T. Suomijärvi

Low-lying collective states in neutron-rich oxygen isotopes via proton scattering ☆

Abstract Proton elastic and inelastic scattering angular distributions to the 2+1 and the 3−1 states for the neutron-rich nucleus 20 O were measured with a secondary beam using the MUST silicon strip detector array. Data for 18 O were also obtained for comparison. A phenomenological analysis has been used to deduce the deformation parameters βp,p′ for the collective excitations. Matter and transition densities were generated from self-consistent QRPA calculations. DWBA calculations using microscopic optical potentials obtained with these densities and the JLM interaction are compared to the data. The isovector character of the 2+1 state in 20 O is confirmed and predictions are discussed for the properties of the heavier neutron-rich oxygen isotopes.

Proton elastic scattering on light neutron-rich nuclei

Abstract Proton-nucleus elastic scattering angular distributions have been measured for 6 He, 7 Li, 10 Be, 11 Be secondary beams. These data, together with other elastic scattering data for proton on unstable nuclei, have been analyzed using phenomenological and microscopic optical model approaches. To reproduce the experimental angular distributions for unstable nuclei, it was necessary to renormalize either the real or imaginary part of the optical potential. The influence of the break-up channels on the results is discussed.

Coupling effects in the elastic scattering of the exotic nucleus 6He on protons

Abstract Cross sections for the elastic scattering of 6He radioactive nuclear beam on proton targets have been measured at 38.3 MeV/nucleon. With a view to test the ability of general optical potentials to reproduce the data for scattering of unstable nuclei, the present results, as well as other existing data for 6,8He, have been analyzed within the framework of the microscopic Jeukenne–Lejeune–Mahaux nucleon–nucleus potential. The angular distributions were found to be best reproduced by reducing the real part of the optical potential. This renormalization can be seen as a consequence of the complex polarization potential produced by the coupling to the continuum due to the weakly bound nature of the unstable nuclei. This effect can be simulated in a phenomenological analysis by a surface potential.

Proton scattering on the radioactive nucleus 20O and the 0gs+→21+ transition in the neutron-rich oxygen isotopes

Abstract Elastic and inelastic proton scattering to the 21+ state of the single closed shell radioactive nucleus 20 O have been measured in inverse kinematics with a beam energy of 30 MeV/u. The matrix element determined for the 0gs+→21+ transition in this work is compared with the corresponding electromagnetic matrix element to determine the ratio of the neutron and proton multipole matrix elements for this transition, Mn/Mp=2.9(4), which is quite different from the value Mn/Mp=N/Z=1.5 that would be expected for a purely isoscalar transition. A comparison of this result to a corresponding result for 18 O suggests that the strength of the core polarizing interactions between the valence neutrons and the core protons and neutrons is changing significantly with mass.

EXCITATION OF GIANT-RESONANCES IN PB-208, SN-120, ZR-90 AND NI-60 BY 84 MEV/NUCLEON O-17 IONS

Abstract Elastic and inelastic scattering of 1435 MeV 17O ions on 208Pb, 120Sn, 90Zr and 60Ni have been measured. Hindrance in the excitation of the first 3− states is observed. Parameters of the isoscalar giant monopole and quadrupole resonances are obtained. The quadrupole resonance exhausts ∼ 55% of the energy-weighted sum rule while the observed monopole resonance corresponds to more than 100% of the sum rule. Strength is also observed at high excitation energy in all targets.

Elastic scattering of 6He and its analysis within a four-body eikonal model

Abstract We have measured the elastic scattering of a 6 He secondary beam on a 12 C target at 41.6 MeV/u. The secondary beam was produced by fragmentation with SISSI, and transported to SPEG. The combined use of SISSI and SPEG allows very good quality data to be obtained in terms of energy and angular resolution. The cross section is analysed within a 4-body (α+n+n+ 12 C) eikonal scattering model which is completely parameter-free. Very good agreement with the data is found.

Search for the signature of a halo structure in the p(6He, 6Li)n reaction

Abstract The elastic scattering p( 6 He, 6 He)p and charge exchange reaction p( 6 He, 6 Li)n have been measured in reverse kinematics with a secondary 6 He beam. The angular distributions for these reactions were obtained. In the case of the charge exchange reaction, the ratio of the cross section for the Gamow-Teller transition to the ground state, and for the Fermi transition to the isobaric analog state is a measure of the relative strength of the two components of the exchange interaction, V στ and V τ . This ratio is found to be compatible with existing systematics for stable T = 1 nuclei, and no clear signature of a halo structure was found in the present data.

Excitation of giant resonances in heavy-ion reactions at intermediate energies: Results from the 40Ar + 90Zr and 208Pb experiments at 41 MeVu☆☆☆

Abstract Giant resonances in 90 Zr and 208 Pb have been studied by inelastic scattering of 40 Ar at 41 MeV/u. Strength distributions and exhausted sum-rule values of different multipolarities are presented. The peak-to-background ratios and differential cross sections from inelastic scattering of heavy ions are compared to those obtained with light projectiles. The influence of Coulomb interaction on the excitation of giant resonances and the possibility to study high multipole resonances by intermediate-energy heavy ions are discussed.

Excitation of giant resonances in 20Ne + 90Zr and 208Pb inelastic scattering at 40 MeV/u☆

The giant resonance region in the inelastic spectra from the reactions 20Ne + 90Zr and 20Ne + 208Pb at 40 MeV/nucleon has been studied with good energy and angular resolutions. The strength distributions of the different multipolarities contributing to the cross section were obtained without making any hypothesis on the excitation energies and shapes of the resonances. In the case of 208Pb the GDR strength was found to be strongly shifted towards lower excitation energies which can be explained by the exponentially decreasing Coulomb excitation probability. Furthermore, indications for a high multipolarity component in the resonance structure were found in both reactions.

High excitation inelastic scattering induced by intermediate energy 40Ar BEAMS

Abstract The inelastic channels of the 40Ar + 40Ca, 90Zn, 120Sn and 208Pb reactions at 44 MeV/nucleon and the 40Ar + 90Zr reaction at 33 MeV/nucleon were measured with a time-of-flight spectrometer in the vicinity of the respective grazing angles. In addition to the well known giant quadrupole resonance which is strongly excited, structures at higher excitation energies are observed in all the studied reactions. The existence and regular spacing of these structures are quantitatively confirmed by auto-correlation, cross correlation and Fourier analyses. The contributions to the spectra of transfer-evaporation reactions are calculated by Monte Carlo methods and are shown to reproduce the measured backgrounds. The excitation energies of the bumps are then shown to vary smoothly with target mass A as A − 1 3 . Interpretations of this phenomenon involving giant resonance and multiphonon excitations and multi-transfer reactions are discussed.