Search

Topical collection : Perspectives on Nuclear Dta for the Next Decade; Expérience GANIL/VAMOS; International audience; Inverse kinematics is a new tool to study nuclear fission. Its main advantage is the possibility tomeasure with an unmatched resolution the atomic number of fission fragments, leading to new observablesin the properties of fission-fragment distributions. In addition to the resolution improvement, the studyof fission based on nuclear collisions in inverse kinematics beneficiates from a larger view with respect tothe neutron-induced fission, as in a single experiment the number of fissioning systems and the excitationenergy range are widden. With the use of spectrometers, mass and kinetic-energy distributions may now beinvestigated as a function of the proton and neutron number sharing. The production of fissioning nucleiin transfer reactions allows studying the isotopic yields of fission fragments as a function of the excitationenergy. The higher excitation energy resulting in the fusion reaction leading to the compound nucleus250Cf at an excitation energy of 45MeV is also presented. With the use of inverse kinematics, the chargepolarisation of fragments at scission is now revealed with high precision, and it is shown that it cannotbe neglected, even at higher excitation energies. In addition, the kinematical properties of the fragmentsinform on the deformation configuration at scission

The projectile fragmentation of a 92 Mo beam onto a beryllium target at an incident energy of 500 A MeV has been studied at the high-resolution spectrometer FRS at GSI. The isotopic production cross-sections of the projectile fragments produced in the reaction have been determined. The data have been compared to the empirical parameterisation, EPAX, and to a microscopic nuclear reaction model, ABRABLA.

Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope 23O. The 22O fragments produced in one-nucleon removal from 23O at 938 MeV/nucleon in a carbon target were detected in coincidence with de-exciting γ rays, allowing to discern between 22O ground-state and excited-states contributions. From the comparison of exclusive experimental momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated in an Eikonal model for the knockout process, and spin and parity assignment of I π = 1/2+ was deduced for the 23O ground state. This result solved the existent experimental discrepancy.

Coulomb breakup of neutron-rich nuclei around mass A ∼ 20 has been studied experimentally using secondary beams (∼ 500–600 MeV/u) of unstable nuclei produced at GSI. The spectroscopic factor deduced for the neutron occupying s 1/2 level in 15C ground state is consistent with the earlier reported value. The data analysis for Coulomb breakup of 17C shows that most of the cross section yields the 16C core in its excited state. For 17–22O, the low-lying E1 strength amounts up to about 12% of the energy weighted dipole sum rule strength depending on neutron excess. The cluster sum rule limit with 16O as a core is almost exhausted for 17,18O, while for more neutron rich isotopes the strength with respect to that limit decreases.

One-neutron removal cross-sections ( \(\)) of 17, 19C in various targets at about 900 MeV/nucleon, and one-proton removal cross-sections ( \(\)) of 8, 10B at about 1400 MeV/nucleon were measured using the fragment separator FRS at GSI. A significant increase of \(\) for 19C compared to its neighbors was observed. The same behavior was found for \(\) for the neutron-deficient nucleus 8B compared to the stable isotope 10B. These results support a one-neutron halo structure in 19C and a one-proton halo in 8B.

Coulomb breakup of neutron-rich Be, B, C, and O isotopes at relativistic energies (400–600 A MeV) was studied with kinematically complete measurements. The ground-state properties of these nuclei were deduced by comparing experimental data for Coulomb-dissociation cross-sections with directbreakup model calculations. The dominant ground-state configurations of 15C, 11Be, 14B, and 23O were found to be 14Cgs(0+) ⊗ v s, 10Begs(0+) ⊗ v s, 13Bgs(3/2+) ⊗ v s,d, and 22Ogs(0+) ⊗ v s, respectively, and 16C(2+) ⊗ v s,d, for 17C. The capture cross-section for 14C(n,λ) 15C relevant in astrophysical scenarios was measured indirectly through Coulomb dissociation.

Results are presented from a one-neutron knockout experiment at relativistic energies of $ \approx 420 A$ MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the $ \nu p_{1/2}$ , $ \nu p_{3/2}$ , (L = 1 and $ \nu f_{7/2}$ , $ \nu f_{5/2}$ (L = 3 neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the $ \nu f_{7/2}$ to the $ \nu p_{3/2}$ orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions $ V_{low k}$ obtained by evolving a chiral N3LO nucleon-nucleon potential.

One-neutron knockout reactions in a 9 Be target have been investigated at relativistic energies, near 700 MeV/u, for a set of sd-shell, neutron-rich nuclei. The experiment was performed in the FRS spectrometer, at GSI. gamma-ray measurements were carried out by means of the MINIBALL gamma-ray spectrometer and allowed the determination of partial cross-sections and branching ratios corresponding to the final states of the emerging knockout fragments. Experimental results are presented for C-17, N-19, O-21 and F-25 projectiles. The role of excited states of the N - 1 fragments in the composition of the ground state of these neutron-rich projectiles is outlined in this work.

Absolute differential cross-sections for elastic p 6He and p 8He scattering were measured in inverse kinematics with secondary beams. In order to supplement data taken for small-angle scattering, differential cross-sections for higher momentum transfer were measured using liquid hydrogen target. Both data sets were analyzed together. They have permitted to deduce the radial shape of the nuclear matter distributions and the root-mean-square radii with the help of Glauber theory. In addition to a phenomenological analysis, used already in the previous work, a model-independent analysis with a help of a Sum-Of-Gaussians (SOG) method has been performed. The experimental p 6;8He elastic scattering cross-sections have also been compared with the predictions from various theoretical nuclear models.

The β-decay of 40Ti was studied. The Fermi and Gamow-Teller strength function was deduced from the observed β-delayed emission of protons and γ-rays and from the measured half-life of 55(2) ms. The results are compared to shell-model predictions and are used to estimate the neutrino-capture rate of the solar-neutrino detector ICARUS.