Atomic nuclei are samples of quantum Fermi fluids. Their size classifies them as mesoscopic systems, whose properties are intermediate between quantum mechanical microscopy and quantum, or classical bulk behavior. The Moretto group has developed a nuclear mesoscopic thermodynamics and have applied it to the interpretation of nuclear reactions in a variety of regimes. For instance, they have predicted, and experimentally found the disappearance of strong quantum structures with increasing excitation energy. Very recently by means of strong scaling techniques they have visualized the presence of temperature dependent shell effects and nuclear superconductivity in the energy dependence of fission probabilities.
The same insight has led them to the prediction and experimental verification of statistical emission of complex fragments (of mass greater then an alpha particle) from excited nuclei. They have tracked this process of fragment emission up to very high energies, where several fragments can be emitted in a single reaction event. This reaction is called multifragmentation.
p> Quantum fluidodynamics has allowed the prediction that multifragmentation may occur through the development of surface instabilities of the Rayleigh kind in peculiar nuclear shapes like disks, tori, and bubbles. In particular the group has discovered a hitherto unknown instability (sheet instability) which manifests itself when two interfaces are closer then the force range. This instability is of a very general nature, and may play an important role on the long term stability of modern layered semiconductor materials.
The Moretto group has also developed new powerful methods for the interpretation of very complex nuclear reactions. These methods have led them to discover very general properties associated with multifragmentation, like "reducibility" and "thermal scaling". For instance, the probability of emitting n fragments can be "reduced" to the probability of emitting a single fragment through a special combinatorial law (binomial). In turn, the one fragment probability yields a linear Arrhenius plot, indicating the thermal scaling characteristic of a Boltzman factor.
Similarly, they have discovered a reducibility equation for the fragment size distributions, which permits to express the n fragment distribution in terms of the one fragment distribution. This equation contains an enthalpic term, associated with the energy necessary to produce a fragment, and an entropic term, which relates to the group properties of the fragmentation decay tree.
Monovariant and bivariant regions have been uncovered in this process, in which the fragmenting system behaves like a saturated or unsaturated vapor. While this approach has been applied so far only to "nuclear" multi- fragmentation, it is clear that it can be applied to a variety of multifragmenting systems, for example degraded polymers