Complex nuclear-structure phenomena in the cooling down of highly excited nuclear systems

Complex structural effects in the nuclide production from the projectile fragmentation of 1 A GeV 238 U nuclei in a titanium target, manifested as an even-odd effect, are reported. The structure seems to be insensitive to the excitation energy induced in the reaction. This is in contrast to the prominent structural features found in nuclear fission and in transfer reactions, which gradually disappear with increasing excitation energy. Most of the features of the results are reproduced using the statistical model of nuclear reactions, treating the pairing correlations in a consistent way both in the masses and in the level densities. The structures appear as the result of the condensation process of heated nuclear matter while cooling down in the evaporation process. As such, it can be considered a manifestation of the passage from the normal liquid phase of the nucleus to its superfluid phase.