Nuclear shape phenomena in heavy nuclei : Experimental studies of octupole deformation and shape-coexistence using Coulomb excitation and lifetime measurements

par Dr Liam Gaffney (KU Leuven)

vendredi 28 mars 2014, 10:30 → 11:30 Europe/Paris

bat 27

Deformation in atomic nuclei has become a familiar topic in nuclear structure physics and axial- and reflection-symmetric quadrupole shapes are prevalent across the chart of nuclides. Shape co-existence, whereby states pertaining to different shapes lie close to each other at low-energy is less common. A large array of experimental evidence has been collected in the neutron-deficient lead region in recent times from beta- and alpha-decay studies, lifetime measurements, laser spectroscopy and, most recently, Coloumb excitation. Even more exotic is the occurence of octupole deformation. There exists strong circumstantial evidence that certain combinations of protons and neutrons will give rise to a reflection-asymmetric pear-like shape. It can be expected that the actinides will possess the strongest octupole collectivity. The nuclei in this region of the chart are unstable meaning that direct information on the electric-octupole transitions connecting excited states in these nuclei have so far not been accessible, with the exception of only the long-lived 226Ra. With the advent of radioactive ion beams and in particular, the ground-breaking ability to post-accelerate the heavy elements radon and radium at REX-ISOLDE, we have recently overcome the challenges limiting our knowledge of octupole collectivity in the actinide region. Coulomb excitation was successfully performed on 220Rn and 224Ra and the E1, E2 and E3 matrix elements connecting the lowest-energy states have been determined. In this seminar I will describe the experiments on 220Rn and 224Ra in detail and discuss more generally the topics of octupole deformation and shape co-existence. I will also review experiments performed on the neutron-deficient mercury isotopes by our collaboration, specifically lifetime measurements using Gammasphere at Argonne and Coulomb-excitation experiments at REX-ISOLDE. Finally, an outline of the on-going HELIOS project, which is utilising in-gas jet laser ionisation to study the heaviest elements, will be presented.