The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ^{48}Ti+^{238}U reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.

For nuclear reactions in which super-heavy nuclei can be formed, the essential difference between the fusion process followed by fission and non-equilibrium processes leading to fission-like fragments is the reaction time. Quite probable non-equilibrium processes, characterized by very short reaction times, are highlighted thanks to mass-angle correlations. However, long lifetime components associated with fission following fusion have been observed with two independent experimental techniques, providing evidence for the formation of compound nuclei with Z = 120 and 124, followed by mass asymmetric fission.

An atomic clock based on x-ray fluorescence yields has been used to estimate the mean characteristic time for fusion followed by fission in reactions 238U + 64Ni at 6.6  MeV/A. Inner shell vacancies are created during the collisions in the electronic structure of the possibly formed Z=120 compound nuclei. The filling of these vacancies accompanied by a x-ray emission with energies characteristic of Z=120 can take place only if the atomic transitions occur before nuclear fission. Therefore, the x-ray yield characteristic of the united atom with 120 protons is strongly related to the fission time and to the vacancy lifetimes. K x rays from the element with Z=120 have been unambiguously identified from a coupled analysis of the involved nuclear reaction mechanisms and of the measured photon spectra. A minimum mean fission time τ(f)=2.5×10(-18)  s has been deduced for Z=120 from the measured x-ray multiplicity.

Fusion-evaporation in the ^124Sn+^136Xe system is studied using a high intensity xenon beam provided by the Ganil accelerator and the LISE3 wien filter for the selection of the products. Due to the mass symmetry of the entrance system, the rejection of the beam by the spectrometer was of the order of 5times10^8. We have thus performed a detailed statistical analysis to estimate random events and to infer the fusion-evaporation cross sections. No signicant decay events were detected and upper limit cross sections of 172 pb, 87 pb and 235 pb were deduced for the synthesis of ^257Rf, ^258Rf and ^259Rf, respectively.

The formation of compound nuclei with Z = 120 and Z = 124 has been evidenced from their very long fission times measured by the blocking technique in single crystals. A possible explanation for the long measured fission times might be found in the temperature dependence of the fission barriers, as predicted by Hartree-Fock-Bogolubov calculations at finite temperature in this super-heavy nucleus domain.

the other, thus forming a giant composite system. Thereby, in such a model, the energy from the collective movement can be transformed into excitation energy via nuclear friction. In the present work, the spectrometer VAMOS was tuned for observing kinematics of deep inelastic reactions. Theoretical calculations indicate that if a long-living component would exist for this reaction, the reaction features would be a large transfer of nucleons associated with a large loss of kinetic energy. The mass flow and energy dissipation in U+U collisions at 7.42A MeV has been already investigated in ref. [1]. From our work we present reactions with such characteristics for different beam energies and compare them with theoretical model calculations [2].

We investigated deep inelastic reactions in 238 U + 238 U collisions at 6.09, 6.49, 6.91, 7.1 and 7.35× A MeV at the VAMOS spectrometer (GANIL). A large transfer of neutrons and protons was observed at all beam energies. For a transfer of more than 10 nucleons the total kinetic energy of the detected fragments becomes independent of the beam energy and reaches values far below the Coulomb barrier for spherical fragments. This points to the formation of a di-nuclear system in the entrance channel which develops an elongated shape and a strong neck. For such reactions we expect an enhanced lifetime of the di-nuclear system which is significantly longer than the time scale for elastic and quasi-elastic reactions. Different theoretical approaches predict delay times of more than 5×10 -21 s for a subset of our data.

An overview of selected recent experimental results on fission times is presented. Evidences for over-damped motion up to saddle point during the fission process of highly excited nuclei have been obtained independently through fission probability, pre-scission multiplicity and direct time measurements. In addition, strong clues have been found for a temperature dependency of friction. Experiments probing transient effects through fission probabilities are presented and the counterbalanced effects of friction and level density parameters are discussed. Promising perspectives for super-heavy stability studies, based on fission time measurements, are presented.

The blocking technique in single crystals is a direct method to investigate the presence of long fission time components. With a lead beam impinging on a germanium single crystal, we tried to produce compound nuclei (CN) with atomic number Z=114 at high excitation energy. Blocking patterns for reaction products are reconstructed with position sensitive detectors at 20 degrees relative to the beam direction. The Z and the energies of all products are measured with Delta E-E telescopes of the 4 pi INDRA array, so that all reaction channels are unambiguously identified. With this setup, we can reach long fission times (>10(-8)s) that can be associated with CN fissions. However, in contrast to previous experiments in which such long fission times could be measured for Z = 120 and 124, no hint of long lifetimes within our sensitivity limit for Z=114 was observed, which may be due to the neutron deficiency of the formed isotopes.

Reaction mechanism analyses performed with a 4pi detector for the systems 208Pb + Ge, 238U + Ni and 238U + Ge, combined with analyses of the associated reaction time distributions, provide us with evidence for nuclei with Z=120 and 124 living longer than 10(-18) s and arising from highly excited compound nuclei. By contrast, the neutron deficient nuclei with Z=114 possibly formed in 208Pb + Ge reactions have shorter lifetimes, close to or below the sensitivity limit of the experiment.

We searched for a long-living component in the collision of 238U+238U between 6.09 A and 7.35 A MeV. The experiment was performed at GANIL using the spectrometer VAMOS, tuned for observing reactions with kinematics similar to quasi-fission events. Theoretical calculations indicate that reactions with strong energy dissipation and a large number of transferred nucleons are correlated to a time delay in the decay of the giant system. We detected events of such type in the focal plane of VAMOS. These events present an excitation function increasing with bombarding energy.

. The blocking technique in single crystals has been applied to reaction time measurements for the 238 U + Ge system at 6.1 MeV/nucleon. Backed up with a reaction mechanism analysis using the INDRA 4π detector, it provides a direct experimental evidence for Z = 124 compound nuclei living longer than 10 -18 s, indicating thus very high fission barriers for this element.

We performed an experiment to search for a signature of a long living component in the collision of U-238 + U-238 between 6.09 and 7.35A MeV. The experiment was performed at GANIL using the spectrometer VAMOS, tuned for observing reactions with kinematics similar to fusion-fission events. Theoretical calculations indicate that if a long living component would exist for this reaction, the most probable fission channel of such a giant system would be via the emission of quasi-lead nuclei. We detected events of such a category in the focal plane of VAMOS. These events present an excitation function growing as a function of the bombarding energy.

Fission time measurements have been undertaken in the super-heavy elements domain around Z=120, using the crystal blocking technique. Complete fusion followed by fission mechanisms have been evidenced from time measurements and detailed kinematics analysis.

It is proposed to use transfer‐induced fission in inverse kinematics coupled to the large acceptance spectrometer VAMOS to identify in atomic and mass number the complete distribution of the fission fragments. The measure of the kinetic properties of the transfer partner allows for determining precisely the excitation of the fissioning system. For the first time, the isotopic yields of the heavy and light fragments may be measured as a function of the excitation energy in neutron‐rich actinides.

"Impact parameter dependent electron capture by dec elerated U 91+ ions at 20 MeV/u using crystal channeling conditions". D. Dauvergne, A. Bräuning-Demian, F. Bosch, H. Bräuning, M. Chevallier, C. Cohen, A. Gumberidze, R. Kirsch, C. Kozhuharov, A. l'Hoir, D. Liesen, P. H. Mokler, J.-C. Poizat, C. Ray, Th. Stöhlker, M. Tarisien, E. Testa, S. Toleikis et M. Toulemonde, Nuclear Instruments and Methods B 205 (2003) 773

Fission times have been measured by the blocking technique in single crystal for uranium nuclei and for super-heavy elements with Z = 120. The fission times measured for uranium nuclei can be reproduced by statistical calculations following the Bohr and Wheeler approach only if a friction coefficient increasing with temperature is considered. In the super-heavy element domain, a discrimination between the fast quasi-fission process and the slow fusion-fission one has been achieved from reaction time measurements. A minimum cross-section sigma = 22 mb for formation of Z = 120 compound nuclei in U-238+Ni reactions at 6.62 MeV/A has been inferred.

With energetic anti proton-light-nucleus interactions it is possible to reach excitation energies exceeding nuclear binding energy. For 1.2 GeV (p) over bar +Cu we observe the onset of vaporization at about 8 MeV/nucleon, but even at 15 MeV/nucleon the probability of vaporization is smaller than 15%. The threshold excitation energy for multifragmentation processes defined as the detection of at least 3 intermediate mass fragments is observed at about 4 MeV/nucleon for 1.2 GeV (p) over bar +Cu and Ag. The probability of multifragmentation is small and even at excitation energy above the total binding energy not larger than 5% and 20% of the corresponding inclusive cross sections.

Fission times of lead and uranium nuclei have been measured at GANIL by the crystal blocking method. The inverse kinematics was used. Fragment atomic numbers and total excitation energies were determined. For data analysis, full Monte-Carlo trajectory calculations were used to simulate the blocking patterns. The effect of post-scission emissions, included in our simulations, is discussed. At high excitation energies, the scissions occur dominantly at times shorter than 10(-19) s, whereas at low excitation energies (E* < 250-300 MeV), scissions occurring at much longer times with sizeable probabilities are observed both for uranium and for lead nuclei, leading to average scission times much longer than those inferred from pre-scission emission. (C) 2002 Elsevier Science B.V. All rights reserved.