1. Motivation: the physics case

In a nuclear collision, varying the energy of the reaction in equivalent to vary the characteristic time of the interaction. This is specially important in the investigation of exotic nuclei. There, one can sepparate the outer nucleons, which form the skin of the halo, and have binding energies about 1 MeV or less, from the inner nucleons, which form the core, and have binding energies of 8 MeV or more. The time scale of the motion of the outer nucleons, which is τ= ħ / B = 6 • 10-22 s, is different from that of the inner nucleons, which is 8•10-23 s. This fact illustrates the importance of doing reactions at different collision energies, to control the time scale of the interaction. There are some phenomena which occur specially at low energies. One of these is the transfer of nucleons. Transfer requires the existence of a common phase space, in coordinates and momenta, of the states in the nucleon in the initial nucleus and in the target. The spread of velocities in projectile and target is about β ≈ ħ l / m c r ≈ 0.1 . So, there would be a momentum mismatch for relative velocities larger than β ≈ 0.2, corresponding to about 20 MeV/ u. Coulomb excitation measurements will be specially important in heavy targets just below the Coulomb barrier, where nuclear effects are small. The energy range which is provided at the Low-Energy Branch allows to investigate the effect of Coulomb and nuclear fields on the time scales which are most relevant for the structure of exotic nuclei. In this sense and as already mentioned above, reactions at low energies are complementary to the experiments performed at the R3B-cave.

It is proposed to study elastic and inelastic scattering, break-up and transfer reactions of exotic nuclei at low energies, from around few MeV/u up to about 20 MeV/u, using the HYbrid DEtector-BALL array HYDE-BALL. Special interest will be devoted to nuclei at the neutron and proton drip line. Targets from light or heavy nuclei will be chosen depending to maximize the effect of nuclear or Coulomb fields. Also, the target nuclei can be chosen considering the properties (binding energy, angular momentum) of the single particle states to which protons and neutrons could be transferred. Such studies should provide angular distributions of elastic, transfer and fusion reactions, providing complementary tests for B(E) values, quadrupole deformations, collective phenomena and nucleon-nucleon correlations at the border lines of nuclear stability.

It is important to note that the Low-Energy Branch is the only facility world-wide, where reactions at kinetic energies from a few MeV/u up to about 20 MeV/u can be performed with nuclides having half-lives down to the sub-microsecond regime. Such opportunities are not available at present/nearest future Radioactive Beam Facilities in Europe.