The early phase of the collisions
A unique probe of the Fermi energy domain is nuclear
MeV) which originates from the most energetic collisions between the nucleons
in the reacting system. These photons probe the initial phase of the reaction
and are a direct manifestation of two-body dissipation. In contrast to
hadrons, photons escape the collision zone without further interaction.
Tracing the Bremsstrahlung spectra as function of impact parameter and
comparing with transport calculations, for beam energies around 100 MeV
per nucleon an initial compression corresponding to a maximum density of
is found. Despite the low yields two-photon correlations can be measured
and the results reveal that, for the larger systems, Bremsstrahlung continues
to be emitted beyond the first chance collisions. This means the photons
can be used as a tracer of the dynamical evolution of the system. The relation
between photon production and the expansion of the system leading eventually
to multifragmentation, will be explored by correlating the Bremsstrahlung
and multifragmentation signals in future experimental studies.
A unique path towards multifragmentation?
The latest experimental results confirm that a generalised
overall binary reaction mechanism persists throughout the energy range
of interest to multifragmentation studies. With increasing centrality an
equilibrated central reaction volume is growing. This allows to identify
and reconstruct projectile and target remnants whose sizes depend on the
impact parameter of the collision and thus it appears to be possible to
determine the size and the excitation energy of the reaction volume with
high accuracy. An adequate investigation, both from experimental and theoretical
points of view, of the collision dynamics of nuclei is strongly needed
in order to understand the evolution of the multifragmentation phenomenon
with impact parameter. The path leading towards a possible equilibrium
state is governed by the far-from equilibrium dynamical evolution of the
nuclear system, necessary to build a single excited piece of nuclear matter
out of two cold colliding objects. In order to achieve a quantitative description
of the equilibrium conditions the contributions to the final observables
from particles emitted prior to equilibrium need to be identified and separated.
Experimentally they are accessible by systematically varying the dynamical
conditions by exploiting various beam energies or projectile and target
sizes. Especially the impact parameter dependence of these phenomena, which
is of extreme importance for a correct determination of the size and excitation
of the observed piece of (equilibrated) nuclear matter, deserves further
studies. A good understanding of the macroscopic, as well as microscopic
nature of the interaction (for example how excitation energy and angular
momentum are shared between the collision partners, what the fluctuations
of such processes are, what the time scales,...) may be crucial to assess
whether the values of excitation, temperature and density, which are the
input for thermodynamical descriptions of multifragmentation are indeed
consistent with the preceding dynamical path of the collision. Only a proper
description of the dynamics as a function of impact parameter will give
confidence in understanding this path.
Fragment production in peripheral reactions
Signals of a transition between mean field behaviour and two-body collisions dominance may have been seen, below 100 MeV per nucleon, in the observation of significant fragment production associated with peripheral collisions. An example is shown in Figure . Detailed studies of these fragments reveal that they originate from a region of velocity space intermediate between projectile and target. It is conjectured that they are produced at the contact zone between the colliding nuclei. Some of these fragments are supposed to break away very early from the system and could be associated with the formation of a neck, others are produced at a later stage from the fission of one of the nuclei but with a strong memory (angular orientation) of the collision dynamics.
The physics behind the production of these fragments (light particles are also seen) is potentially very interesting since the neck region could have quite different properties from those of the bulk. This could be an inroad for the study of nuclear matter at variable density and isospin ratios. The respective influence of surface and volume mechanical instabilities and the role of fluctuation terms can potentially be investigated. Indeed first results obtained at GANIL have shown that in fragments originating from the neck region neutrons are strongly enriched. The use of dynamical models in studying these results has met with a certain degree of success in reproducing the average characteristics of this fragment production. But many aspects are still beyond the scope of these models: fragment production, multiplicities and size, free neutron and proton densities, etc.