Charge exchange and knockout reactions induced by sn isotopes at relativistic energies

Resumo

In this work we propose to make use of peripheral heavy-ion collisions induced by stable and non-stable relativistic projectiles to investigate nuclear and nucleonic excitations in nuclei. We propose to induce these excitations by means of isobaric charge-exchange reactions and nucleon knockout. Charge-exchange reactions are spin-isospin transitions which can provide information on the isovector component of the nuclear force. These excitations may take place in two different energy regimes. At low energies this quasi-elastic charge exchange lead to nuclear excitations (Fermi, Gamow-Teller, spin-dipole, quadruple, giant resonance etc. ) and are understood as a virtual charged meson exchange between projectile and target nucleus. At higher energies the process is explained by the excitation of nucleonic resonances and their subsequent decay emitting real charged mesons escaping from the nuclear medium. In the present work we concentrate in the later case. We propose then to investigate baryon excitations in asymmetric nuclear matter. The peripheral character of these reactions, imposed by the escaping condition for the emitted mesons, could be used to investigate the relative abundance of protons and neutrons at the nuclear periphery. In this work we also investigate nucleon knockout processes. These reactions have been largely used to investigate the single-particle structure and the radial extension of nuclei far from stability. Moreover, we make use of these reactions to investigate nuclear excitations in proton-removal collisions. The investigation of these processes was done experimentally, running an experiment at the GSI (Germany) facilities. In this experiment we used stable (112Sn , 124Sn ) and unstable (110Sn , 120Sn and 122Sn) tin projectiles accelerated at 1000 MeV. The systematic study of the energy dependece of charge exchange reaction used a beam of 112Sn at different energies : 1000, 700 and 400 AMeV. In addition, we used different targets ( C, CH2, Cu and Pb) to systematically investigate these reactions according to the projectile isospin and energy and target nature. The projectile reaction residues were identified in atomic and mass number using a high-resolution zero-degree magnetic spectrometer, the Fragment Separator (FRS). This spectrometer also allowed us to measure with high accuracy the longitudinal momentum of the projectile residues. From these measurements we could defined two observables to characterize isobar charge-exchange and nucleon knockout reactions, the cross sections of these reaction channels and the energy lost by the projectile residues. The missing energy spectra measured with the FRS for the projectile residues issued in isobar charge-exchange reactions showed two clear components. These components correspond to the quasi-elastic charge-exchange and the baryon excitation. Moreover, an accurate deconvolution technique developed in the framework of this investigation allowed us to identified several baryonic excitations, the Delta and Roper (N*(1440)) resonances and a third one not yet identified. We could also observe a clear dependence of the cross sections of these reaction channels with the neutron excess in the target and projectile nuclei. The systematic investigation of proton and neutron knockout reactions showed a clear dependence of the cross sections of these reaction channels with the projectile neutron excess. At large neutron excess proton knockout induces larger excitations and vice versa.