GAMMA SPECTROSCOPY OF TRANS-LEAD NUCLEI

Abstract

Since its discovery by Rutherford et al. in 1911, the atomic nucleus has been posing many challenges to understand its structure. Continuous theoretical and experimental efforts are still underway in the field of fundamental as well as applied nuclear physics to understand this femto-meter sized object. An atomic nucleus is a unique and mysterious quantum-mechanical system which involves the dynamics of neutrons and protons under the influence of strong interaction. The internal structure of this quantal system varies greatly with the number of its constituent nucleons. These variations in the nucleus are associated with the changes in nuclear excitation spectrum and its decay properties. Thus, the motivation of nuclear structure investigations is to comprehend the physical structure of nucleus in terms of underlying nucleon configurations through the observed emission spectrum of γ rays. There are various approaches to investigate the nuclear structure viz. scattering, radioactive decay and heavy-ion fusion etc. In almost all the approaches, information regarding the excited nuclear states is extracted from the gamma rays emitted in the process of de-excitation. The excited states established thereby yield a wealth of information regarding the nuclear structure. The decay studies are of special importance when information on non-yrast states in the daughter nuclei is desired. However, such studies can provide knowledge only on the states with spin near to that of the decaying state i ii of the parent nucleus. On the other hand, high-spin studies are mainly accomplished through heavy-ion fusion-evaporation reactions. In this type of reactions, a projectile is bombarded on the target to produce a compound nucleus which further decays into residual nuclei through various evaporation channels. These residual nuclei decay to the ground state via cascade of γ rays which consists myriad facts about the nuclear energy states and its related phenomena. High-spin gamma-ray spectroscopy has established several interesting aspects of nuclear structure such as band termination, octupole deformation, magnetic rotation, etc. This thesis comprises the results from the decay and high-spin studies of nuclei lying in the trans-lead region. In the first study, the EC/β+ decay of the N = 126, 213Fr nucleus populating states in 213Rn, was investigated for the first time at the CERN isotope separator on-line (ISOLDE) facility with the help of gamma-ray and conversion-electron spectroscopy. Five new α-decay branches from the 213Fr ground state to 209At have also been identified. Shell model calculations have been performed to understand the newly observed states in 213Rn. In the second study, the high spin states in 216Fr were established using the 208Pb(11B, 3n) fusion-evaporation reaction at 57 MeV of the beam energy. The γ rays, following de-excitation of residual nuclei, were detected with the help of Indian National Gamma Array (INGA) at IUAC, New Delhi. 55 new γ transitions comprising 30 energy states have been identified in this work. The newly identified levels have established the simplex partner of a previously reported band leading to parity doublets with small energy splitting, indicating near-static octupole deformation. The observed levels do not follow regular pattern of rotational bands indicating low quadrupole collectivity. Staggering in E1 transition energies and B(E1)/B(E2) ratios is noted. The enhancement of octupole correlations in 216Fr is attributed to the availability of a neutron orbital with a K = 3/2 component. With this observation, 216Fr is identified as the first doubly-odd nucleus in which dominance of octupole correlations over quadrupole collectivity is observed.