The atomic nucleus: fissile liquid or molecule of life?

The atomic nucleus is generally described as a drop of quantum liquid. In particular, such liquid-like behaviour explains nuclear fission and applies especially to heavy nuclei such as uranium. The so-called liquid drop mass formula is a typical textbook model in nuclear physics. On the other hand, light nuclei can behave like tiny molecules – or clusters – made up of neutrons and protons within the nucleus. This molecular aspect at the femtometre scale makes it possible to understand the stellar nucleosynthesis of ¹²C and consequently of heavier elements such as oxygen.

So far, both the “molecular nucleus” and the “liquid nucleus” views have co-existed. Now, a team from the Institut de Physique Nucléaire d’Orsay (Université Paris-Sud/CNRS) and the French Atomic Energy Commission (CEA), in collaboration with the University of Zagreb, has proposed a unified view of these two aspects. By using relativistic-energy density functionals, the researchers have demonstrated that, although a light nucleus can show molecule-like behaviour (tending towards the crystalline state), heavier nuclei take on a more liquid-like behaviour.

The team took inspiration from neutron stars – remnants of core-collapse supernovae that are composed mainly of neutrons with a few protons. Inside the crust of neutron star, matter passes from being a nucleonic crystalline medium to becoming a nuclear-liquid medium. Thanks to this analogy, the team identified a mechanism of transition from the liquid to the crystalline state in the nucleus.

When the interactions between neutrons and protons – through the depth of the confining nuclear potential – are not strong enough to fix them within the nucleus, the latter is in a quantum-liquid-like state where protons and neutrons are delocalized. Conversely, in a crystalline state, neutrons and protons would be fixed at regular intervals within the nucleus. The nuclear molecule is interpreted as being an intermediate state between a quantum liquid and a crystal. In the long term, the aim is to attain a unified understanding of these various states of the nucleus.