The gamma-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 La-139 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 gamma-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 La-139 and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

In nuclear network calculations especially the p-nucleus 92Mo is frequently underproduced. Since experimental data of the photodisintegration of 92Mo so far do not exist, it was necessary to measure the reaction yields with the photoactivation method using the brems-strahlung facility ELBE at FZ Dresden-Rossendorf. Also the reaction 100Mo(γ,n) could be tested while irradiating Mo samples of natural composition. The photon fluence was measured with nuclear resonance fluorescence (NRF) of strong and well-known transitions in 11B. As a normalization standard the photoactivation reaction 197Au(γ,n) was used. For the 65 s half-life of 91mMo a fast pneumatic delivery was used. The photoactivation yields of the (γ,p) and (γ,n) reactions are compared to yield integrals of the bremsstrahlung spectral shape folded with the cross sections derived from Hauser-Feshbach nuclear model calculations using the TALYS program and cross sections from earlier experiments.

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. Recent studies show that extra γ‐ray strength near the neutron separation energy Sn (pygmy resonance) affects the stellar reaction rate strongly. In this work, the photoabsorption cross section for 139La below Sn was measured using bremsstrahlung produced at the electron accelerator ELBE of Eorschungszentrum Dresden‐Rossendorf with an electron beam of 11.5 MeV kinetic energy. Experimental result of 139La is presented.

A. Makinaga,1,* R. Schwengner,2 G. Rusev,3 F. Dönau,2 S. Frauendorf,2,4 D. Bemmerer,2 R. Beyer,2 P. Crespo,2,† M. Erhard,2,‡ A. R. Junghans,2 J. Klug,2,§ K. Kosev,2 C. Nair,2,‖ K. D. Schilling,2 and A. Wagner2 1Department of Physics, Konan University, Okamoto 8-9-1, Higashinada, Kobe 658-8501, Japan 2Institut für Strahlenphysik, Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany 3Department of Physics, Duke University, and Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA 4Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA (Received 4 June 2010; published 16 August 2010)

In explosive nucleosynthesis temperatures are high enough for photodisintegration reactions to occur, e.g. leading to the production of p-process nuclei. I n order to understand the reaction rates of element production and element disruption we start ed an experimental program at the new bremsstrahlung facility of the superconducting electr on accelerator ELBE of FZ DresdenRossendorf. The bremsstrahlung facility and the detector s etup are designed such that the scattering of photons from nuclei and the photodisintegration o f uclei around the particle separation energies can be studied under optimized background conditi s. In activation measurements with bremsstrahlung at end-point energy from 10.0 to 16.5 MeV ( γ,p), (γ,n) and (γ,α) reactions of 92,100Mo have been studied. Our activation yields can be described within a factor 2-3 or better with calculations using the cross sections from recent Haus er-Feshbach models.

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 photoactivation. After a proper correction for multistep processes the observed quasicontinuous spectra of scattered photons show a remarkably good match to the photon strengths derived from nuclear photoeffect data obtained previously by neutron detection and corrected in absolute scale by 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 parametrization developed previously which is based on it. This parametrization 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 cosmic nucleosynthesis. It also has impact on the accurate analysis of neutron capture data of importance for future nuclear energy systems and waste transmutation.

Dipole‐strength distributions in the stable even‐mass molybdenum isotopes up to the neutron‐separation energies have been studied in photon‐scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE at the Research Center Dresden‐Rossendorf, Germany, and with mono‐energetic photon beams at the High Intensity Gamma‐ray Source facility at Triangle Universities Nuclear Laboratory. In order to determine the dipole‐strength distribution, statistical methods were developed for the analysis of the measured spectra. The data obtained for the stable even‐mass molybdenum isotopes from the present (γ,γ’) experiments are combined with (γ,n) cross sections from the literature resulting in a photoabsorption cross section covering the full range from about 4 to 15 MeV, which is of interest for nuclear structure as well as for nuclear astrophysics network calculations. Novel information about the low‐energy tail of the Giant Dipole Resonance and the energy spreading of its strength is der...

The dipole response of the $N=50$ nucleus $^{90}\mathrm{Zr}$ 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 ($\ensuremath{\gamma}$, $n$) 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 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 heavy neutron deficient p-nuclei are produced in explosi ve stellar environments via photo- disintegration reactions like ( γ,n), (γ,p) and (γ,α) on r- or s- seed nuclei. The reaction rates of p-nuclei are mostly based on theoretical parameterizations using statistical model calculations. We study experimentally the photodisintegration rates of heavy nuclei at the bremsstrahlung fa- cility of the superconducting electron accelerator ELBE of FZ Dresden-Rossendorf. Photoactiva- tion measurements on the astrophysically relevant p-nuclei 92Mo and 144Sm have been performed with bremsstrahlung end-point energies from 10.0 to 16.5 MeV. The activation yields are com- pared with calculations using cross sections from recent Hauser-Feshbach models. The sensitivity of the statistical models to the input ingredients like phot on strength function, optical potentials are tested against the experimental activation yield.

The ${}^{197}\mathrm{Au}(\ensuremath{\gamma},n)$ reaction is used as an activation standard for photodisintegration studies on astrophysically relevant nuclei. At the bremsstrahlung facility of the superconducting electron accelerator ELBE (Electron Linear accelerator of high Brilliance and low Emittance) of Forschungszentrum Dresden-Rossendorf, photoactivation measurements on $^{197}\mathrm{Au}$ have been performed with bremsstrahlung endpoint energies from 8.0 to 15.5 MeV. The measured activation yield is compared with previous experiments as well as with calculations using Hauser-Feshbach statistical models. It is shown that the experimental data are best described by a two-Lorentzian parametrization with taking the axial deformation of $^{197}\mathrm{Au}$ into account. The experimental ${}^{197}\mathrm{Au}(\ensuremath{\gamma},n)$ reaction yield measured at ELBE via the photoactivation method is found to be consistent with previous experimental data using photon scattering or neutron detection methods.

Electromagnetic dipole-strength distributions up to the particle separation energies are studied for the stable even–even nuclides 92,94,96,98,100Mo in photon-scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf. The influence of inelastic transitions to low-lying excited states has been corrected by a simulation of γ cascades using a statistical model. After corrections for branching ratios of ground-state transitions, the photon-scattering cross sections smoothly connect to data obtained from (γ, n) reactions. With the newly determined electromagnetic dipole response of nuclei well below the particle separation energies, the parametrization of the isovector giant dipole resonance is done with improved precision.

R. Schwengner ∗a, G. Rusev a, N. Benouaret a,b, R. Beyer a, F. Dönau a, M. Erhard a, E. Grosse a,c, A. R. Junghans a, J. Klug a, K. Kosev a, C. Naira, N. Nankov a,d, K. D. Schilling a and A. Wagner a aInstitut für Strahlenphysik, Forschungszentrum DresdenRossendorf, 01314 Dresden, Germany bFaculté de Physique, Université des Sciences et de la Techno logie d’Alger, 16111 Bab-Ezzouar-Alger, Algerie cInstitut für Kernund Teilchenphysik, Technische Univers ität Dresden, 01062 Dresden, Germany dInstitute for Nuclear Research and Nuclear Energy, BAS, 178 4 Sofia, Bulgaria E-mail: r.schwengner@fzd.de

Dipole-strength distributions in the nuclides Mo-92, Mo-98 and Mo-100 have been investigated in photon-scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. A simulation of gamma cascades was performed in order to estimate the distribution of inelastic transitions to low-lying states and thus to deduce the primary dipole-strength distribution up to the neutron-separation energies. The absorption cross sections obtained connect smoothly to (gamma, n) cross sections and give novel information about the low-energy tail of the Giant Dipole Resonance below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation (QRPA) in a deformed basis. Photoactivation experiments were performed at various electron energies to study the 92 Mo(gamma, n), Mo-92(gamma,p), 92 Mo(gamma, n) and Mo-100(gamma, n) reactions. The deduced activation yields are compared with theoretical predictions.

The ELBE electron beam at Forschungszentrum Rossendorf, Dresden, with energies up to 40 MeV, will be used to produce intense neutron beams.The neutron radiator consists of a liquid * Speaker PoS(FNDA2006)015 PoS(FNDA2006)015 Development of a neutron ToF source at the ELBE accelerator J. Klug * Electron Linear accelerator with high Brilliance and low Emittance PoS(FNDA2006)015 PoS(FNDA2006)015 Development of a neutron ToF at the ELBE accelerator