N. Frascaria

Multiple phonon excitation in nuclei: experimental results and theoretical descriptions

Abstract This report is devoted to the studies of multiphonon excitations in nuclei, it contains in fact two reports: an experimental one presenting a review of the observation of multiphonon excitations and a theoretical one centered on the description of such states. The present knowledge about giant resonances in nuclei is first briefly presented and the relative merits of different probes to excite such states are illustrated. The existence of giant resonances built on excited states is stressed. In this context, multiphonon states, i.e. a giant resonance built on top of other giant resonances, are expected. Several paths can be followed to obtain experimental evidence for multiphonon states: inelastic nuclear and Coulomb excitations and charge exchange reactions. The status of the search for isoscalar multiphonon excitations by means of the strong nuclear potential produced by heavy ions is presented and novel experimental signatures of the double excitation of the giant quadrupole resonance are reported. Coulomb excitation induced by relativistic heavy ions is presented as another fruitful approach to the study of the double dipole and recent experimental results using the very selective (π+,π−) double charge exchange reactions are shown and discussed. Calculations for these different excitation mechanisms are presented. A review of the theoretical descriptions of the properties of the multiphonon states is then presented. Hydrodynamical and phenomenological models are first discussed. The mean-field approaches are summarized. The random phase approximation is introduced to discuss giant resonances while several extensions of the meanfield concepts are presented in order to deal with large amplitude motion and multiphonons. Finally, boson mapping techniques are used to introduce a general treatment of multiple excitations. The different approaches consistently predict that such multiple collective excitations in nuclei should closely follow a harmonic pattern. Therefore, the energy of the phonons is predicted to be additive. This conclusion allows to discuss the properties of the width of the multiphonon strength in a simple manner and the observed width is predicted to be close to the quadratic sum of the single-phonon widths. General conclusions are drawn and new prospects are discussed.

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.

The Giant Dipole Resonance in hot Sn nuclei

Abstract We have studied the Giant Dipole Resonance (GDR) built on the excited states of 112Sn formed in the fusion of 19F with 93Nb. The excitation energy of the compound nucleus 112Sn was 130 and 152 MeV. High-energy γ-rays were detected in a 7-element BaF2 cluster and 4 NaI(Tl) scintillators. Coincidences with the evaporation residues detected in two parallel-plate avalanchecounters ensured that the γ-rays were associated with fusion events. In order to study the characteristics of the GDR, we have performed analyses of the γ-ray spectra within the statistical model, using a continuous evolution of the GDR centroid energy with the mass and the temperature of the emitting nucleus, and a nuclear temperature dependent level density parameter. The theoretical prescriptions concerning the evolution of the GDR width with excitation energy have been tested. We show that, in order to reproduce the γ-spectra, the increase of the width must be more rapid than the two theoretical predictions. The width has to reach large values, of the order of 15 to 20 MeV above 100 MeV of excitation energy. These values are greater than the saturation value of about 12 MeV which has been published following the analyses using constant parameters. On this condition, the shape of the spectra can be reproduced with or without a saturation of the width. It is therefore not possible to say if the increase of the width is due to angular momentum effects or to collisional effects.

CHARGE-EXCHANGE REACTION INDUCED BY 6HE AND NUCLEAR DENSITIES

Abstract The charge-exchange reaction p( 6 He, 6 Li)n is studied in reverse kinematics with a secondary 6 He beam at 41.6 MeV/nucleon. The transition connecting the ground state of 6 He to its isobaric analog state in 6 Li has been measured and analyzed in the context of a microscopic calculation. The angular distribution is shown to be sensitive to the nuclear interaction potential and to the radii of the density distribution of 6 He. It is shown that only a coherent microscopic analysis may disentangle the different contributions.

Elastic scattering and charge exchange reaction with light neutron rich exotic beams

Abstract The elastic scattering of 6 He, 10,11 Be secondary beams on a (CH 2 ) 3 target and the charge exchange reaction p( 6 He, 6 Li)n have been measured. A microscopic optical potential was used to reproduce the proton-nucleus elastic scattering data. No clear signature of a halo structure was found in the present data for the elastic scattering and charge exchange reaction, due to the lack of large angle measurement.

Neutron decay of the excitation-energy region up to 60 MeV, excited by heavy ion scattering. (I) 208Pb

Abstract The neutron decay of the continuum in 208 Pb, excited by small-angle inelastic scattering of 84 MeV/nucleon 17 O ions in the range from 1.5° to 4.5°, has been measured. Statistical decay was found to dominate the excitation-energy interval studied, up to 60 MeV. In the excitation-energy interval of about 10 to 30 MeV a distinct direct-decay component to the hole states in 207 Pb and a decay pattern resembling pre-equilibrium decay was observed. The angular correlation of the hole-state population suggests that in addition to the direct decay to the hole states in 207 Pb, there is another, unidentified process populating the same states.

Neutron decay of the excitation-energy region up to 60 MeV, excited by heavy-ion scattering. (II) 90Zr and 124Sn

The neutron decay of the continuum in 90Zr and 124Sn, excited by inelastic scattering of 84A MeV 17O ions in the range from 1.5 to 4.5°, has been measured. Statiscal decay was found to dominated the excitation-energy interval studied, up to about 50 MeV. In the excitation-energy interval from the neutron threshold up to about 35 MeV a distinct direct-decay component to the hole states in 89Zr and 123Sn was observed. In addition, a decay pattern resembling pre-equilibrium decay was observed for 90Zr. In the excitation energy spectrum of 90Zr gated by low-energy (< 2.5 MeV) neutrons, indications of a resonance structure is observed at about 27 MeV.

The Giant Dipole Resonance at very high temperatures

Abstract Gamma-rays emitted from hot nuclei of mass around 115 and excitation energies above 300 MeV, formed in the 36 Ar + 90 Zr at 27 MeV/u, 36 Ar + 94 Zr at 32 MeV/u and 36 Ar + 98 Mo at 37 MeV/u, have been measured with the MEDEA multidetector in coincidence with evaporation residues. The γ-ray yield from the decay of the Giant Dipole Resonance is independent of excitation energy and of bombarding energy.

The Giant Dipole Resonance built on highly excited states — results of the MEDEA experiment

Abstract Gamma-rays, light charged particles and evaporation residues emitted from hot nuclei formed in the 36 Ar + 90 Zr reaction at 27 MeV/u have been measured with a nearly 4π barium fluoride multidetector. It is shown that hot Sn-like nuclei with a range of excitation energies between 300 and 600 MeV are produced. The γ-ray yield from the decay of the Giant Dipole Resonance in these nuclei is shown to remain constant over this excitation energy range. The measured γ-ray spectra are compared with statistical calculations encompassing several recent theoretical models for the quenching of gamma-ray emission from the dipole resonance at very high temperatures.

Charge-exchange reaction with light neutron-rich beams

The charge exchange reaction p(, )n was studied in reverse kinematics with a secondary beam at 41.6 MeV/nucleon. The transitions connecting the ground states and the one leading from the ground state of to its isobaric analogue state in have been measured and analysed. Their ratio is a measure of the relative strength of the two components of the exchange interaction, and . No clear signature of a halo structure is found from this ratio, since it is found to be compatible with existing systematics for stable T = 1 nuclei. The sensitivity of the (p,n) charge exchange angular distributions to the nuclear interaction potential and to the rms radii of the density distribution of is studied.