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Results of the experiments aimed at the study of fission and quasi‐fission processes in the reactions 12C+204Pb, 48Ca+144,154Sm, 168Er, 208Pb, 238U, 244Pu, 248Cm; 58Fe+208Pb, 244Pu, 248Cm, and 64Ni+186W, 242Pu are presented. The choice of the above‐mentioned reactions was inspired by the experiments on the production of the isotopes 283112, 289114 and 283116 at Dubna using the same reactions. The 58Fe and 64Ni projectiles were chosen since the corresponding projectile‐target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U‐400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time‐of‐flight spectrometer of fission fragments CORSET and the neutron multi‐detector DEMON. The role of shell effects and the influence of the entrance channel asymmetry and the deformatio...

Results of the experiments aimed at the study of fission and quasi-fission processes in the reactions ^12C+^204Pb, ^48Ca-^144Sm, ^154Sm, ^168Er, ^208Pb, ^238U, ^244Pu, ^248Cm ; ^58Fe+^208Pb, ^244Pu, ^248Cm, and ^64Ni+^186W, ^242Pu are presented in the work. The choice of the above mentioned reactions was inspired by recent experiments on the production of the isotopes (283)112, (289)114 and (283)116 at Dubna [1], [2] using the same reactions. The Fe-58 and ^64Ni projectiles were chosen since the corresponding projectile-target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the ^400U accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time-of-flight spectrometer of fission fragments CORSET[3] and the neutron multi-detector DEMON[4], [5]. The role of shell effects and the influence of the entrance channel on the mechanism of the compound nucleus fusion-fission and the competitive process of quasi-fission are discussed.

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z = 82-122 formed in the reactions with ^48Ca and ^58Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the ^400U accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro, (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments, fission, quasi-fission and evaporation residues cross sections, multiplicities of neutrons and gamma quanta and their dependence on the mechanism of formation and decay of compound systems have been studied.

Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia I Universite Libre de Bruxelles, Bruxelles, Belgique 21nstitut de Recherches Subatomiques, CNRS-IN2P3, Strasbourg, France 'Department of Physics, University of Messina and INFN, Italy "Institute of Nuclear Physics of the National Nuclear Center of Kazakhstan 480082 Almaty, Kazakhstan 'Institute of Physics SASc, 84228 Bratislava, Slovakia

Fusion-evaporation and fusion-fission cross sections have been measured in the reaction 48 Ca+ 154 Sm from well below to well above the Coulomb barrier. The comparison of fusion cross sections for this reaction with previous data corresponding to 16 O+ 186 W, leading to the same compound nucleus 202 Pb * , puts in evidence an interesting competition between sub-barrier enhancement, due to the strong couplings related to the 154 Sm deformation, and above barrier suppression for the 48 Ca induced reaction. The fusion hindrance mechanism has been attributed to the strong quasi-fission component observed in the mass-energy distributions of fission fragments.

A recent publication reports that heavy-ion fusion cross sections at extreme subbarrier energies show a continuous change of their logarithmic slope with decreasing energy, resulting in a much steeper excitation function compared with theoretical predictions. We show that the energy dependence of this slope is partly due to the asymmetric shape of the Coulomb barrier, that is its deviation from a harmonic shape. We also point out that the large low-energy slope is consistent with the surprisingly large surface diffusenesses required to fit recent high-precision fusion data., 9 pages, 3 eps figures

We have studied the mass and energy distributions of fission fragments from the two reactions ${}^{12}\mathrm{C}{+}^{204}\mathrm{Pb}$ and ${}^{48}\mathrm{Ca}{+}^{168}\mathrm{Er}$ that lead to the same compound nucleus ${}^{216}{\mathrm{Ra}}^{*}.$ Despite the fact that the excitation energy was around 40 MeV in both cases, the contribution from asymmetric fission in the first reaction is only around 1.5% but is about 30% in the second. This marked increase in the yield of asymmetric products is connected to the quasifission process, in which important shell effects become evident. The mass-energy distributions are interpreted in terms of an independent decay mode competing with the normal fusion-fission process and possibly leading to a significant suppression of the fusion cross section itself.

Near-barrier excitation functions have been measured for evaporation-residue production and fission in the 12C + 204,206,208Pb and 48Ca + 168,170Er systems that lead to the compound nuclei 216,218,220Ra*. A Pronounced suppression of evaporation-residue production is observed for the more symmetric combinations, 48Ca + 168,170Er. We relate this to the significant quasifission components already observed for these systems.

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z = 82-122 formed in the reactions with Ca-48 and Fe-58 ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multidetector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments ; fission, quasifission, and evaporation residue cross sections ; and multiplicities of neutrons and gamma-quanta and their dependences on the mechanism of formation and decay of compound systems have been studied.

Heavy-ion fusion barrier distributions are now routinely obtained directly from experimental data. Measurements of the total γ-ray multiplicity for the fusion channels of the 58 Ni + 60 Ni system, which has striking yet well understood barrier structures, confirm the theoretical predictions that very high spins can be populated at energies close to (and even below) the nominal Coulomb barrier. The mapping from multiplicities to spin populations shows that structures in the barrier distribution are still evident in the γ-ray results.

Opening Talk: Nuclear Structure at Border Lines (Yu Ts Oganessian) Hard Photons: A Probe of Dynamical Effects in Heavy Ion Collisions at Intermediate Energy (R Alba et al.) Studying Exotic Nuclei Through Direct Reactions (Y Blumenfeld) Isospin Effects on Instabilities and Fragmentation Mechanisms (M Colonna et al.) New Opportunities with Beams of Rare Isotopes in the US (C K Gelbke) Preliminary Results and Future Activities at the GARFIELD Apparatus (F Gramegna et al.) Mean-Field Calculations of Super-Heavy Elements (P H Heenen) Recent Experiments and Plans for the Synthesis of Superheavy Elements at the GSI SHIP (S Hofmann) Nuclear Fission at Border Lines (M G Itkis et al.) Many-Body Theory at Extreme Isospin (H Lenske et al.) Probing Correlations in Many-Body Haloes (F M Marques Moreno) Theoretical Approaches and Experimental Evidences for Liquid-Vapor Phase Transitions in Nuclei (L G Moretto et al.) Study of Halo-Structure in Radioactive He and Li Nuclei with Proton Elastic Scattering (A V Dobrovolsky et al.) Effects of the Shell Structure in Reactions Leading to the Same Compound Nucleus or Different Isotopes (A K Nasirov et al.) New Magic Number, N=16, Near the Neutron Drip Line (A Ozawa) Experiments on Super-Heavy Nuclei at GANIL (J Peter et al.) Thermodynamics of Hot Nuclei: Multifragmentation and Phase Transition (M F Rivet) Structure and Properties of Superheavy Nuclei (I Muntian & A Sobiczewski) Backtraced Neutron Multiplicities and Capture Dynamics in the Superheavy Region (L Stuttge et al.) Astrophysical S Factors from Asymptotic Normalization Coefficients (R E Tribble et al.) and other papers.

Just over a century after the discovery of radioactivity by Becquerel and of new elements by the Curies, we have begun to master the techniques of performing experiments with radioactive beams. And, of course, the search for other new elements goes on. At the same time the exploitation of γ-ray arrays of increasing power pushes observation limits lower and lower in the search for new phenomena at high spins. Ancillary devices, capable of giving cleaner channel selection, also permit the study of higher-mass systems and of more exotic charge-to-mass ratios even with stable beams.

Through precision measurements of fusion cross sections at energies close to the Coulomb barrier and through the application of the method of “experimental barrier distributions” which these permit, many recent advances have been made in our understanding of the dynamical processes occurring during a heavy-ion collision. It is now clear that the target and projectile reach one another in superpositions of states which correspond to different orientations for rotational nuclei or to different induced deformations for vibrational nuclei. The creation of a neck of neutron matter has also long been postulated and by studying the isotopic dependence of the fusion reaction, some recent results in the 10Ca+90,96Zr systems appear to confirm this result. For large Z1Z2 a type of extra-push effect can arise from the same inelastic entrance-channel effects which enhance the fusion of lighter systems, though this will be absent in cases where the enhancement arises from neutron transfers.

A fascinating challenge in nuclear structure physics is to find the maximum atomic numberZ— the number of protons — for which a relatively stable nucleus can exist. The question is, just how many stable elements is it possible to make?

High precision fusion excitation functions have been measured for the 16 O+ 58 Ni and 16 O+ 62 Ni systems from which fusion barrier distributions have been evaluated. Coupled-reaction-channels (CRC) calculations, which describe elastic and quasi-elastic scattering, also satisfactorily reproduce the fusion cross sections and barrier distributions. The small value of Z 1 Z 2 in this case leads to barrier distributions with relatively little structure. However, in conjunction with the detailed elastic scattering data for these systems, this allows us to elucidate the role of previously ignored states in 16 O in pushing the entire distribution to lower energies. These shifts are consistent with derived magnitudes of polarization potentials for both systems.

Exact many-body calculations have been performed in a large Nilsson-model basis using a two-body monopole-pairing force. The diagonalisations are reduced to a manageable size by exploiting the symmetry of the general pairing Hamiltonian and time-reversal invariance. The results have been successfully applied to a study of multi-quasiparticle states in the Hf-W region. An extension of this work, calculating nuclear masses, allows a comparison with the experimental neutron-pairing energiesP N. These have been studied in detail for the Kr isotopes. Two important conclusions emerge (i) the monopole-pairing term must be generalized to a state-dependent interaction; good results being obtained for a delta-function force (ii) the small nuclear deformations relevant to this mass region appear to vary smoothly, as given by laser spectroscopy experiments, rather than showing the fluctuations of microscopic-macroscopic calculations.

The extraction of representations of the fusion barrier distribution from backward-angle, quasi-elastic, elastic and transfer excitation functions is discussed. Such excitation functions have been measured for , and projectiles incident on a variety of targets. The results are compared with representations obtained from fusion excitation functions. Varying in their sensitivity, all representations show evidence of the barrier structure. Differences between the scattering and the fusion representations can be related to the effects of coupling to residual, weak reaction channels.

Using the above-barrier fusion probabilities of Berriman and the appropriate extra-push energies, we re-investigate the quasi-fission dynamics of reactions leading to the compound nucleus 220 Th. A consistent description is obtained in terms of the entrance- channel barriers that are known to determine the initial capture process. The different dynamical deformations that give rise to these barriers significantly influence the potential- energy landscapes onto which the captured system is injected.

The regulatory step Start in the cell cycle of the budding yeast Saccharomyces cerevisiae is inhibited by nalidixic acid (Nal). To study this inhibition, mutations were identified that alter the sensitivity of yeast cells to Nal. Nal-sensitive mutations were sought because the inhibitory effects of Nal on wild-type cells are only transient, and wild-type cells naturally become refractory to Nal. Three complementation groups of Nal-sensitive mutations were found. Mutations in the first complementation group were shown to reside in the ARO7 gene, encoding chorismate mutase; tyrosine and phenylalanine synthesis was inhibited by Nal in these aro7 mutants, whereas wild-type chorismate mutase was unaffected, These aro7 alleles demonstrate 'recruitment', by mutation, of an innately indifferent protein to an inhibitor-sensitive form. The Nal-sensitive aro7 mutant cells were used to show that the resumption of Nal-inhibited nuclear activity and cell proliferation takes place while cytoplasmic Nal persists at concentrations inhibitory for the mutant chorismate mutase. Mutations in the second complementation group, nss2 (Nal-supersensitive), increased intracellular Nal concentrations, and may simply alter the permeability of cells to Nal. The third complementation group was found to be the ERG6 gene, previously suggested to encode the ergosterol biosynthetic enzyme sterol methyltransferase. Mutation or deletion of the ERG6 gene had little effect on the inhibition of Start by Nal, but prevented recovery from this inhibition. Mutation of ERG3, encoding another ergosterol biosynthetic enzyme, also caused Nal sensitivity, suggesting that plasma membrane sterol composition, and plasma membrane function, mediates recovery from Nal-mediated inhibition of Start.

The low-spin band structure of neutron-deficient 115 54Xe has been studied following a low-energy (212.3 MeV) 60Ni(58Ni,2pnγ) reaction performed at IReS, Strasbourg, with the Garel+ spectrometer. The relative excitation energies of νh11/2 (isomeric), νg7/2, and νd5/2 bandheads have been established.

We study in detail the barrier distributions extracted from large-angle quasi-elastic scattering of heavy ions at energies near the Coulomb barrier. Using a closed-form expression for scattering from a single barrier, we compare the quasi-elastic barrier distribution with the corresponding test function for fusion. We examine the isocentrifugal approximation in coupled-channels calculations of quasi-elastic scattering and find that for backward angles, it works well, justifying the concept of a barrier distribution for scattering processes. This method offers an interesting tool for investigating unstable nuclei. We illustrate this for the $^{32}$Mg + $^{208}$Pb reaction, where the quadrupole collectivity of the neutron-rich $^{32}$Mg remains to be clarified experimentally., Comment: 26 pages, 10 eps figures

A great many physical processes are mediated by the penetration or surmounting of a potential barrier. Some notable examples are the thermionic emission of electrons, chemical reactions, tunnelling of Cooper pairs through Josephson junctions, nuclear ‘burning’ in stars etc. Often the potential in question is not simply a function of the position of the intruding particle, but is a complicated landscape determined by the static structure of the physical objects concerned as well as by their dynamical response to their mutual interactions. For example, in a Josephson junction, the penetration of the barrier is strongly influenced by the excitation of lattice vibrations while catalysed chemical reactions proceed via complex routes which present a series of lower activation energies. In a physical system with a well-defined temperature, the details of the potential landscape may be obscured by the Maxwell-Boltzmann distribution of energies. The fusion of heavy nuclei can, however, be achieved over a controlled range of well-defined beam energies to reveal fine details of some of these effects. To date the major new experimental work in this area has been performed at the ANU, Canberra and at the INFN, Legnaro. As well as yielding valuable information on nuclear structure (detailed information on nuclear shapes etc.) and some totally unexpected reaction dynamics (complexity of the induced surface oscillations, neutron flow), these experiments and their theoretical analysis give considerable insights into the general problem of ‘tunnelling in the presence of an environment’.

Using a simple but reasonably realistic form for the long-range part of the real nucleus-nucleus potential some simple approximations are derived for the nuclear parts of the WKB phase shift and classical deflection function for the elastic scattering of two heavy ions. The formulae are presented in the form of universal functions which allow a rapid evaluation of the approximate “rainbow” angle and angular momentum. The numerical simplicity of this form of the nuclear phase allows a rapid determination of the corresponding cross section using the partial wave expansion. The cross sections show a shift of the quarter-point to forward angles with respect to the corresponding cross section in which to real nuclear potential is ignored. This suggests that critical angular momenta evaluated from the usual quater-point recipe will generally be an overestimate. It is shown that for very heavy ions the nuclear potential may give a large enhancement of the peak in the elastic cross section and indirect evidence for this effect is cite. For lighter ions the oscillatory structure of the cross sections appears to be dominated by absorption rather than by the real potential.

Most of the recent major developments in our understanding of nuclear structure physics stem from experiments performed with heavy-ion beams. The word 'ion' indicates the need to form a charged–state of an atom in order to accelerate its nucleus; 'heavy' generally means anything more massive than an α-particle.

International audience; Recent experimental data showing the interplay between fusion hindrance effects and the onset of the quasi-fission mechanism are presented. The aim of such investigations is understanding the role of entrance channel properties on the competing fusion-fission and quasi-fission processes, also in connection with the possible implications for superheavy element production. In this context, the role of mass asymmetry, nuclear deformation and shell effects is discussed.

Experimental results on the fusion inhibition effect near the Coulomb barrier due to the onset of the quasi-fission mechanism are presented. The investigation was focused on reactions induced by $^{48}$Ca projectiles on different heavy targets and comparing them to reactions induced by light ions such as $^{12}$C and $^{16}$O leading to the same compound nuclei. Cross sections and angular distributions of evaporation residues and fission fragments have been measured.