GAMMA-RAY SPECTROSCOPY OF NEUTRONDEFICIENT NUCLEI AROUND DOUBLY-MAGIC 208PB

Abstract

The atomic nucleus is a many-body quantal system consisting of protons and neutrons, collectively known as nucleons. Since the discovery of the atomic nucleus, examining the structure of nuclei has been a fascinating but challenging task. The reason lies in the dynamics of the constituent nucleons under the in uence of the strong nuclear force. The interactions between the nucleons lead to unique structural phenomena of atomic nuclei, which have no parallels in other systems. The structures of the nuclei are mainly governed by the valence nucleons outside the shell closures and the interactions between them. The nuclei with a few valence nucleons outside the shell closures exhibit near-spherical shapes, which are mainly attributed to the intrinsic degrees of freedom (i.e., particle-hole excitations). On the other hand, onset of deformation is observed with increasing number of valence nucleons or holes, which originates due to the collective degrees of freedom. The nuclei between the near-spherical and deformed regions, known as transitional nuclei, are of particular interest as the interplay between the two basic modes of excitations results in the diverse structural phenomena. This thesis comprises of results from two in-beam -ray spectroscopy experiments performed using heavy-ion fusion-evaporation reactions to study high-spin states, isomers and shape evolution in the transitional nuclei in the region around the doubly-magic 208Pb. The experiments were performed using the Indian National Gamma Array (INGA) at Inter University Accelerator Center (IUAC), New Delhi. The multiparameter data were acquired using CAMAC and VME based data acquisition systems and written into ROOT tree format after energy calibration. ix x The calibrated data were sorted into various 2- and 3-dimensional histograms, which were further analyzed using ROOT and RADWARE. The rst study reports level structure in the transitional nucleus 215Fr investigated using the 208Pb(11B, 4n)215Fr reaction. An extended level scheme of 215Fr up to 55/2~ and 4.8 MeV excitation energy, with the addition about 50 new -ray transitions, is proposed. Previously established isomers and their half-lives, except for the 47/2+ state, are revisited. The discrepancy in the half-life of the 39/2􀀀 state is resolved, and its half-life is revised to 11.4(14) ns. The observed level structures are interpreted in the framework of the shell model approach. An overall good agreement is observed between the experimental results and the shell-model calculations performed using the CD-Bonn NN interaction derived from the Vlow-k renormalization approach. A weak coupling of the odd-proton to the even-even core is observed to account for the level structure at lower energies, which strongly resembles a decoupled non-rotational band. A new positive-parity sequence is also established which is observed to originate from the coupling of the i13=2 proton at low excitation energy. In the second study, high-spin states and isomers in 207At were investigated using the 198Pt(14N, 5n)207At reaction. Yrast and near-yrast states above the known 25/2+ isomer in 207At are established for the rst time. The level scheme is extended up to 47/2~ and 6.5 MeV with the addition of about 60 new -ray transitions. The half-life of the 25/2+ isomer is revisited and a value of T1=2 = 107.5(9) ns is deduced. Evidence of a hitherto unobserved 29/2+ isomer in 207At is presented. A systematic study of B(E3) values for the transitions de-exciting the 29/2+ isomer in the neighboring odd-A At isotopes suggests a half-life in the 2􀀀4.5 s range for this state. The experimental results are compared with large-scale shell-model calculations performed using the KHM3Y e ective interaction in the Z = 50􀀀126, N = 82􀀀184 model space and an overall good agreement is noted between the theory and the experiment. A qualitative comparison of the low-lying excited states and the isomers with analogous states in neighboring nuclei provides further insight into the structure of 207At.