Spectroscopy of exotic reflection-asymmetric atomic nuclei

Atomic nuclei can assume different shapes depending on the numbers of neutrons and protons that they possess. Many nuclei are spherical but some take on shapes that are deformed like a rugby ball (stretched sphere) or a pumpkin (squashed sphere). In some cases atomic nuclei can become reflection-asymmetric or pear shaped. This occurs in certain localized regions of the nuclear chart, where the neutrons and protons occupy specific orbitals which drive towards a reflection shape. Specifically the orbitals of interest have a difference in both orbital and total angular momentum of 3 hbar. Nucleons occupying these orbitals can interact by the octupole interaction which gives rise to a reflection-asymmetric or octupole-deformed shape. On the nuclear chart, this occurs close to nucleon numbers 34, 56, 88, and 126. Any nucleus with these numbers of nucleons is susceptible to octupole correlations and in extreme cases permanent octupole deformation. The nuclei that possess the strongest octupole correlations have proton number Z close to 88 and neutron number N close to 126, which are the light actinide region. This region includes the neutron deficient radon (Z=86), radium (Z=88), thorium (Z=90), and uranium (z=92) nuclei. Over the past few decades a number of these nuclei have been studied in experiments and they have demonstrated the spectroscopic features of octupole deformation such as low-lying negative-parity states, interleaving bands with opposite parities, and strong electric-dipole transitions. Recent calculations have shown that the region of octupole deformation in the light-actinide region maybe more extended than previously thought - that is, it may include the neutron-deficient plutonium (Z=94) and curium (Z=96) isotopes. To date, these exotic nuclei have been out of the reach of experimental investigation, but new experimental techniques are making these nuclei accessible.

This PhD project will involve the study of exotic atomic nuclei in the light actinide region with Z values on the upper side of the region with Z values greater than about 94. The PhD project will use different techniques to study these nuclei, for example including decay spectroscopy where excited states are populated following alpha decay in heavy-ion fusion-evaporation reactions. Such experiments will be performed using an array of radiation detectors at the focal plane of the RITU recoil mass spectrometer at the University of Jyväskylä Accelerator Laboratory in Finland. Other experiments will also be performed using state-of-the-art gamma-ray spectrometers such as Digital-Gammasphere at Argonne National Laboratory near Chicago and using the new AGATA gamma-ray tracking spectrometer at the Legnaro National Laboratory near Padova in Italy. The work undertake in this PhD project will include one or more such experiments, including planning for the experiment with simulations and modelling, setting up and running the experiment, and analysis of data. The results of the PhD project will lead to a better understanding of the development and evolution of exotic reflection-asymmetric deformations in atomic nuclei.