High-sensitivity studies of E1 and M1 transitions observed in the reaction 138 Bað; 0 Þ at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HIS facility. The electric dipole character of the so-called ''pygmy'' dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 ''spin-flip'' mode was observed for the first time in N ¼ 82 nuclei. DOI: 10.1103/PhysRevLett.104.072501 PACS numbers: 21.10.Hw, 21.60.Àn, 23.20.En, 24.70.+s In stable and unstable neutron-rich nuclei a resonancelike concentration of dipole strength is observed at excitation energies around the neutron-separation energy [1][2][3][4][5][6][7][8][9][10]. This clustering of strong dipole transitions has been named the pygmy dipole resonance (PDR). In hydrodynamic and collective approaches, it was suggested that an oscillation of a small portion of neutron-rich nuclear matter relative to the rest of the nucleus is responsible for the generation of pygmy resonances [11,12]. Further, in microscopic models based on the quasiparticle-random-phase approximation, relativistic (RQRPA) and nonrelativistic (QRPA), the position and the distribution of the PDR have been investigated [13][14][15][16]. From the analysis of transition densities, the unique behavior of the PDR mode is revealed, making it distinct from the well-known giant dipole resonance (GDR) [17]. The systematic studies of the PDR over isotonic and isotopic chains of nuclei indicate a correlation between the observed total BðE1Þ strength of the PDR and the neutron-to-proton ratio N=Z [5,8,14,15]. In addition, it has been suggested that the PDR is independent of the type of nucleon excess (neutron or proton) [13,15].The existence of the PDR mode near the neutron threshold has also important astrophysical implications. For example, the reaction rate of the (, n) and (n, ) reactions in explosive nucleosynthesis of certain neutron deficient heavy nuclei may be significantly enhanced by the PDR [18]. Furthermore, for very neutron-rich exotic nuclei, the PDR is an important topic of study at the new generation of radioactive ion beam facilities [19].In many cases the interpretation of the PDR excitation is based on the assumption of negative parity for the majority of the J ¼ 1 states. However, there has not been a systematic experimental verification that all the dipole states observed in the entire PDR region are indeed 1 À states. The parity was measured directly in certain energy intervals, e.g., off-axis bremsstrahlung or Compton polarimetry [20]. The advantage of using 100% linearly polarized photon beams for parity identification has been recently demonstrated [21][22][23][24], which opens new opportunities for unveiling the character of the nuclear dipole response.On the other hand, in heavy mass nuclei there should be M1 strength located in the same excitation energy region as the PDR, i.e., in the low-energy tail of the GDR [17]. A major ex...

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J(π)=1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ(N)(2). The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Properties of low-energy dipole states in232 Th have been investigated with the nuclear resonance fluorescence technique. The present work used mono-energetic γ-ray beams at energies of 2 -4 MeV from the high-intensity γ-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 Th were determined and compared with quasi-particle random phase approximation calculations. Large number of E1 transitions were observed for the first time in actinide nuclei with summed strength of 3.28(69)×10−3 e 2 fm 2 . The observed summed M 1 strength of 4.26(63) µ 2 N is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei.