Shape coexistence is one of the most intriguing subjects in nuclear structure, and has been studied extensively for several decades. The concept was initially developed for nuclei around doubly closed shells, however, it is now recognised to appear throughout the nuclear landscape. Shape coexistence combines collective and microscopic models, and guides us towards better understanding and improved models for nuclear structure. Electric monopole (E0) transitions, which occur between nuclear states of equal spin and parity, are useful for identifying shape coexistence. This is because monopole transition strengths are sensitive probes of differences in the nuclear mean square charge radius. Since E0 transitions do not carry angular momentum, single γ-emission is forbidden, and the energy is instead dissipated by conversion electrons or internal pair creation. The ANU Super-e spectrometer combined with the Miel Si(Li) array is an excellent tool for measuring electrons and electron-positron pairs, up to energies of several MeV. Two different projects relying on E0 measurements with the Super-e setup will be presented in this seminar.
The first project is focussed on shape coexistence in the N=Z=28 region. Shape coexistence was investigated by monopole transition strengths in 54,56,58Fe (Z=26, N=28,30,32), deduced from conversion electron, internal pair conversion, γ-ray, and lifetime measurements. The lifetime data were obtained from DSA measurements at the University of Kentucky, USA. Additional γ and γγ measurements were also carried out using the CAESAR array.
The second project involves the 0+ Hoyle state in 12C, a shape coexisting 3α cluster-state, which greatly enhances the stellar formation of carbon (fusion of three α particles). The Hoyle state is unbound with an α-decay branch of 99.96%, and stable carbon is only formed following either an E0 transition to the 0+ ground state or an E2 transition to the first excited 2+ state. The focus of this project is not on the shape coexistence itself, but the rate of the 3α→12C process, which can be deduced from the radiative width of the Hoyle state. The current value of the radiative width has been found indirectly, and has an uncertainty of about 10%. Our aim is to reduce the uncertainty to an estimated 5%, by using a direct measurement of the E2/E0 pair transition ratio.
The motivation, experimental setup, methods, and results of both projects using the Super-e will be discussed in detail.