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| Dissertation / PhD Thesis | GSI-2019-00716 |
2009
Abstract: As a response to an external perturbation, a nucleus can be collectively excited. Amongthose collective excitations exist the giant resonances, which have been extensively studiedin stable nuclei. However, their evolution as the isospin asymmetry increases is not so wellknown. Theoretical models predict the appearance of a new component in the isovectorelectric dipole response, the pygmy dipole resonance. Recently, experimental methods ofinvestigating giant resonances for unstable nuclei have been developed. The observation ofa concentration of strength below the giant dipole resonance (GDR) has been reported inneutron-rich nuclei, in particular, by the LAND collaboration at GSI, on neutron-rich Snisotopes.The pygmy dipole resonance, which can macroscopically be interpreted as an oscillationof the neutron-skin against an isospin-saturated core, provides valuable information forthe nuclear structure of instable nuclei. We report on another experiment performed ina different mass region, i.e. the neutron-rich Ni nuclei. The experiment is based on theheavy-ion induced electromagnetic excitation at relativistic energies (around 500 MeV/u)technique. This excitation mechanism is almost identical to that of real photons.The interaction is studied in a kinematically complete setup. The projectile is detected andidentified before hitting the target. All the products of the decay from the resonance aredetected and their momenta measured.Recent development increases the modular behaviour of the setup. Particularly the devel-opment of new electronic modules and data acquisition system. A libraries-based softwarefor VME modules has been developed.After the experiment, all the detectors have been calibrated and integrated cross sectionshave been determined. Using measurements on C, Sn and Pb targets, a model for thenuclear contribution to the cross section has been established.The measured one-neutron and one-neutron-one-proton electromagnetic dissociation crosssection from the stable 58Ni, 127(6) mb, is in agreement with the published cross section127(12) mb, measured with a direct γ absorption.One- and two-neutron electromagnetic dissociation cross sections have been determined for67−69Ni. Compared with a semi-classical calculation, the measurements present a shift ofthe dipole strength to lower energies. This shift cannot be explained by an underlyinggiant quadrupole resonance. Calculations were done using a simple model for the pygmydipole resonance, located at 9 MeV and 11 MeV, according to results of a random-phaseapproximation prediction and of a measurement using (γ, γ′) reaction. The observed excesswould be consistent with a pygmy dipole resonance located at 11 MeV and exhausting8.2(9)% of the energy weighted sum rule, or exhausting 5.2(6)% of the sum rule if thepygmy were located at 9 MeV.Although further analysis is required in order to determine the details of its strength dis-tribution, the appearance of a low-lying (“pygmy”) dipole mode in 68Ni seems evident fromthe present experiment.
Note: Dissertation, University of Strasbourg (France), 2009
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