A focus on shape coexistence in nuclei

By Colin Adcock on February 5, 2016 • ( 1 )

Guest Editors Kris Heyde and John L Wood

John L Wood (Georgia Institute of Technology) and Kris Heyde (Ghent University) are two of the world’s experts on nuclear shapes and shape coexistence.

Now, they have Guest Edited JPhysG’s latest focus issue, but what is it all about?

A classic way to think of an atomic nucleus is as a sphere. However this isn’t the whole picture, and once you look a bit closer it is clear that nuclei come in all sorts of different shapes that depend on the properties of each nuclei.

Indeed it turns out many nuclei actually can’t be considered spherical, even in their ground state. They are generally quadrupole deformed; often oblate or prolate to varying degrees. This can be imagined somewhat by squashing a sphere so it becomes ellipsoidal. There’s more to it though; more complex shapes can also occur like octupole deformed nuclei (can be pear shaped) and perhaps even hexadecapole deformation, which are a rather odd set of shapes that I struggle to describe. These shapes of course aren’t what we would think of as true hard shapes macroscopically, but are instead defined by the density and mean field of the nuclei.

This idea of nuclear shapes and pear shaped nuclei is explained very nicely by Peter Butler recently in a 100s science video over at Physics World. Well worth a quick watch for a more scientific outline of the phenomena.

As it turns out, a variety of shapes can be seen in the same nucleus, and it all depends on energy states. Nuclei want to be in the lowest energy state they can, and they will deform in the process if its energetically cheaper to be in such a state. So far so good. However, when two minima energy states with the nucleus are very close together they may well have different associated shapes, meaning that it’s possible for two shapes to seemingly coexist.

To describe this idea a bit more accurately I defer to the words of JPhysG Editorial Board member Alfredo Poves, as he writes in his Foreword:

“Shape coexistence is a very peculiar nuclear phenomenon consisting in the presence in the same nuclei, at low excitation energy, and within a very narrow energy range, of two or more states (or bands of states) which: (a) have well defined and distinct properties, and, (b) which can be interpreted in terms of different intrinsic shapes.”

This is a topic that has spawned a great deal of interest since its discovery some 60 years ago, and the idea of shape coexistence in nuclei has gone from being thought of as a rare scenario to something that appears to be much more universal in nature. It does however remain a challenging field to study: although new models and tools are making it easier to theoretically predict where shape coexistence may occur, many of the nuclei which are predicted to host such coexisting states are difficult to study experimentally because they are difficult to produce or are too short lived.

One particularly interesting avenue is the study of the nuclear chart around ¹⁸⁶Pb, a famous case of triple shape coexistence between spherical, oblate and prolate states. This nuclei essentially has three minima very close together each providing a distinctive shape. This lead to increased interest in studying shape coexistence in more exotic nuclei far from stability in the search to understand shape coexistence and the underlying physics of nuclear shapes. All of the discovered coexisting states have so far been quadrupole deformed – there has been no evidence of shape coexistence involving octupole or higher deformed states.

Our new issue in JPhysG looks at coexistence in a meaningful way, looking in new and promising directions for the study of the field. We hope you enjoy reading it.

Focus on shape coexistence in nuclei
Guest Editors: John L Wood and Kris Heyde

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