Session O02: Nuclear Structure and Shell Evolution

Evolution of Nuclear Shaps

The nucleus is one of the nature’s most fascinating quantal many-body systems. It exhibits several unique behaviors and is characterized by some fundamental properties, one of which is the shape. While closed-shell nuclei are commonly associated with spherically symmetric shapes, those in-between closed shells possess varying degrees of spheroidal deformation, with the quadrupole degree of freedom being the most common deviation from spherical symmetry. Nuclei with quadrupole deformation can either have axial symmetry, where one distinguishes between prolate and oblate shapes, or axially asymmetric shapes. In addition, nuclei can also adopt reflection-asymmetric or octupole-deformed shapes at low excitation energies and, in some cases, configurations corresponding to different shapes can coexist at similar energies. Moreover, nuclear shapes are sensitive to the underlying structure and thus, can change from one nucleus to another. Typically, these changes are a result of the rearrangement of orbital configuration of nucleons or the dynamic response to rotation. Predicting this coexistence phenomenon and shape evolution as a function of the nucleon number and angular momentum is extremely challenging from a theoretical point of view. Thus, determining nuclear shapes experimentally and tracking their evolution represents a stringent test of theoretical models and at the same time, constrains our understanding of the microscopic origin of nuclear shape changes. In this talk, I will present a general overview of how nuclear shapes are inferred and highlight recent experimental and theoretical progress towards understanding nuclear shape evolution in certain regions of the nuclear chart.   

First observation of a new partner band in mass 200 region near Z$=$82 shell closure as a signature of triaxiality.

The experimental observation of the wobbling motion as a signature of triaxial shapes in nuclei is of recent interest. The recent observation of wobbling in $^{183,187}$Au nuclei boosted up the search for such exotic shape in heavier mass region. The deformation driving effect of $\nu $i$_{13/2}$ causes axial and non-axial shapes in Hg nuclei around A $=$ 190. An experiment was performed at VECC, Kolkata, India, with 36-MeV $\alpha $ and using VENUS {\&} INGA Clover array which reports a new $\Delta $I $=$ 2, E2 band in $^{199}$Hg which decays to the yrast $\nu $i$_{13/2\, }$band via a set of $\Delta $I $=$ 1, E2-like transitions with large $\delta $ mixing, a signature of wobbling. A signature partner band was also found. This would be the first example of wobbling in this region and first such case with a neutron-hole configuration.   

Exploring the Level Structure of $^{59}$Co

Fusion evaporation reaction induced by a beam of 43MeV $^{14}$C on a $^{48}$Ti target has been used to populate the high spin levels in $^{59}$Co. Emitted $\gamma $ rays were detected using the FSU clover array which consists of 6 high purity germanium clover detectors (with BGO shields for Compton suppression) and a few single crystal detectors all placed at 3 different angles. The directional correlation from oriented states (DCO ratio) was measured using ratios of intensities from detectors at 90$^{o}$ and 135$^{o}$.The polarization was also measured, which in addition to the DCO ratio was used to determine the spin and parity of the new energy levels in the $^{59}$Co. Previously known spins in the $^{59}$Co were confirmed and their parity was assigned. Level scheme of the $^{59}$Co has been extended to 11139keV with J$^{\pi } \quad =$ 31/2$+$. The result was compared with the theoretical shell model calculations within the fp-g$_{9/2}$ shells valence space and was found to agree up to the single particle excitation.~Using same beam on $^{50}$Ti target. preliminary results of $^{61}$Co are also presented here. This work was supported by the U.S. National Science Foundation under grant number Phy-2012522.