Date of Completion

7-30-2020

Embargo Period

7-30-2020

Keywords

nuclear reactions, nuclear structure, two-neutron adding, magnesium, shell model, experimental, HELIOS

Major Advisor

Alan Wuosmaa

Associate Advisor

Richard Jones

Associate Advisor

Peter Schweitzer

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The structure of nuclei outside the region of stability is an area of ongoing experimental research. Recent investigations in the region around 32Mg have discovered inversions in the usual ordering of shell-model states. From these discoveries, theories of the evolution towards this "island of inversion" predict low-lying deformed intruder states for several nuclei in the region. One such nucleus, which exists in the region of transition between the normal and inverted hierarchies, is 28Mg. In this nucleus the ground state is expected to be mixed, i.e., it is a superposition of nearby 0+ neutron-pairing configurations. An excited intruder 0+ state will differ from surrounding states in shell-model configuration and shape, a property known as shape coexistence. Multi-nucleon transfer is known to be sensitive to the static deformation of a nucleus. It is also sensitive to both the amplitude and phase of configuration-mixed states, and enhances transfer to those states which are similar to the ground state of the target plus two nucleons in single-particle orbitals. This makes it a valuable tool for investigating the properties of 28Mg. The two-neutron transfer reaction 26Mg(t,p)28Mg has been used to study the properties of the ground state and excited 0+ states. This experiment was carried out at Argonne National Laboratory using the HELIcal Orbit Spectrometer (HELIOS). HELIOS was designed to overcome the difficulties of measuring reactions in inverse kinematics. Experiments in inverse kinematics consist of a heavy particle, in this case 26Mg, incident on the light particle (3H). Because multi-nucleon transfers are more complex than single-particle transfers, a calculation of the nuclear structure must guide the understanding of which configurations will be strongly populated. Shell-model calculations have been used to evaluate the structure-related transfer amplitudes, which were used as input into distorted-wave Born approximation (DWBA) calculations of the reaction. The results of this analysis are the relative contributions of different configurations to the ground state and excited 0+ states, which provide a stringent test of the theoretical prediction of shape coexistence and intruder states.

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