The study of reaction mechanisms in heavy-ion collisions between 20 and 100 MeV/nucleon is inextricably linked to that of multi-fragment emission or multifragmentation. At these energies, most reactions lead to the copious production of intermediate masss fragments (IMF) with atomic numbers Z>=3. Understanding the significance of this multi-fragment production is one of the major goals of current nuclear physics research :
- is multi-fragment emission a signature of the exploration of the coexistence region of the nuclear matter phase diagram ? Among the different signals which are currently the most studied by the GANIL community are
negative heat capacity
charge correlations and spinodal decomposition
- what is the importance of the role played by dynamical effects in these collisions where there is strong competition between the nuclear mean field and nucleon-nucleon collisions ? Experimental evidence for such effects includes
light particle and IMF production at mid-rapidity
determination of evaporated and non-evaporated particles
non-spherical multifragmenting sources in central collisions
Statistical models such as SMM (Statistical Multifragmentation Model of Bondorf et al) or MMMC (Metropolis Microcanonical Multifragmentation Code of Gross et al) use thermodynamical arguments to simulate the disassembly of an equilibrated excited source. A good agreement is found in many cases by including in the statistical model some dynamical effects, such as collective flow energy and deformed source topologies.
Dynamical models such as BUU/BNV/Landau-Vlasov... (based on the so-called nuclear Boltzmann equation), BOB (based on the Boltzmann-Langevin transport theory), or QMD/AMD... (Quantum/Antisymmetrized... Molecular Dynamics) are approximate solutions of the quantum N-body problem, and therefore give an incomplete description of the collisions. Different models reproduce different aspects of the collisions with varying success.