D. Cortina

Measurements of interaction cross sections for light neutron-rich nuclei at relativistic energies and determination of effective matter radii

Abstract We measured the interaction cross sections ( σ I ) of 10,11B, 12–20C, 14–23N, 16–24O and 18–26F on carbon targets at energies of around 950 A MeV. We then deduced the effective matter radii of the nuclei by a Glauber-model analysis. Based on the assumption of a core plus a valence neutron structure, we applied a Glauber-model analysis for a few-body system adapted for nuclei with an odd neutron number. We also deduced the effective nucleus-matter densities as well as some spectroscopic information for selected nuclei. Evidence for a one-neutron halo structure was found for 22N, 23O and 24F, as well as 19C.

Slowing down of relativistic few-electron heavy ions

Abstract Experimental data on energy loss and energy-loss straggling of fully ionized relativistic heavy ions agree well with the theory of Lindhard and Sorensen (LS). However, when heavy ions penetrate matter with many fluctuating ionic charge states the theoretical description is more complicate and less accurate. A novel exploratory step to contribute to a better understanding of the slowing down of heavy ions has been done with the present experiment in an energy region where the atomic interaction is still dominated by bare and few-electron projectiles. In the energy range of 100–1000 MeV/u we measured with the high-resolution magnetic spectrometer FRS the stopping powers, the energy-loss straggling and the charge-state distributions of 197 Au, 208 Pb and 209 Bi projectiles in charge-state equilibrium interacting with solids ranging from beryllium to lead targets. Additionally, charge exchange cross-sections have been measured to support a better analysis and interpretation of the energy-loss data. The experimental results on stopping power and energy-loss straggling clearly demonstrate the contribution of ions in different charge states. A novel application of the slowing down of relativistic exotic heavy ions is presented.

Measurements of the interaction cross sections for Ar and Cl isotopes

We have measured the interaction cross sections (σI) of 31–40Ar and 31–37Cl on carbon targets at energies of around 950 A MeV. The effective matter radii for these nuclei were deduced by a Glauber-model analysis. Combining our matter radii with measured charge radii for Ar isotopes, we could deduce the proton-skin thicknesses for the 32–40Ar isotopes, which were found to increase monotonically with decreasing neutron number. The larger radius of the proton drip-line nucleus 31Ar suggests an anomalous structure for this nucleus. In addition, using NaI(Tl) arrays surrounding the carbon target, we measured γ-rays emitted from excited states in these isotopes. In this way we could deduce the upper limits for the inelastic cross sections (σinela) on carbon targets at energies of around 950 A MeV.

Production cross-sections of light neutron-rich nuclei from 40Ar fragmentation at about 1 GeV/nucleon

Abstract Using a primary beam of 40 Ar at ∼1A GeV impinging on a Be target, the production cross-sections of light neutron-rich fragments from projectile fragmentation were measured at the projectile-fragment separator FRS at GSI. The experimental cross-sections were obtained for isotopes of the elements B to F both close to stability and near the neutron drip line. These data are compared to the results of the empirical parametrization EPAX. We also compare the results to those measured previously at LBL. As an additional result, the particle instability of 26 O has been confirmed.

Transfer reactions in inverse kinematics: An experimental approach for fission investigations

Inelastic and multinucleon transfer reactions between a U-238 beam, accelerated at 6.14 MeV/u, and a C-12 target were used for the production of neutron-rich, fissioning systems from U to Cm. A Si telescope, devoted to the detection of the targetlike nuclei, provided a characterization of the fissioning systems in atomic and mass numbers, as well as in excitation energy. Cross sections and angular and excitation-energy distributions were measured for the inelastic and transfer channels. Possible excitations of the targetlike nuclei were experimentally investigated for the first time, by means of gamma-ray measurements. The decays from the first excited states of 12C, B-11, and Be-10 were observed with probabilities of 0.12-0.14, while no evidence for the population of higher-lying states was found. Moreover, the fission probabilities of U-238, Np-239 and Pu-240,Pu-241,Pu-242 and Cm-244 were determined as a function of the excitation energy.

Giant resonances in unstable oxygen isotopes

Abstract Electromagnetic and nuclear breakup of the neutron-rich Oxygen isotopes ranging from A = 17 to A = 22 is studied experimentally in reactions at energies around 600 MeV/u. The beams were produced in fragmentation reactions and separated by the GSI Fragment Separator FRS. By measuring the four-momenta of all decay products after inelastic scattering and neutron decay of the projectile, the excitation energy is determined. From the differential cross sections dσ dE ∗ for electromagnetic excitation, the E1-strength distributions can be deduced. For 18,20,22 O, low-lying dipole strength is observed, exhausting about 5% of the Thomas Reiche Kuhn sumrule for energies up to 5 MeV above the continuum threshold.

Coulomb breakup of psd-shell neutron-rich nuclei

Inelastic scattering of loosely bound nuclei by Coulomb interaction at intermediate energies (400?600 MeV/nucleon) has been utilized as a spectroscopic tool for exotic nuclei. The observed electromagnetic dipole (E1) strength above the one neutron threshold of neutron-rich C, Be, B and O isotopes can be explained by a non-resonant transition of a neutron into the continuum. The shape of these strength distributions reflects properties of the wavefunction of the released neutron in the nucleus and hence ground-state properties of these isotopes. Neutron capture cross-sections such as for the 14C(n,?) 15C reaction which are of astrophysical relevance can be deduced indirectly.

Continuum excitations in neutron-rich Oxygen isotopes

Electromagnetic and nuclear excitations of the neutron-rich Oxygen isotopes ranging from A=17 to A=22 are studied experimentally in reactions at energies around 600 MeV/u. By measuring the four-momenta of all decay products the excitation energy is determined. From the differential cross sections for electromagnetic excitation, the E1-strength distributions can be deduced. For 18,20,22O, low-lying dipole strength is observed, exhausting about 5% of the energy weighted TRK sumrule for energies up to 5 MeV above the continuum threshold.

Production and beta half lives of nuclei close TI the nucleosynthesis R-process path at N=126

Cold‐fragmentation reactions of 208Pb projectiles impinging a beryllium target at 1 A GeV have been used to produce more than 190 nuclei “south” of lead, 25 of them for the first time. The beta half lives or upper limits of 13 of these new nuclei have been determined using position and time correlations between the nuclei and the betas in an active stopper. The measured half lives can only be described when first forbidden transitions are considered.

The dipole response of nuclei with large neutron excess

The dipole response of neutron-rich nuclei in the mass range from A = 10 to A = 22 and with mass to charge ratios of 2.5 to 2.8 has been invesitigated experimentally utilizing electromagnetic excitation in heavy-ion collisions at beam energies around 600 MeV/u.