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.
J-P Ebran, E Khan, T Nikšić and D Vretenar 2012 Nature 487 341.