The nuclides Mo-92, Mo-98, and Mo-100 have been studied in photon-scattering experiments by using bremsstrahlung produced at an electron energy of 6 MeV at the ELBE accelerator of the Forschungszentrum Rossendorf and at electron energies from 3.2 to 3.8 MeV at the Dynamitron accelerator at the University of Stuttgart. Six dipole transitions in Mo-98 and 19 in Mo-100 were observed for the first time in the energy range from 2 to 4 MeV. The experimental results are compared with predictions of the shell model and with predictions of the quasiparticle random-phase approximation (QRPA) in a deformed basis. The latter show significant contributions of isovector-orbital and isovector-spin vibrations. The change of the magnetic dipole strength in the isotopic chain of the even-mass isotopes from Mo-92 to Mo-100 is discussed. The calculations within the QRPA are extrapolated to the particle-separation energies to estimate the possible influence of M1 strength on the stability of the nuclides against photodissociation in cosmic scenarios.

Systematic nuclear resonance fluorescence (NRF) experiments on all nine stable (seven even-even and two odd-mass) Xe isotopes have been performed at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator. For the first time thin-walled, high-pressure gas targets (about 70 bar) with highly enriched target material were used in NRF experiments. Precise excitation energies, transition strengths, spins, and decay branching ratios were obtained for numerous states, most of them previously unknown. The systematics of the observed E1 two-phonon excitations (2(+)circle times 3(-)) and M1 excitations to 1(+) mixed-symmetry states in the even-even isotopes are discussed with respect to the new critical point symmetry E(5). The fragmentation of these fundamental dipole excitation modes in the odd-mass isotopes Xe-129,Xe-131 is shown and discussed. In the even-even nuclei several low-lying E2 excitations were observed.

The nuclides 98Mo and 100Mo have been studied in photon-scattering experiments by using bremsstrahlung produced from electron beams with kinetic energies from 3.2 to 3.8 MeV. Six electromagnetic dipole transitions in 98Mo and 19 in 100Mo were observed for the first time in the energy range from 2 to 4 MeV. A specific feature in the two nuclides is the de-excitation of one state with spin J = 1 to the 0+ ground state as well as to the first excited 0+ state, which cannot be explained in standard models. We present a model that allows us to deduce the mixing coefficients for the two 0+ shape-isomeric states from the experimental ratio of the transition strengths from the J = 1 state to the 0+ ground state and to the 0+ excited state.

A new facility for the production of polarized bremsstrahlung has been built at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. The bremsstrahlung facility and the detector setup are designed such that the background radiation due to scattering of photons and production of neutrons is minimized allowing for experiments close to and above particle separation energies in nuclei. First results of photon-scattering and photo-dissociation experiments on 92,98,100Mo are presented. The results are compared to recent cross-section calculations for astrophysical networks.

Dipole and quadrupole excitations in the semimagic N=50 nucleus Sr-88 were investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC with bremsstrahlung of an end-point energy of 6.8 MeV. Many new dipole excitations could be identified, and their reduced excitation probabilities were determined. The experimental findings are discussed in the context of quasiparticle-phonon-model and shell-model calculations. A breaking of the N=50 core is essential to describe the structure of the observed excitations. The two-phonon quadrupole-octupole J(pi)=1(-) state exhibits unusual features which are presently not understood.

Dipole and electric quadrupole excitations in Pb-204,Pb-206,Pb-207,Pb-208 have been measured up to 6.75 MeV in resonant photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC using two Euroball-Cluster detector modules. In Pb-208, 14 excited states have been populated; in Pb-206, the decays of 41 states have been detected. Information about 45 heretofore unknown excited states in Pb-204 could be measured as well as eleven known levels in Pb-207. The extracted dipole strength distributions are discussed within phenomenological ("pygmy resonance") and microscopic models (quasiparticle-phonon model). A strong fragmentation and a small shift of the detected E1 strength towards higher energies is observed with the opening of the neutron shell closure. (C) 2003 Published by Elsevier B.V.

A nuclear resonance fluorescence experiment with two highly efficient EUROBALL Cluster detectors has been performed on the γ-soft nucleus 194Pt. Dipole excitations were observed between 2 and 4 MeV excitation energy. They are tentatively interpreted as the main fragments of the scissors mode based on the measured excitation strengths and a comparison to microscopic calculations in the framework of the quasiparticle random phase approximation (QRPA). The data indicate large differences to the neighbouring isotope 196Pt: a doubling of the observed dipole strength and a shift of the energy centroid by about 600 keV. None of the currently available models is able to reproduce these features consistently in both nuclei.

Excited states of the nucleus Br-79 were investigated via the reaction Ge-76(Li-7,4n) at a beam energy of 35 MeV. Coincidence data of emitted gamma rays were measured with an arrangement of six EUROBALL CLUSTER detectors. The E2 bands built on the 9/2(+) and 3/2(-) states were extended up to J=37/2 at Eapproximate to8.8 MeV. The M1 band starting with a 15/2(-) state at 2.6 MeV was observed up to J=(29/2) at E=6.4 MeV. Crossover E2 transitions within this band were observed for the first time. Mean lifetimes of 17 levels were deduced using the Doppler-shift-attenuation method. The M1 band can be described within the tilted-axis-cranking model on the basis of the tilted three-quasiparticle configuration pi(g(9/2)) nu(g(9/2)) nu(fp) which has a triaxial shape. This band appears as a mixed case including contributions of both magnetic and collective rotation.

The nucleus Se-70 was studied using the Ca-40(Ca-40, 2alpha2p) reaction at a beam energy of 185 MeV. Gamma rays were measured with the EUROBALL III spectrometer. The known positive-parity bands have been extended and one new band of positive parity and two of negative parity have been identified. These bands are interpreted in terms of the cranked Nilsson-Strutinsky approach. Calculations suggest that the two negative-parity bands, which have the same signature, are both based on a configuration with two protons and three neutrons lifted from the fp shell to the g(9/2) orbital, but at different nuclear shapes. This represents a shape coexistence at high spin.

Dipole excitations in the semimagic N=50 nucleus Rb-87 were investigated at the Stuttgart Dynamitron facility using bremsstrahlung with an end-point energy of 4.0 MeV. The widths Gamma or the reduced excitation probabilities B(Pi1)up arrow of 18 states were determined for the first time. The magnetic dipole excitations are well reproduced in the framework of the shell model, however, these calculations cannot describe the observed electric dipole excitations. The 1/2(+) state at 3060 keV is proposed to be the weak coupling of an f(5/2) proton hole to the 3(-) octupole vibrational state in the N=50 core Sr-88. The relatively strong E1 transition from that state to the ground state is explained as mainly the neutron h(11/2)-->g(9/2) transition. The breakup of the N=50 core and neutron excitations into the h(11/2) shell are essential to describe electric dipole excitations, but neutron-core excitations do not play an important role for the structure of magnetic dipole excitations.

Dipole excitations in the spherical nucleus Sr-88, observed in the energy region 4.5 MeV less than or equal to E-x less than or equal to 5.5 MeV, are proposed to be scissors-like excitations on top of the dynamically deformed quadrupole vibrational state.