A. Krugmann

Dipole polarizability of 120 Sn and nuclear energy density functionals

The electric dipole strength distribution in 120Sn between 5 and 22 MeV has been determined at RCNP Osaka from a polarization transfer analysis of proton inelastic scattering at E_0 = 295 MeV and forward angles including 0{\deg}. Combined with photoabsorption data an electric dipole polarizability \alpha_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established \alpha_D(208Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.

Low-energy electric dipole response in 120Sn

Electric dipole strength in 120 Sn below the neutron threshold has been extracted from proton inelastic scattering experiments at Ep = 295 MeV and at forward angles including 0 . The strength distribution is very dierent from the results of a 120 Sn(; 0 ) experiment and peaks at an excitation energy of 8.3 MeV. The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and is more than three times larger than what is observed with the (; 0 ) reaction. This implies a strong fragmentation of the E1 strength and/or small ground state branching ratios of the excited 1 states.

First Measurement of Collectivity of Coexisting Shapes Based on Type II Shell Evolution: The Case of ^{96}Zr.

BACKGROUND Type II shell evolution has recently been identified as a microscopic cause for nuclear shape coexistence. PURPOSE Establish a low-lying rotational band in ^{96}Zr. METHODS High-resolution inelastic electron scattering and a relative analysis of transition strengths are used. RESULTS The B(E2;0_{1}^{+}→2_{2}^{+}) value is measured and electromagnetic decay strengths of the 2_{2}^{+} state are deduced. CONCLUSIONS Shape coexistence is established for ^{96}Zr. Type II shell evolution provides a systematic and quantitative mechanism to understand deformation at low excitation energies.

Wavelet signatures of K -splitting of the Isoscalar Giant Quadrupole Resonance in deformed nuclei from high-resolution (p,p ′ ) scattering off 146, 148, 150 Nd

Abstract The phenomenon of fine structure of the Isoscalar Giant Quadrupole Resonance (ISGQR) has been studied with high energy-resolution proton inelastic scattering at iThemba LABS in the chain of stable even-mass Nd isotopes covering the transition from spherical to deformed ground states. A wavelet analysis of the background-subtracted spectra in the deformed 146, 148, 150 Nd isotopes reveals characteristic scales in correspondence with scales obtained from a Skyrme RPA calculation using the SVmas10 parameterization. A semblance analysis shows that these scales arise from the energy shift between the main fragments of the K = 0 , 1 and K = 2 components.

Shell evolution and E2 collectivity: new spectroscopic information

The way how the complex nuclear forces form atomic nuclei from their constituent protons and neutrons represents the core topic of nuclear structure research. The evolution of nuclear structure across the nuclear chart is dominated by the evolution of single-particle orbitals as a function of the numbers of protons and neutrons that make up the nuclei. The understanding of shell evolution has been pushed especially by the Tokyo group around T. Otsuka. It is supported by vast experimental evidence in particular from the structure of neutron-rich nuclei that have become accessible due to new facilities for intense beams of radioactive ions. Recently, the concept of Type II Shell Evolution has been introduced. It emphasizes, that the effective single-particle energies of nucleon orbitals systematically depend on the nucleonic configuration in each individual nuclear state due to the mutual interaction between the nucleons. This general mechanism can be particularly pronounced in key-nuclei with certain numbers of protons and neutrons, for which Type II Shell Evolution may lead to drastic nuclear structure effects such as even shape coexistence and first-order shape phase transitions like in the chain of Zr isotopes [1]. We report here on the first evidence [2] for shape coexistence caused by Type II Shell Evolution which is firmly based on the measurement of absolute E2 transition rates. The data have been obtained in highresolution inelastic electron scattering spectroscopy of the nucleus Zr at the Superconducting Darmstadt Linear electron Accelerator (S-DALINAC).The data [2] will be presented and the mutual impact of proton and neutron valence-shell configurations will be discussed with respect to the formation of collective structures. Further information on the proton-neutron degree of freedom of collective valence-shell excitation can be obtained from the properties of the scissors mode in deformed nuclei. While it originates from the nuclear quadrupole deformation its dominant decay mode is by enhanced M1 transitions due to its isovector character. Infact, the E2 decay strength of the scissors mode of deformed nuclei has been unknown until recently. We have recently measured the (small) E2 decay strength of the J = 1 band head of the scissors mode to the ground state band by γ-ray polarimetry in Nuclear Resonance Fluorescence [3]. The data will be presented and discussed. *We thank all those who have contributed to the experiments, in particular the accelerator crew, students, and doctoral researchers at the S-DALINAC and in the spectrometer group at TU Darmstadt, and those at HIγS, TUNL. We thank T. Otsuka for numerous discussions. We gratefully acknowledge the financial support by the German Research Council (DFG) under grant No. SFB 1245 and by the German Ministry for Education and Research (BMBF) under grant No. 05P15RDEN9. [1] T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016). [2] C. Kremer et al., Phys. Rev. Lett. 117, 172503 (2016). [3] T. Beck et al., Phys. Rev. Lett. (2017), to be published.

High precision electron scattering off 96Zr

We report on our recent electron scattering experiment off 96Zr. The B (E2; 01 +→22 +) transition strength is extracted in a relative analysis and allows the determination of all decay strengths of the 22 + state. The strong collectivity of the 22 + → 02 + transition establishes the 02 + state and the states build on top of it as an isolated deformed band.

E2 strengths and transition radii difference of one-phonon 2 + states of 92 Zr from electron scattering at low momentum transfer

Background: Mixed-symmetry 2 + states in vibrational nuclei are characterized by a sign change between dominant proton and neutron valence-shell components with respect to the fully symmetric 2 + state. The sign can be measured by a decomposition of proton and neutron transition radii with a combination of inelastic electron and hadron scattering [C. Walz et al., Phys. Rev. Lett. 106, 062501 (2011)]. For the case of 92 Zr, a difference could be experimentally established for the neutron components, while about equal proton transition radii were indicated by the data. Purpose: Determination of the ground-state (g.s.) transition strength of the mixed-symmetry 2 + state and verification of the expected vanishing of the proton transition radii difference between the one-phonon 2 + states in 92 Zr. Method:Differential cross sections for the excitation of one-phonon 2 + and 3 − states in 92 Zr have been measured with the (e, e � ) reaction at the S-DALINAC in a momentum transfer range q � 0.3‐0. 6f m −1 . Results: Transition strengths B(E2;2 + → 0 + ) = 6.18(23), B(E2;2 + → 0 + ) = 3.31(10), and B(E3;3 − → 0 + ) = 18.4(1.1) Weisskopf units are determined from a comparison of the experimental cross sections to quasiparticle-phonon model (QPM) calculations. It is shown that a model-independent plane wave Born approximation (PWBA) analysis can fix the ratio of B(E2) transition strengths to the 2 +,2 states with a precision of about 1%. The method furthermore allows to extract their proton transition radii difference. With the present /

Direct Evidence for Proton-Neutron Out-of-Phase Coupling in Mixed-Symmetry States from Scattering Experiments on92Zr

The coupling of the giant quadrupole resonance to valence space configurations is shown to dominantly contribute to the formation of low-lying quadrupole collective structures in vibrational nuclei with symmetric and mixed-symmetric character with respect to the proton-neutron degree of freedom. Experimental evidence is obtained from electron- and proton scattering experiments on the nucleus 92Zr that are directly sensitive to the relative phase of valence space amplitudes by quantum interference.

Electric dipole response in 120Sn

In high-resolution (,) experiments under 0° the complete B(E1) strength distribution can be studied in stable nuclei. At the Research Center of Nuclear Physics in Osaka, Japan, the cross sections and observables for the polarization transfer of E1 and M1 excitations in 120Sn were measured for scattering angles Θ = 0° − 4° in an excitation energy range of 5 - 25 MeV. From the present measurement the complete B(E1) strength distribution and the branching ratios of the PDR to the ground state can be extracted. The experimental setup, the principle of backgound subtraction and first results on the E1 strength are presented.

Electric dipole response in Sn-120

In high-resolution (,) experiments under 0° the complete B(E1) strength distribution can be studied in stable nuclei. At the Research Center of Nuclear Physics in Osaka, Japan, the cross sections and observables for the polarization transfer of E1 and M1 excitations in 120Sn were measured for scattering angles Θ = 0° − 4° in an excitation energy range of 5 - 25 MeV. From the present measurement the complete B(E1) strength distribution and the branching ratios of the PDR to the ground state can be extracted. The experimental setup, the principle of backgound subtraction and first results on the E1 strength are presented.