Nuclear Physics – Nuclear Structure Research



* Study of chiral symmetry

Chirality, or handedness, in nuclear structure is has been predicted to occur dynamically when roughly three equal components of angular momentum are present on all axes. Such a situation can occur in
triaxial nuclei, as the core angular momentum is aligned along the axis of intermediate length, and proton and neutron angular momenta align with the remaining axes. Of particular interest for the group at iThemba LABS is the influence of residual proton-neutron interactions on the properties of proposed chiral bands.

* Search for exotic shapes

It is well known that nuclei can take quadrupole shapes, similar to a rugby ball or a pumpkin, or that they can even be slightly pear shaped (octupole), but shapes that are less well established are
hyperdeformed shapes and tetrahedral shapes. The hyperdeformed shape corresponds to an extremely elongated nucleus, three times longer than it is wide, which in the actinide region, is also expected to
be pear shaped. The AFRODITE array is ideally suited to this search, as it has a high efficiency (~10%) in the energy region of interest (~100keV) to observe transitions in such a rotational band.
Tetrahedral shapes have been predicted in the mass 160 region, the prime candidates being rotational bands which were formally regarded as corresponding to the rotation of an octupole vibration.

* Pairing isomers

A programme of study at the lab is aimed at establishing the true identity of the rotational bands based on the second 0+ state in the Sm and Gd region. Traditionally these have been interpreted as beta-
vibrational bands, or examples of shape-coexistence. A body of evidence now supports the alternative interpretation of a pairing isomer, based on the occupation of the 11/2-[505] orbital.

K600 Magnetic Spectrometer

Hadronic scattering at intermediate energies of a few 100 MeV per nucleon serve as a classical tool for the investigation of giant resonances. There are various topics of current interest, such as the investigation of the fine structure of the Spin-Dipole and Isoscalar Giant Quadropole Resonance to help with the identification of the excitation and decay modes of the giant resonances.

Inelastic proton scattering as well as transfer reactions with very high resolution at extremely forward angles including zero degrees is a very powerful tool for studying E1 and M1 excitations in nuclei. In order to extract the E1 strengths from measured data, it is vital to measure the energy dependence of the cross section, which in the case of E1 excitations depends strongly on the incident beam energy. The physics motivation is explained by relating the dipole transitions to the nucleosynthesis and other aspects in supernovae. Model calculations show that the amount of E1 strength significantly affects the nucleosynthesis scenario in the Type-II supernovae.
The M1 resonances are strongly related to neutrino-induced reactions in the supernova. These processes are relevant to the energy flow during supernova nucleosynthesis by neutral currents, and dynamic simulations of the evolution of a supernova.

Some exotic excitation modes that are excited through hadronic scattering, such as mixed symmetry states and stretched states, can be studied with the high energy resolution capabilites of the K600 magnetic spectrometer.


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