Inelastic and multi-nucleon transfer reactions between a 238 U beam, accelerated at 6.14 MeV/u, and a 12 C target were used for the production of neutron-rich, fissioning systems from U to Cm. A Si telescope, devoted to the detection of the target-like nuclei, provided a characterization of the fissioning systems in atomic and mass numbers, as well as in excitation energy. Cross-sections, angular and excitation-energy distributions were measured for the inelastic and transfer channels. Possible excitations of the target-like nuclei were experimentally investigated for the first time, by means of γ-ray measurements. The decays from the first excited states of 12 C, 11 B and 10 Be 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 238 U, 239 Np and 240,241,242 Pu and 244 Cm were determined as a function of the excitation energy.

The isotopic-yield distributions and kinematic properties of fragments produced in transfer-induced fission of 240Pu and fusion-induced fission of 250Cf, with 9 MeV and 45 MeV of excitation energy respectively, were measured in inverse kinematics with the spectrometer VAMOS. The kinematic properties of identified fission fragments allow to derive properties of the scission configuration such as the distance between fragments, the total kinetic energy, the neutron multiplicity, the total excitation energy, and, for the first time, the proton- and neutron-number sharing during the emergence of the fragments. These properties of the scission point are studied as functions of the fragment atomic number. The correlation between these observables, gathered in one single experiment and for two different fissioning systems at different excitation energies, give valuable information for the understanding and modeling of the fission process.Comment: 12 pages, 8 figure

Reliable neutron-induced-reaction cross sections of unstable nuclei are essential for nuclear astrophysics and applications but their direct measurement is often impossible. The surrogate-reaction method is one of the most promising alternatives to access these cross sections. In this work, we successfully applied the surrogate-reaction method to infer for the first time both the neutron-induced fission and radiative capture cross sections of 239 Pu in a consistent manner from a single measurement. This was achieved by combining simultaneously measured fission and γ-emission probabilities for the 240 Puð 4 He; 4 He 0 Þ surrogate reaction with a calculation of the angular-momentum and parity distributions populated in this reaction. While other experiments measure the probabilities for some selected γ-ray transitions, we measure the γ-emission probability. This enlarges the applicability of the surrogate-reaction method.

Total fission cross sections of 208 Pb induced by protons have been determined at 370A, 500A, and 650A MeV. The experiment was performed at GSI Darmstadt where the combined use of the inverse kinematics technique with an efficient detection setup allowed us to determine these cross sections with an uncertainty below 6%. This result was achieved by an accurate beam selection and registration of both fission fragments in coincidence which were also clearly distinguished from other reaction channels. These data solve existing discrepancies between previous measurements, providing new values for the Prokofiev systematics. The data also allow us to investigate the fission process at high excitation energies and small deformations. In particular, some fundamental questions about fission dynamics have been addressed, which are related to dissipative and transient time effects.

Transfer-and fusion-induced fission in inverse kinematics has proved to be a powerful tool to investigate nuclear fission, widening information on the fission fragments and access to unstable fissioning systems with respect to other experimental approaches. An experimental campaign is being carried out at GANIL with this technique since 2008. In these experiments, a beam of 238 U, accelerated to 6.1 MeV/u, impinges on a 12 C target. Fissioning systems from U to Cf are populated through inelastic scattering, transfer, and fusion reactions, with excitation energies that range from a few MeV up to 46 MeV. The use of inverse kinematics, the SPIDER telescope, and the VAMOS spectrometer allow the characterization of the fissioning system in terms of mass, nuclear charge, and excitation energy, and the isotopic identification of the full fragment distribution. This work reports on new data from the second experiment of the campaign on fission-fragment yields of the heavy actinides 238 U, 239 Np, 240 Pu, 244 Cm, and 250 Cf, which are of interest from both fundamental and application points of view.

The two fragments produced in fission reactions induced by 208 Pb projectiles impinging on a liquid hydrogen target at 500A MeV have been fully identified in coincidence. The experiment was performed at GSI Darmstadt, where the combined use of the inverse kinematics technique with an efficient detection setup permitted one to detect and to fully identify the fission fragments in a range from Z = 27 to Z = 52. The corresponding isotopic cross sections and velocities of the fission fragments were measured with high accuracy. The results are compared to state-of-the-art model calculations that reproduce the measured observables. Correlations between the two fragments were used to assess the role of charge polarization and the excitation energy gained by the nascent fragments using the neutron excess of the final fragments. The analysis of the average velocities of the fission fragments allowed us to parametrize the distance between the two fission fragments at scission as a function of the size of the fissioning system.

International audienceThe nuclear level density is one of the main ingredients for the statistical description of the fission process.In this work, we propose to constrain the description of this parameter by using fission reactions induced byprotons and light ions on 208Pb at high kinetic energies. The experiment was performed at GSI (Darmstadt),where the combined use of the inverse kinematics technique with an efficient detection setup allowed us tomeasure the atomic number of the two fission fragments in coincidence. This measurement permitted us to obtainwith high precision the partial fission cross sections and the width of the charge distribution as a function of theatomic number of the fissioning system. These data and others previously measured, covering a large range infissility, are compared to state-of-the-art calculations. The results reveal that total and partial fission cross sectionscannot unambiguously constrain the level density at ground-state and saddle-point deformations and additionalobservables, such as the width of the charge distribution of the final fission fragments, are required

No abstract