Microscopic description of giant electric and magnetic multipole resonances in closed-shell nuclei

Sanchez-Dehesa, J.

Book/Report

FZJ-2018-01035

Microscopic description of giant electric and magnetic multipole resonances in closed-shell nuclei

Sanchez-Dehesa, J. (Corresponding author)FZJ*

1977
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 1425, 147, [45] p. (1977)

Please use a persistent id in citations: http://hdl.handle.net/2128/17624

Report No.: Juel-1425

Abstract: In the last few years one of the most exciting phenomena in nuclear physics has been the observation of the so-called giant multipole resonances. One can define a Giant Multipole Resonance (GMR) as any nuclear state or group of overlapping states which has the following properties: i) It exhausts a large fraction of the energy-weighted sum rule (EWSR) strength allowed for that multipole and type (isoscalar or isovector) of excitation ii) It appears in a systematic way in many nuclei ii) It is concentrated in a relatively narrow energy region Contrary to the resolved discrete low-lying ("bound") states (i. e. those characterized by excitation energies of order 1 MeV) which depend on the detailed shell structure of nuclei, the giant resonances, whose characteristic energies are of order 10 MeV, have structure governed by gross or average nuclear properties. These collective multipole states are then caracteristic normal modes of nuclei. Nowadays apart from the well known electric isovector giant dipole resonance, recent experimental evidence has firmly established the existence of an isoscalar giant quadrupole resonance over the whole periodic table. Moreover there is some evidence for other electric resonances such as the isoscalar monopole and octupole and the isovector quadrupole states as well as for the magnetic dipole excitations. To identify these normal modes and elucidate their properties is a fundamental task for the understanding of nuclei. It is of special interest the identification of the giant magnetic dipole resonance because it can supply a very accurate test of the spin-dependent part of the effective nucleon-nucleon interaction and the giant monopole "breathing" or compressional mode from which one can directly extract the nuclear compressibility which is very difficult to obtain otherwise. In the present work all of these normal modes in the nuclei $^{12}$C, $^{16}$O, $^{90}$Zr and $^{208}$pb are investigated with the use of the zero-range density-dependent Migdal force in framework of a ne\lJ method which we have called Core-Coupling Random-Pahse-Approximation (in short, Core-Coupling RPA). This method is developed in section III. It is a special type of two'particle - two hole RPA approach which has two important advantages compared to the conventional one: first we get more physical insight into the structure of giant multipole resonances and secondly, we avoid the diagonalization of very large matrices. The latter permit us to treat almost equally easily light, medium and heavy nuclei, In this procedure the GMR's are described by means ofquasi rticle-quasihole excitations, i. e. in terms of conventional 1p-1h [...]

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