The dipole response of the N = 50 nucleus 90 Zr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 7.9, 9.0, and 13.2 MeV. We identified 189 levels up to an excitation energy of 12.9 MeV. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (γ, n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range of 6 MeV to 11 MeV. Calculations within the framework of the quasiparticle-phonon model ascribe this strength to a vibration of the excessive neutrons against the N = Z neutron-proton core, giving rise to a pygmy dipole resonance.

A global study of the electric dipole strength in and below the isovector giant dipole resonance (GDR) is presented for mass numbers A>80. It relies on the recently established remarkably good match between data for the nuclear photo effect to novel photon scattering data covering the region below the neutron emission threshold as well as by average resonance neutron capture (ARC). From the wide energy coverage of these data the correlation of the GDR spreading width with energy can be studied with remarkable accuracy. A clear sensitivity to details of the nuclear shape, i.e. the β-and γ-deformations, is demonstrated. Based hereon a new parameterization of the energy dependence of the nuclear electric-dipole strength is proposed which -with only two new parameters -allows to describe the dipole strength in all heavy nuclei with A>80. Although it differs significantly from previous parameterizations it holds for spherical, transitional, triaxial and well deformed nuclei. The GDR spreading width depends in a regular way on the respective resonance energy, but it is independent of the photon energy.Key properties of nuclei are their mass and shape as well as their response to electromagnetic radiation. Photo-nuclear processes were among the first nuclear reactions studied [1] and their appreciable strength has triggered the conclusion [2] that they are likely to play an important role for the cosmic nucleosynthesis: In the intense photon flux during high temperature cosmic scenarios particle emission thresholds are reached leading to the photo-disintegration of previously formed nuclides. For a full assessment of photon induced processes the knowledge of the underlying smooth strength is similarly important as the "pygmy" structures observed in that energy range [3][4][5]. Finally, photon strength functions influence not only cosmic processes but they also are of importance for the detailed understanding of radiative neutron capture [6]: To analyze γ-spectra following capture the photon strength has to be known up to threshold. A detailed knowledge of neutron-induced processes is of practical importance for future systems dedicated to transmute nuclear waste as well as new concepts on nuclear reactors.The electric dipole strength in heavy nuclei is mainly concentrated in the isovector giant dipole resonance (GDR). The centroid energy E 0 of the GDR is related to the symmetryenergy constant J and the surface-stiffness Q -as determined in a fit of the finite range droplet model (FRDM) [7] to the nuclear masses -with the effective nucleon mass m* as an additional parameter [8]. The energy-integrated dipole strength is determined by rather general quantum mechanical considerations leading to the Thomas-Reiche-Kuhn (TRK) sum rule [9]. Thus it is mainly the width of the GDR and its detailed shape which are of interest for further study. Very recently a covariant calculation based on density functional theory has resulted in a satisfactory description of the GDR for spherically symmetric nuclei [10]. Although equi...

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 dipole response of the magic N = 50 nucleus 88 Sr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0, 13.2, and 16.0 MeV. We identified 160 levels up to an excitation energy of 12 MeV. By using polarized photons linear polarizations of about 50 γ transitions were measured that enabled parity assignments to the corresponding states. In the energy range of 6-12 MeV we identified only one M1 transition; all other transitions have E1 character. Thus, E1 character was proven for 63% of the total dipole strength of the observed levels in the given energy range. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (γ, n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength at 6-11 MeV may be considered as an indication for a pygmy dipole resonance.