NUCLEAR STRUCTURE INVESTIGATIONS IN TRANSITIONAL REGION AROUND DOUBLY-MAGIC 208Pb

Abstract

It has been more than a century since the discovery of the atomic nucleus, however, the quest for understanding the various facets of this finite fermionic system still remains an intriguing task. Extensive experimental and theoretical studies over the years have revealed that the shape and structure of the atomic nucleus are primarily governed by the underlying interactions between the nucleons. Depending on the number of nucleons outside the shell closures and available orbitals near the Fermi surface, nuclei can exhibit a wide range of high-spin phenomena. For instance, the structure of nuclei near the shell closures are mainly governed by the single-particle excitations. On the other hand, the properties of nuclei with a large number of valence nucleons are influenced by the collective excitations. Thus, the transitional nuclei, which lie in between the extremes of the spherical and well-deformed regions, are particularly interesting as their structures include contributions from both the single-particle and collective modes of excitations and the competition between these basic modes reveals several interesting phenomena in such nuclei. This thesis is based on the results of two high-spin studies which were carried out for investigating the structure of transitional nuclei beyond the doubly-magic 208Pb. The experiments related to the thesis work were performed at Inter-University Accelerator Centre, New Delhi using heavy-ion fusion-evaporation reactions. The gamma rays emitted in the deexcitation of residual nuclei were detected using the Indian National Gamma Array and the data were acquired using CAMAC and VME based data acquisition systems. After calibration, the data were written into a ROOT tree format and further sorted into various two- and three-dimensional histograms for detailed analysis. The properties of the excited states were obtained by employing several γ- ray spectroscopic techniques which include establishing the coincidence relationships between the observed gamma rays, determining their multipolarities and extracting the half-lives of the isomeric states using centroid-shift and decay curve analyses. ix In the first work of the thesis, the structure of the transitional nucleus 216Fr has been investigated using the 208Pb(11B, 3n)216Fr reaction. Three new isomers, one at low excitation energy and two at higher energies, have been identified in this doublyodd nucleus. The properties of the low-lying (11+) isomer [T1/2 = 9.6(14) ns] are compared with those of the similar isomeric states in neighboring odd-odd nuclei. The experimental results are also compared with the predictions of the shell-model calculations and a fair agreement is observed between the experimental and calculated excitation energies of the (11+) isomeric state and the states to which it decays. However, the deviation between the measured and calculated reduced transition probabilities suggests contribution from effects other than the single-particle degrees of freedom in these states. Furthermore, the evidence of two high-spin [Iπ > (18+)] isomers with T1/2 = 7.8(14) ns and 89(9) ns is also presented. These isomers at high excitation energies reflect a pronounced change in the structure above the previously established level scheme, where signatures of octupole correlations were evident. In the second study, the high-spin states in 217Ra have been investigated using the 208Pb(12C, 3n)217Ra reaction. The level structure of 217Ra has been extended with the addition of about 20 new transitions. In addition, the discrepancies between the placements of several transitions reported in the earlier studies are resolved. Moreover, a new negative-parity sequence has been established at low excitation energies, which hints at the expected parity-doublet structures in 217Ra. Since the presence of parity-doublet structures reflects octupole correlations, theoretical calculations using reflection-asymmetric triaxial particle rotor model have been performed. A comparison of the observed features of the simplex bands in 217Ra with the properties of the similar bands in neighboring nuclei and the predictions of the calculations suggests that 217Ra exhibits an intermediate behavior between the extremes of spherical and octupole-deformed nuclei. Furthermore, the simplex bands are observed to terminate at intermediate energies and the structures lying above reflect the dominance of the single-particle excitations at higher excitation energies.