Stephane Lalkovski
PhD Thesis:
High-spin states in 108,110Pd produced as fission fragments. Ground-γ band mixing effects

The thesis is dedicated to the study of excited states in 108,110Pd nuclei produced as fission fragments in 31P+176Yb induced fission reaction. Both nuclei are of particular interest as they are placed in a nuclear mass region, where a variety of shapes is observed - spherical, gamma-unstable, triaxial and axially deformed prolate shapes. In fact, these two nuclei are part of an isotopic chain which shows a relatively "stable" gamma-unstable behaviour. All even-even Pd isotopes with A>108 have gamma-soft shapes which is reflected in their band structures. The relative displacement of the odd-spin levels with respect to the even one in the gamma-band is used as a sign of the type of triaxiality. This effect is known as the odd-even staggering and it is very sensitive to the deviation from axial symmetry of the shape.

The newly-observed gamma-band states allowed us to make a full systematic of the gamma-band level evolution in Pd isotopic chain from A=108 to the heaviest known neutron-rich Pd isotope - 118Pd as well as to compare it with that in Ru and Mo isotopic chains. An attempt to describe their ground state bands (gsb) and gamma-bands in the framework of the Davidov's and Wilets and Jean's models was made. It is shown that through these models neither the staggering effect in the gamma-bands nor the relative displacement of gamma-band levels with respect to the gsb can be reproduced accurately. Thus the systematic behaviour of the gsb-gamma bands naturally led us to the suggestion that the staggering effect can be interpreted as a result of the strong ground state band - gamma band interaction which causes the relative displacement of the even-spin states with respect to the odd ones. Thus we needed a model in which both bands are coupled into the same multiplet. Such a scheme proposes the Vector Boson Model (VBM). It is applicable for the near-SU(3) nuclei but not for Pd isotopes. In order to apply the VBM formalism for Pd, Ru and Mo nuclear region a phenomenological modification is performed. The staggering effect is used to test it. It is observed that for the nuclei from SU(3) region the staggering effect has smaller amplitude than for the nuclei far from the region of good rotors. This effect is considered as a manifestation of the neutron mid-shell filling. The systematic study of the staggering effect in Mo, Ru and Pd isotopic chain shows that it changes from 64 for Mo isotopic chain to 68/70 for Pd nuclei.

Besides the low lying positive-parity levels a number of Mo, Ru, Pd and Cd nuclei have negative-parity bands in their spectra, which are interpreted as rotational semi-decoupled bands, based on (νh11/2,πd5/2g7/2) configuration. This fact inspired us to search for analogical bands in 108Pd and 110Pd nuclei.

The existence of gamma-band and rotational bands, based on neutron quasi-particle configurations makes these nuclei good laboratories for study of the nuclear collectivity and their interplay with single-particle excitations. They are used in the thesis as a test for different collective models.

Table of Contents

Chapter I
Concerns the principles of the gamma-ray detection from the large multidetector array EUROBALL. Experimental analysis of data treatment in a composite Cluster detector is presented.
Chapter II
Introduces the main features of the induced fission fragment spectroscopy. Formation and decay of the highly excited fragments are discussed here. Some physical properties which could be deduced from the gamma-rays emitted from the highly spinning fission fragments are outlined.
Chapter III
Deals with the experimentally obtained high-spin states in 108,110Pd nuclei, produced as fission fragments in fusion/fission reaction.
Chapter IV
Gives the interpretation of high-spin states in neutron-rich Pd isotopes. A systematic of even- and odd-parity states in even-Pd nuclei is presented. The collective bands in neighbouring odd-Pd isotopes are also analyzed.
Chapter V
presents the new phenomenological model for ground state band - gamma-band mixing interaction. It is applied for all nuclei with known gamma-bands in 40<Z<50 region.

Some theoretical models used in the thesis are represented in a number of Appendices A-F.