Burda, Oleksiy (2008)
Nature of Mixed-Symmetry 2+ States in 94Mo from High-Resolution Electron and Proton Scattering and Line Shape of the First Excited 1/2+ State in 9Be.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The present work contains two parts. The first one is devoted to the investigation of mixed-symmetry structure in 94Mo and the second one to the astrophysical relevant line shape of the first excited 1/2+ state in 9Be. In the first part of the thesis the nature of one- and two-phonon symmetric and mixed-symmetric 2+ states in 94Mo is investigated with high-resolution inelastic electron and proton scattering experiments in a combined analysis. The (e,e′) experiments were carried out at the 169° magnetic spectrometer at the S-DALINAC. Data were taken at a beam energy Ee = 70 MeV and scattering angles Θe = 93° - 165°. In dispersion-matching mode an energy resolution ΔE = 30 - 45 keV (full width at half maximum) was achieved. The (p,p′) measurements were performed at iThemba LABS, South Africa, using a K600 magnetic spectrometer at a proton energy Ep = 200 MeV and scattering angles Θp = 4.5° - 26°. Typical energy resolutions were ΔE ≈ 35 keV. The data are compared to calculations in the quasiparticle-phonon model, shell model and interacting boson model with proton-neutron degrees of freedom. The combined analysis reveals a dominant one-phonon structure of the transitions to the first and third 2+ states, as well as an isovector character of the transition to the one-phonon mixed-symmetric state within the valence shell by the different momentum-transfer dependence in electron and proton scattering and the analysis of the microscopic wave functions. Excitation of the two-phonon states is sensitive to admixtures of one-phonon components, which are found to be small. Quantitatively consistent estimates of the one-phonon admixtures are obtained from both experimental probes when two-step contributions to the proton scattering cross sections are taken into account.In the second part of the thesis the line shape of the first excited 1/2+ state in 9Be is studied. Spectra of the in 9Be(e,e′) reaction were measured at the S-DALINAC at an electron energy Ee = 73 MeV and scattering angles of 93° and 141° with high energy resolution up to excitation energies Ex = 8 MeV. The form factor of the first excited state has been extracted from the data. The momentum transfer dependence of its form factor is well reproduced by the modern no-core shell model calculations. The astrophysical relevant 9Be(γ,n) cross sections have been extracted from the (e,e′) data. The resonance parameters of the first excited 1/2+ state in 9Be are derived in a one-level R-matrix approximation. The deduced resonance parameters averaged over all available (e,e′) data are ER = 1.748(6) MeV and ΓR = 274(8) keV in agreement with the latest direct (γ,n) experiment. However, the extracted B(E1) strength is a factor of two smaller than found in the latter indicating a violation of Siegert′s theorem at the photon point.

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