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.

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.

Two methods based on bremsstrahlung were applied to the stable even Mo isotopes for the experimental determination of the photon strength function covering the high excitation energy range above 4 MeV with its increasing level density. Photon scattering was used up to the neutron separation energies S n and data up to the maximum of the isovector giant resonance (GDR) were obtained by photo-activation. After a proper correction for multi-step processes the observed quasi-continuous spectra of scattered photons show a remarkably good match to the photon strengths derived from nuclear photo effect data obtained previously by neutron detection and corrected in absolute scale using the new activation results. The combined data form an excellent basis to derive a shape dependence of the E1 strength in the even Mo isotopes with increasing deviation from the N = 50 neutron shell, i.e. with the impact of quadrupole deformation and triaxiality. The wide energy coverage of the data allows for a stringent assessment of the dipole sum-rule, and a test of a novel parameterization developed previously which is based upon. This parameterization for the electric dipole strength function in nuclei with A>80 deviates significantly from prescriptions generally used previously. In astrophysical network calculations it may help to quantify the role the p-process plays in the cosmic nucleosynthesis. It also has impact on the accurate analysis of neutron capture data of importance for future nuclear energy systems and waste transmutation.PACS numbers: 24.30. Cz, 26.50.+x, 27.60.+j, 28.90.+i. Temporary ID: es2009jun29_093 + ) now at INFN, Padova, Italy *) now at Vattenfall, Stockholm, Sweden I. INTRODUCTION: DIPOLE STRENGTH IN HEAVY NUCLEIThe response to electromagnetic radiation plays an important role not only for the fundamental understanding of nuclei but also for the de-excitation processes following nuclear reactions.Details of the dipole strength may significantly affect the de-excitation path after neutron capture in heavier nuclei and it is thus of importance for calculations designed to predict properties of advanced nuclear systems and devices aiming for the transmutation of radioactive nuclear waste.A similar impact is expected on calculations for the cosmic nucleosynthesis, especially for high temperature scenarios where photo-nuclear processes are likely to play an important role. The socalled p-process may be the origin of more than 30 neutron-deficient nuclides not resulting from neutron-capture reactions. They may eventually be produced in the intense photon flux leading to the photo-disintegration of previously formed heavier nuclides [1,2].Nuclear photo-effect calculations are usually based on the statistical Hauser-Feshbach theory [2] and these need information on the photon strength functions λ f . As the electric dipole mode E1 contributes the most, the photo-disintegration cross sections like ( γ ,n) are often used as a measure for the electric dipole strength above the neutron threshold S n . Above the particl...

The γ -ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. To test model predictions, the photoabsorption cross section of 139 La up to the neutron-separation energy was measured using bremsstrahlung produced at the electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf with an electron beam of 11.5 MeV kinetic energy. The experimental data were analyzed by applying Monte Carlo simulations of γ -ray cascades to obtain the intensities of the ground-state transitions and their branching ratios. We found a significant enhancement of electric dipole strength in the energy range from 6 to 10 MeV that may be related with a pygmy dipole resonance. The present data are combined with photoneutron cross sections for 139 La and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

Double-differential cross sections for light-ion (p, d, t, 3 He and α) production in oxygen induced by 96 MeV neutrons are reported. Energy spectra are measured at eight laboratory angles from 20• to 160• in steps of 20• . Procedures for data taking and data reduction are presented. Deduced energy-differential and production cross sections are reported. Experimental cross sections are compared to theoretical reaction model calculations and experimental data at lower neutron energies in the literature. The measured proton data agree reasonably well with the results of the model calculations, whereas the agreement for the other particles is less convincing. The measured production cross sections for protons, deuterons, tritons and alpha particles support the