"The atomic nucleus is a complex, many-body quantal system consisting of protons and neutrons. The study of nucleus is not only of fundamental importance in our quest to understand the world around us and its origin, but it also provides the tools for a variety of applications from energy to medicine. Although the nucleus is being studied for more than 100 years since its discovery by Rutherford, a thorough understanding of its quantum structure is far from being complete. Technological breakthroughs in the last decades have opened up several entirely new and exciting scientific frontiers. One of these frontiers is the production and study of isotopes with extreme neutron to proton ratios (exotic nuclei). These studies have revealed strong modifications in the ordering of single-particle orbitals in exotic nuclei as compared to the ones predicted by the original Shell Model of Mayer, Haxel, Suess and Jensen which in turn have dramatic consequences how the heavy elements, ie. beyond Ni and Fe, were created in the universe in the so-called rapid neutron capture (or r-) process. This research project aims at the study of neutron-rich exotic nuclei in the mass A~100-110 region through gamma-ray spectroscopy using fusion-fission and Coulomb excitation experiments. The experiments are able to determine the shape and deformation of these nuclei. The results of these measurements should help to understand the residual interactions responsible for changing shell structure in neutron rich nuclei and allow for a stringent test of various nuclear structure models, which are used to predict properties of even more neutron-rich isotopes relevant for the r-process.
Neutron-rich nuclei are key players in the creation of elements in several astrophysical scenarios, e.g. supernovae explosions and neutron star mergers. In particular the neutron star merger scenario has attracted strong interest in view of the first experimental observation of this process from the observation of gravitational waves. It should also be noted that the ""afterglow"" of the merger in the gamma ray spectrum was correctly predicted by nuclear structure calculations as early as 2010. However, the nuclei/isotopes produced in these violent events are far from being accessible in the laboratory for at least decades to come if ever. Therefore, we completely rely on theoretical predictions, which we have to test with (less exotic) isotopes available in the laboratory. The experiments realised in this project and their results are precision tests of nuclear structure models for moderately exotic nuclei. As one example the sudden onset of deformation in the chain of Zr isotopes, although known experimentally for many year, was never described correctly by many different theoretical approaches, e.g. the nuclear shell model or so-called mean-field models with varying effective interactions. Our experiment on 98Zr confirmed that a shape transition occurs suddenly between mass number 98 and 100, and our collaborators from the U. of Tokyo are able to describe this effect as a shape phase transition with their modern version of the shell model, the so-called Monte-Carlo Shell Model.
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