Just as a dense liquid, when heated, can exist in an intermediate vapor phase
(liquid + gas) before turning into a dilute gas, atomic nuclei too can exist
in a mixed phase of heavy fragments and light clusters before dissociating
into their constituent nucleons. Unlike ordinary liquid, nuclei is a two
component fluid made of neutrons (N) and protons (Z). The two component
nature of nuclei can significantly alter the chemical composition of the
fragments and the light clusters. In particular, the distillation/fractionation
for asymmetric nuclear matter (N/Z > 1) can result in a dilute and neutron
rich gas (nucleons and light clusters), and a dense and symmetric liquid
(heavy fragments) phase.
The figure on the right shows an experimental evidence of the relative increase in the neutron richness of the light clusters formed in Fe + Ni and Fe + Fe reactions as a function of their N/Z. With increasing N/Z of the system, the excess neutrons are easily available to form neutron rich clusters. With increasing beam energy, the clusters are produced with higher excitation energy, which boils of the excess neutrons making them less neutron rich.