmetries.