Structure of Bi isotopes close to the 208Pb doubly-magic core

Abstract

The atomic nucleus, being a dense system of protons and neutrons, can be considered as a 'laboratory' in which three fundamental interactions strong, electromagnetic and weak, can be studied. Although much experimental data concerning the structure and characteristics of the atomic nucleus have been collected, a theoretical description which would explain all the observed phenomena is still incomplete. This is in part because the nucleon-nucleon interaction has very complex characteristics due to the fact that nucleons are not fundamental particles so they have an intrinsic structure (thus, the description of nuclear forces must take into account that nucleon-nucleon interactions are the result of interactions between the quarks). It is also because the nucleus, as a system of many strongly interacting nucleons obeying the Pauli exclusion principle, demonstrates a very high degree of complexity. Moreover, electromagnetic and weak interactions manifesting in the atomic nucleus are the source of additional complications in its description. As a consequence, progress in theory must go together with experimental investigations, which results in a strong connection between theory and experiment in nuclear physics. New theoretical concepts dictate which experiments would be the most effective in verifcation of a particular model. On the other hand, measurements can inspire theory to gain better parameter values from its models. To describe the atomic nucleus as a system of more elementary constituents (nucleons), one needs to know the wave function being the solution of wave equation for such a system. Due to the diffculties mentioned above, one needs to use simplifed models instead of the exact description. One of them is the shell model, which explains many experimental observations, such as magic numbers of nucleons: 2, 8, 20, 28, 50, 82, 126, the spin-parity values of the ground states of many nuclei, as well as the structure of excitations of nuclei in the region of magic nuclides. While the shell model with a classic set of orbitals works well near doublymagic nuclei lying close to the stability valley, in the exotic regions of the nuclear chart; i.e., in the regions remote from stability, the situation is different - the structure of singleparticle energy levels changes and new energy gaps may show up while the classical ones may disappear.One way to trace all these changes, is to undertake systematic investigations of excited structures along a chain of neutron-rich isotopes to the description of which the shell model can be applied. In the present work, we have chosen as the objective of study the series of Bi isotopes near the doubly-closed nucleus 208Pb. We have investigated the 205;206;210Bi nuclei, which have one valence proton and from four neutron holes to one neutron particle with respect to the doubly-magic core 208Pb. Since 208Pb is considered to be one of the best doubly-closed cores due to remarkably wide energy gaps which separate proton shells at Z=82 and neutron shells at N=126, the structure of the 205;206;210Bi nuclei is an excellent testing ground for modern shell-model calculations. In the present work, information about the high-spin yrast structures in the 205;206;210Bi isotopes has been extended. In particular, the aim of the work is the identifcation of high spin states arising from valence particles/holes excitations and from core excitations in 205;206;210Bi. Also, the spectroscopic data on the low-spin excitations in 210Bi were acquired in a neutron-capture reaction. The neutron rich nuclei are diffcult to reach for spectroscopy studies, because they cannot be produced in fusion-evaporation reactions. The access to excited structures at high spin in those nuclides is possible thanks to a method which relies on using deep-inelastic collisions (DIC) of heavy ions - this method has been developed at IFJ PAN. The main object of interest in the presented thesis are high-spin structures in Bi isotopes. The experiments aimed at investigating those structures were performed at Argonne National Laboratory, where Bi nuclei were populated in deep-inelastic reactions with the use of 76Ge and 208Pb beams on 208Pb target. During such reactions, the nuclei come to a close contact and much kinetic energy is dissipated giving rise to internal excitation energy. In the exit channel one has then two products excited to relatively high energy and spin. Since thick targets were used, the products were stopped inside the target and most of the rays that were measured with the use of the Gammasphere multidetector germanium array, appeared in the spectra as sharp lines - they were emitted from nuclei at rest. The second experiment, performed at the Institute Laue-Langevin in Grenoble, was devoted to the low-spin structure of the 210Bi nucleus. In this case 210Bi was produced in cold-neutron capture on 209Bi. The spectroscopic measurements in the non-yrast low-energy region of 210Bi could be performed. In the first chapter, an introduction to the structure of the atomic nucleus is presented - it includes: the characteristics of nuclear forces and the foundations of the shell model as well as the calculation methods and computer codes used nowadays. The second chapter provides a short description of the region of interest - the region around doubly-magic 208Pb - with emphasis on Bi isotopes. The third chapter presents description of the reactions leading to the nuclei of interest, the experiments which were performed, and the methods of analysis of the coincidence and angular distributions and correlations of -ray data. In the fourth chapter, the experimental results are discussed. The fifth chapter is devoted to comparisons of the experimental results with predictions based on shell-model calculations involving the presently available two-body shell-model interactions. The last part contains a summary.