Scheikh Obeid, Abdulrahman (2013)
Excitation strengths and transition radii differences of one-phonon quadrupole excitations from electron scattering on 92,94Zr and 94Mo.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In the framework of this thesis electron scattering experiments on low-energy excitations of 92Zr and 94Zr were performed at the S-DALINAC in a momentum transfer range q = 0.3 − 0.6 fm^−1. The nature of one-phonon symmetric and mixed-symmetric 2+ and 3− states of 92Zr was investigated by comparison with predictions of the quasi-particle phonon model (QPM). Theoretical (e, e′) cross sections have been calculated within the distorted wave Born approximation(DWBA) to account for Coulomb distortion effects. The reduced strengths of the one-quadrupole phonon states and the one-octupole phonon state have been extracted. The similarity of the momentum-transfer dependence of the form factors between the 2+ states supports the one-phonon nature of the 2+2 state of 92Zr. A new method based on the Plane Wave Born Approximation (PWBA) for a model-independent determination of the ratio of the E2 transition strengths of fully symmetric (FSS) and mixed-symmetry (MSS) one-phonon excitations of heavy vibrational nuclei is introduced. Due to the sensitivity of electron scattering to charge distributions, the charge transition-radii difference can be determined. The basic assumptions (independence from the ratio of Coulomb corrections and from absolute values of transition radii) are tested within the Tassie model, which makes no specific assumptions about the structure of the states other than collectivity. It is shown that a PWBA analysis of the form factors, which usually fails for heavy nuclei, can nevertheless be applied in a relative analysis. This is a new promising approach to determine the ground state transition strength of the 2+ MSS of vibrational nuclei with a precision limited only by the experimental information about the B(E2; 2+1 → 0+1) strength. The PWBA approach furthermore provides information about differences of the proton transition radii of the respective states, containing independent information about the mixed-symmetry character of 2+ states and the sign change of leading valence shell components between FSS and MSS. For 92Zr, the proton transition radii difference of the 2+1 and 2+2 states agree with zero within about 0.5 fm, consistent with predictions of a sign change of the leading neutron component in the wave function of the mixed-symmetry 2+2 state, while the leading proton components remain unchanged. The extracted B(E2) strength of the transition of the 2+ 2 state of 94Zr using the new method is smaller than the B(E2) value of the 2+1 state in contrast to a previous but in agreement with a recent (n,n′γ) experiment. The transition radii difference in 94Zr is −0.24(34) fm compatible with zero. Additional data available on 94Mo were analyzed showing that the momentum transfer dependence of Coulomb corrections leads to a difference of more than 10% between the lowest and the highest measured momentum transfer. If corrected by the Tassie model, a transition radii difference R = −0.13(5) fm differing from zero is found. This can be interpreted as a sign of an increase of the leading proton two-quasiparticle component in the wave function of the mixedsymmetry 2+ state. The B(E2) ratio for 94Mo, 0.193(1), differs from literature, 0.138(2).

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