ISBN 978-3-8439-1045-3

978-3-8439-1045-3, Reihe Physik

Matthias Hackstein
Feasibility Studies of Recoil-Distance Doppler-shift Experiments at Energies around 200 MeV/u and the Determination of Absolute Transition Strengths in 186Hg

204 Seiten, Dissertation Universität Köln (2012), Softcover, A5

Zusammenfassung / Abstract

Nuclear structure physics is an exciting field which offers through several thousand different isotopes a vast testing ground to investigate some of the fundamental properties of nature. The measurement of lifetimes of excited nuclear states provides an observable, which allows to derive properties of the inner structure of atomic nuclei. The Recoil-Distance Doppler-shift (RDDS) technique has been used extensively in the past to determine lifetimes of excited nuclear states and continues to be an important tool in the field of nuclear structure physics, as more and more exotic nuclei are accessible to this technique. This work is dedicated to the advancement of this technique, its implementation to an important nuclear physics facility as well as its application to the case of 186Hg.

Within this work, a first RDDS experiment using relativistic ion beams was performed at the PRESPEC set-up at the GSI Helmholtzzentrum für Schwerionenforschung to test the feasibility of such experiments under the conditions of this set-up. A successful application of the RDDS technique to this kind of experiments would allow lifetime studies in exotic nuclei that are not accessible otherwise.

It was possible to reproduce the known lifetime of the first excited state in 54Cr in a commissioning experiment, albeit with a large uncertainty. Therefore, in principle, the feasibility of these experiment could be shown.

Further, the RDDS technique is used to investigate the nuclear structure of neutron deficient 186Hg, where two bands with different properties are assumed to coexist at similar energies. Lifetimes of the yrast band up to J = 10 hbar were measured and compared to theoretical calculations using the formalism of the Interacting Boson Model (IBM) in conjunction with an Energy Density Functional (EDF) model. Furthermore, a two-band mixing calculation was performed to test if these bands in 186Hg can be described with a rotational model and a constant deformation.

The third part of this work considers a particular aspect of the analysis of RDDS experiments. The influence of the velocity distribution of the recoils on the resulting lifetimes is investigated in detail. It was found that the velocity distribution affects the measured lifetime. The effect is analysed analytically and specific conditions were found in which the velocity does not influence the resulting lifetimes.