M. A. G. Alvarez

Toward a global description of the nucleus-nucleus interaction

Extensive systematizations of theoretical and experimental nuclear densities and of optical potential strengths extracted from heavy-ion elastic scattering data analyses at low and intermediate energies are presented. The energy dependence of the nuclear potential is accounted for within a model based on the nonlocal nature of the interaction. The systematics indicates that the heavy-ion nuclear potential can be described in a simple global way through a double-folding shape, which basically depends only on the density of nucleons of the partners in the collision. The possibility of extracting information about the nucleon-nucleon interaction from the heavy-ion potential is investigated.

A parameter-free optical potential for the heavy-ion elastic scattering process☆

Abstract Thirty elastic scattering angular distributions for seven heavy-ion systems, in wide energy ranges, have been studied with the aim of systematizing the optical potential, real and imaginary parts, in a global way. The framework is: (i) an extensive systematization of nuclear densities, (ii) the energy dependence of the bare potential accounted by a model based on the nonlocal nature of the interaction, and (iii) the real and imaginary parts of the optical potential assumed to have the same radial shape.

Long range absorption in the scattering of 6He on 208Pb and 197Au at 27 MeV

Quasi-elastic scattering of 6He at E_lab=27 MeV from 197Au has been measured in the angular range of 6-72 degrees in the laboratory system employing LEDA and LAMP detection systems. These data, along with previously analysed data of 6He + 208Pb at the same energy, are analyzed using Optical Model calculations. The role of Coulomb dipole polarizability has been investigated. Large imaginary diffuseness parameters are required to fit the data. This result is an evidence for long range absorption mechanisms in 6He induced reactions. Comment: 10 pages, 10 figures, minor corrections. To appear in Nucl. Phys. A

Experimental determination of the ion-ion potential in the N = 50 target region: A tool to probe ground-state nuclear densities

Abstract Precise elastic and inelastic differential cross sections have been measured for the 16 O + 88 Sr , 90,92 Zr, 92 Mo systems at sub-barrier energies. From a coupled channel data analysis, the corresponding “experimental” bare potentials have been determined. The comparison of these potentials with those derived from double-folding theoretical calculations and the high energy (96 Mev/nucleon) elastic scattering data analysis indicate that the method is a very sensitive probe of the ground-state nuclear densities in the surface region.

Effect of the 18O nuclear density on the nuclear potentials of the 18O+58,60Ni systems

Abstract Quasi-elastic, inelastic, one- and two-neutron transfer differential cross sections have been measured for the 18 O+ 58,60 Ni systems at sub-barrier energies. The corresponding bare potentials have been determined at interaction distances larger than the respective barrier radii, and the results have been compared with those previously obtained for systems involving the 16 O as projectile. The detected difference between the 18 O and 16 O nuclear potentials has allowed the determination of the nuclear density that corresponds to the two extra neutrons of the 18 O nucleus.

The heavy-ion nuclear potential: determination of a systematic behavior at the region of surface interaction distances

Abstract Precise elastic scattering differential cross sections have been measured for the 16 O + 120 Sn , 138 Ba , 208 Pb systems at sub-barrier energies. The corresponding “experimental” nuclear potentials have been determined at interaction distances larger than the Coulomb barrier radii. These experimental potentials have been compared with our earlier results for other systems, and with theoretical calculations based on the double-folding and liquid-drop models. We have shown that the nuclear potentials have a systematic behavior at the surface region. The present results for the 16 O + 208 Pb system are used to extend earlier studies of the dispersion relation to sub-barrier energies.

Proton-proton correlations observed in two-proton decay of 19Mg and 16Ne

Proton-proton correlations were observed for the two-proton decays of the ground states of 19 Mg and 16 Ne. The trajectories of the respective decay products, 17 Ne + p + p and 14 O + p + p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.

α-particle production in the scattering of 6He by 208Pb at energies around the Coulomb barrier

New experimental data from the scattering of 6He + 208Pb at energies around and below the Coulomb barrier are presented. The yield of breakup products coming from projectile fragmentation is dominated by a strong group of α particles. The energy and angular distribution of this group have been analyzed and compared with theoretical calculations. This analysis indicates that the α particles emitted at backward angles in this reaction are mainly due to two-neutron transfer to weakly bound states of the final nucleus.

Long range effects on the optical model of 6He around the Coulomb barrier

We present an optical model (OM) analysis of the elastic scattering data of the re- actions 6 He+ 27 Al and 6 He+ 208 Pb at incident energies around the Coulomb barrier. The bare part of the optical potential is constructed microscopically by means of a double folding procedure, using the Sao Paulo prescription without any renormal- ization. This bare interaction is supplemented with a Coulomb dipole polarization (CDP) potential, which takes into account the effect of the dipole Coulomb interac- tion. For this CDP potential, we use an analytical formula derived from the semiclas- sical theory of Coulomb excitation. The rest of the optical potential is parametrized in terms of Woods-Saxon shapes. In the 6 He+ 208 Pb case, the analysis confirms the presence of long range components, in agreement with previous works. Four-body Continuum-Discretized Coupled-Channels calculations have been performed in or- der to better understand the features of the optical potentials found in the OM analysis. This study searches to elucidate some aspects of the optical potential of weakly bound systems, such as the dispersion relation and the long range (attractive and absorptive) mechanisms.

Performance of upstream interaction region detectors for the FIRST experiment at GSI

The FIRST (Fragmentation of Ions Relevant for Space and Therapy) experiment at GSI has been designed to study carbon fragmentation, measuring 12C double differential cross sections (∂2σ/∂θ∂E) for different beam energies between 100 and 1000 MeV/u. The experimental setup integrates newly designed detectors in the, so called, Interaction Region around the graphite target. The Interaction Region upstream detectors are a 250 μm thick scintillator and a drift chamber optimized for a precise measurement of the ions interaction time and position on the target. In this article we review the design of the upstream detectors along with the preliminary results of the data taking performed on August 2011 with 400 MeV/u fully stripped carbon ion beam at GSI. Detectors performances will be reviewed and compared to those obtained during preliminary tests, performed with 500 MeV electrons (at the BTF facility in the INFN Frascati Laboratories) and 80 MeV/u protons and carbon ions (at the INFN LNS Laboratories in Catania).