Hot nuclear matter
At GANIL we study nuclear matter under extreme conditions of temperature, pressure and isospin in order to investigate the nuclear equation of state. This is an essential ingredient in models of stellar evolution (supernovae, neutron stars, etc.) as well as for improving our fundamental knowledge of nuclei. To create such extreme conditions in the laboratory, we use collisions of heavy ions accelerated by the GANIL cyclotrons. Using more and more efficient detector arrays to collect the large numbers of reaction products, we can reconstruct the history of each collision and compare it with the predictions of different theoretical models.
The GANIL beams allow the study of heavy ion collisions from just above the Coulomb barrier up to 100 MeV/nucleon. This is exactly the adequate energy domain to investigate the phase coexistence region predicted by numerous theoretical models for nuclear matter, which is a strong constraint for the equation of state. Scanning this intermediate energy range, we observe:
- low-energy collisions produce moderately excited hot nuclei which decay essentially by evaporation of light particles and fission. Statistical models are in reasonable agreement with experimental data for these decay modes.
- for intermediate energies a new decay mode becomes important: multi-fragment emission, characterised by a copious number of intermediate mass fragments with atomic number Z>=3.
- at higher energies the nuclei undergo complete disassembly into light particles, called vaporisation.
These observations, confirmed by many different experiments throughout the world, can be interpreted as the signature of a phase transition of excited nuclear matter. In order to do so, one must first correctly disentangle the properties of equilibrated excited nuclear systems produced in the reactions from the dynamical (non-thermal) processes which become also important in this energy domain.