What Are Nuclear Phase Transformations?

Nuclear phase transformations are changes in the collective state of nuclear matter that occur when ensembles of nucleons are driven to extremes of temperature, density, or isospin composition. Just as ordinary matter transitions between solid, liquid, and gas phases, nuclear matter undergoes analogous transformations: a liquid-to-gas transition at nuclear densities and temperatures of several MeV, and a dissolution of nucleonic structure into a quark-gluon plasma at densities several times the nuclear saturation density and temperatures above roughly 150 MeV. Understanding these transformations connects nuclear physics to astrophysics, heavy-ion collision experiments, and the thermodynamic properties of matter in the early universe.

The study of nuclear phase transformations belongs to nuclear thermodynamics, the branch of nuclear theory concerned with how large assemblies of nucleons respond to changes in temperature and pressure. It draws on quantum many-body theory, relativistic mean-field models, and lattice quantum chromodynamics (QCD), as well as on experimental data from accelerator-based heavy-ion collisions.

Phase Transitions in Nuclear Matter

At ordinary conditions, nuclear matter exists in a liquid-like state with a saturation density of approximately 0.17 nucleons per cubic femtometer (fm3), where the attractive and repulsive components of the nucleon-nucleon interaction are in balance. When nuclear matter is heated to temperatures of a few MeV or expanded below saturation density, the liquid becomes mechanically unstable and undergoes a first-order liquid-gas phase transition analogous to the boiling of water. Within the coexistence region, the nuclear liquid and gas phases are in equilibrium; both theory and experiments at intermediate-energy heavy-ion facilities have identified signatures of this transition through the pattern of fragment production (multifragmentation) in nuclear collisions.

The critical point for the nuclear liquid-gas transition is estimated to lie at a temperature near 16 to 18 MeV and a density roughly 0.3 to 0.4 times the saturation value. Research at Lawrence Berkeley National Laboratory describes how the nuclear equation of state governs the character of this transition and its connection to the broader phase diagram of nuclear matter.

Nuclear Thermodynamics and the Equation of State

The equation of state (EOS) of nuclear matter relates pressure, energy density, and temperature across the full range of densities and compositions encountered in nature, from the near-saturation densities of atomic nuclei to the several-times-saturation densities expected at the cores of neutron stars. The symmetry energy, which quantifies how the EOS changes as the neutron-to-proton ratio increases, plays a central role in both the structure of neutron-rich isotopes and the properties of neutron star matter.

Heavy-ion collisions at intermediate and relativistic beam energies are the primary laboratory tool for probing the dense, hot nuclear matter EOS. At facilities including GSI/FAIR in Germany and RHIC at Brookhaven National Laboratory, collisions between heavy nuclei compress matter to several times saturation density for brief periods, and measurements of particle production, collective flow, and kaon yields constrain the stiffness of the EOS. An arXiv review of the equation of state and phase transitions in nuclear matter compiles the theoretical frameworks and experimental constraints available from these experiments.

At very high temperatures and densities, the phase transition to a quark-gluon plasma (QGP) marks the deconfinement of quarks and gluons from individual nucleons into a freely interacting medium. Evidence for QGP formation in ultrarelativistic heavy-ion collisions at RHIC and the LHC at CERN has been established through observations including collective flow, jet quenching, and the suppression of quarkonium states. Nuclear phase transition research published in the European Physical Journal discusses the proton flow constraints on the high-density EOS from these experiments.

Applications

Nuclear phase transformations have applications in a wide range of fields, including:

• Neutron star structure and maximum mass calculations in astrophysics
• Core-collapse supernova dynamics and neutron star merger simulations
• Quark-gluon plasma production and characterization in ultrarelativistic heavy-ion collisions
• Nuclear fuel behavior under extreme reactor accident conditions
• Cosmological models of the early universe and the baryon asymmetry