Investigation of the octupole degree of freedom with Coulomb excitation

Tuesday 23 April 2019, 2.00PM

Speaker(s): Liam Gaffney, University of Liverpool

The enhancement of the octupole degree of freedom in nuclei leads to a reflection asymmetric nuclear shape. This is expected to occur around the octupole magic numbers of N,Z = 34, 56, 88 and 134, where the Fermi surface lies close to single-particle orbitals with quantum numbers [l,j] and [l-3,j-3]. This enhances particle-hole interactions in the octupole part of the nucleon-nucleon force and is the driver of octupole collectivity.
Coulomb excitation is more than 50 years old as a technique, but has never been more alive in nuclear-structure physics. That is due in a large part to the advent of high-efficiency germanium arrays for the detection of gamma-rays and the availability of post-accelerated exotic beams from ISOL facilities. 
The beauty of the technique lies in its sensitivity to the nuclear shape, both via the spectroscopic and intrinsic quadrupole moments extracted using the reorientation effect and the quadrupole-sum-rules method, respectively [1]. Coulex remains unique in being able to provide such observables for excited states and is now the go-to method for studying phenomena such as shape coexistence [2].
Higher-order deformations also become accessible and octupole moments can also be obtained [1]. Ground-breaking experiments were made at REX-ISOLDE studying the magnitude of octupole deformation in 224Ra and 220Rn [3]. The former was determined to have a significant enhancement of the octupole moment and follows as only the second direct measurement of the deformation (after 226Ra) in a nucleus classified as octupole deformed in its ground state.
With the completion recent HIE-ISOLDE upgrade in 2018, radioactive ions beams from ISOLDE can now be post-accelerated to energies of up to 10 MeV/u. A campaign over the past two years has been carried out to study multiple isotopes predicted to be either octupole deformed or octupole vibrational. In this seminar, I will present recent experimental data on the Coulomb excitation of 142,144Ba, 222,228Ra and 222,224,226Rn, as well as reviewing the current status of experiments around the world. Comparisons will also be made to a variety of different theoretical models and the implications for studies of atomic electric dipole moments (EDMs) will be briefly discussed.
[1] M. ZieliƄska, L.P. Gaffney, K. Wrzosek-Lipska, E. Clément, T. Grahn, N. Kesteloot, P. Napiorkowski, J. Pakarinen, P. Van Duppen, and N. Warr, Eur. Phys. J. A 52, 99 (2016).
[2] K. Wrzosek-Lipska and L.P. Gaffney, J. Phys. G Nucl. Part. Phys. 43, 24012 (2016).
[3] L.P. Gaffney et al, Nature 497, 199 (2013).

Location: P/T/111