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.
Additional Information
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William G. Lynch, 1997
