Nuclear Multifragmentation: low density phase transitions.

Theoretically, there is little doubt that infinite nuclear matter undergoes a transition from a liquid to a gaseous phase and supports a mixed phase equilibrium at temperatures up to about 17 MeV. Some of the recent experimental evidence for the rise and fall of fragment production  (fragments are light nuclei like Carbon or Oxygen) with increasing incident energy for collisons of much heavier Krypton (Kr) and Gold (Au) nuclei is shown in the figure to the right. This measured trend is consistent with the theoretical predictions shown below for the yield of fragments within an equilibrated phase mixture of fragments (droplets) and nucleons (gas) as a function of temperature. These calculations predict the maximum in the fragment yield to occur for systems undergoing a phase transition. Physically, the phase mixture is assumed to consist of a collection of NIMF fragments embedded in a gas of nucleons, all at thermal equilibrium.

Research at the NSCL is currently being directed at testing this and other similar theoretical models. One prediction of such models, namely that the fragment occurs at low density over a short time time scale, has been experimentally confirmed. On the other hand, the assumption that the entire system is a thermal equilibrium (i.e. every thing is at the same temperature) does not seem to be very accurate at the highest beam energies. For the moment, it appears that systems are much closer to thermal equilibrium at the lower incident energies, but more accurate tests of the attainment of thermal equilibrium in low energy collisions need to be made. We are currently conducting these tests and at the same time trying to determine the densities and temperatures at which this phase transition in nuclear matter occurs.

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William G. Lynch, 1997