Nuclear (thermo-)dynamics

Giant resonances
The giant resonances are now known since 50 years for the dipole mode and only less than 20 years for the other modes. Macroscopically they are interpreted as vibrations of the various nuclear fluids (protons/neutrons or spin-up/down).
In microscopic descriptions the giant resonances are  interpreted as coherent particle-hole excitations (ie single-particle excitations).
In all the different approaches the giantresonances are understood as quantum harmonic oscillators.
Today the properties of Giant resonances are actively studied both experimentally and theoretically. Of particular interest are the structures of collective modes,their life time and various decay Channels. also recently the discovery of new nuclear structures such as the neutron halo has suggested the possible existence of novel vibrationnal modes such as the soft dipole a vibration of the core against the halo.

Hot Giant resonances
In the 80’s were discovered the giant resonances built on the compound nucleus(hot nucleus). Classically it is natural to consider the vibrations of a hot drop of nuclear liquid. However a quantum description of such states is difficult. The microscopic theories like the RPA describe such state as a coherent single-particle excitations.
The  properties of the hot giant resonances are not yet understood. Of particular interest are the evolution of the width and the observed disappearance of this mode above a given temperature (about 5MeV for tin nuclei). These features have been widely investigated by our group partly in collaboration with the experimental groups of Orsay, Saclay and Catania.

Multiphonons
The giant resonances are interpreted as vibrations of nuclei. From the quantum point of view one expects that not only the first vibration quantum (the giant resonances themselves) will be observable but also the higher quanta (the multiphonons). These new modes have been recently observed in several experiments in particular at GANIL. Their properties such as their energy and width are in good agreement with the naive picture of independent phonon excitations.

However, it seems that their excitation probability is systematically bigger than the theoretical predictions. Those theories correspond to the simple picture shown below of an harmonic oscillator forcedby an external field. We have proposed that the problem could be that the excitation dynamics of giant resonances are in fact non-linear (and anharmonic).

Multifragmentation
Interactions are like Van der Waals forces with an attractive part at long distance and a short range repulsive component (hard core). Therefore one expects for the nuclear matter a typical phase diagram with a liquid and a gas phase. The gas phase could be for example the dilute plasma of protons and electrons as observed in stars or the nucleons evaporated from a hot nucleus while the liquid phase can be found in nuclei or in neutron stars. 
The determination of this phase diagram is one of the major issues in Nuclear Physics. One of the idea is that a piece of matter suddenly quenched in the coexistence region would spontaneously break its symmetries and will develop fragments(as in the figure).This multifragmentation needs to be treated using new theoretical technics dealing with statistical dynamics out of equilibrium

Stochastic approaches
The multifragmentation needs to be treated using newtheoretical technics dealing with statistical dynamics out of equilibrium.Inparticular the theoretical tools should be able to generated an ensemble ofdifferent partitions from an unique initial condition.
One of the methods developed for this type of stochastic approaches is the projection on a subspace of global variables. The disregard degrees of freedom then plays the role of a fluctuating heat-bath.

The evolution with the temperature of an excited nucleus scission time is a powerful way to get information on the nuclear dissipation processes involved during the fission process [1].
An experimental program has been undertaken at GANIL in order to measure in aquite direct way, using the blocking technique in single crystals, the fission times as a function of the fissioning nucleus characteristics (excitation energy, mass, charge, angular momentum).
The first results [2,3] show that fission time scales inferred inprevious experiments from pre- or post-scission particle multiplicities [1] are underestimated by orders of magnitude, due the lack of sensitivity of these experiments to long lifetime components.

The principle of fission time measurement [4] by the blocking technique can be briefly summarized as follows. When a single crystal is used as a target, blocking effects give rise to dips in the angular distributions of fission fragments detected around the directions ofaxes and planes of the crystal. The dips arise from the repulsive potential due to the atomic rows or planes that modifies the fission fragment trajectories within the crystal.
The distribution of the scission distances from the axis or plane of the crystal determines the blocking patterns. This technique has only a restricted range of time sensitivity. On the one hand, if the fissioning nuclei travel through the crystal on a distance on the average much larger than the spacing between two rows or planes, no dip is observed in the angular distribution. On the other hand, if the scissions occur on average within the thermal vibration domain of the atoms of the crystal, the potential experienced by the fission fragments is smeared and the dips in the angular distributions do not depend anymore on the fission times.The typical sensitivity range for the present experiments is between 10^-19s and $10^-16s.

Contact: Maurice Morjean – morjean@ganil.fr

References :
[1] D.Hilscher and H. Rossner, Ann. Phys.Fr. 17, 471 (1992)
[2] M. Morjean et al., Nucl. Phys.A630, 200c (1998)
[3] F. Goldenbaum et al., Phys. Rev. Lett. 82, 5012 (1999)
[4] W. M. Gibson, Ann. Rev.Nucl. Sci. 25, 465 (1975)