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6 pages; International audience; Isomeric states in 253No have been investigated by conversion-electron and gamma-ray spectroscopy with the GABRIELA detection system. The 31 micro second isomer reported more than 30 years ago is found to decay to the ground state of 253No by the emission of a 167 keV M2 transition. The spin and parity of this low-lying isomeric state are established to be 5/2+. The presence of another longer-lived isomeric state is also discussed.

Measurements of fusion-evaporation cross-sections for the system 48Ca + 154Sm have been performed in the sub- and near-barrier energy range. Barrier-passing cross-sections have been obtained by adding recently measured capture-fission cross-sections at the same energies, and the barrier distribution for capture has been extracted. The data have been analyzed within a coupled-channel model, and a large subbarrier cross-section enhancement is observed, due to the ground-state prolate deformation of 154Sm. The 48Ca + 154Sm capture cross-sections are compared to existing data on 16O + 186W fusion, leading to the same CN, where a few higher-energy points have also been measured. The evaporation residue cross-sections for the two systems above the barrier indicate that complete fusion is inhibited for 48Ca + 154Sm by ≈ 40% in that energy region, with respect to 16O + 186W.

Fusion-fission and fusion-evaporation cross sections have been measured in a large energy range for the 4 8 Ca+ 1 6 8 Er, 1 5 4 Sm reactions. The comparison of the reduced evaporation data for such reactions and for collisions induced by light projectiles leading to the same compound nuclei 2 1 6 Ra* and 2 0 2 Pb* puts in evidence a fusion hindrance, which is particularly large for the first composite system. This fusion suppression is consistent with a noticeable contribution coming from quasi-fission events observed in the mass-energy distribution of fission fragments. The asymmetric shape of the quasi-fission component has been explained in terms of shell effects in the exit channel, favouring the population of closed shell fission fragments. The comparison of 4 8 Ca+ 1 5 4 Sm with preliminary data on 4 8 Ca+ 1 4 4 Sm seems to suggest that the target deformation plays a role in the onset of the quasi-fission mechanism.

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, 12C + 204Pb and 48Ca + 168Er, leading to the same compound nucleus 216Ra at the excitation energy E*∼40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the 48Ca + 168Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process.

Background: The nuclear structure of colliding nuclei is known to influence the fusion process. Couplings of the relative motion to nuclear shape deformations and vibrations lead to an enhancement of the sub-barrier fusion cross section in comparison with the predictions of one-dimensional barrier penetration models. This enhancement is explained by coupled-channels calculations including these couplings. The sub-barrier fusion cross section is also affected by nucleon transfer channels between the colliding nuclei.Purpose: The aim of the present experiment is to investigate the influence of the projectile and target nuclear structures on the fusion cross sections in the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ and $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ systems.Methods: The experimental and theoretical fusion excitation functions as well as the barrier distributions were compared for these two systems. Coupled-channels calculations were performed using the ccfull code.Results: Good agreement was found between the measured and calculated fusion cross sections for the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ system. The situation is different for the $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ system where the coupled-channels calculations with no nucleon transfer clearly underestimate the fusion cross sections below the Coulomb barrier. The fusion excitation function was, however, well reproduced at low and high energies by including the coupling to the neutron pair-transfer channel in the calculations.Conclusions: The nuclear structure of the colliding nuclei influences the fusion cross sections below the Coulomb barrier for both $^{40}\mathrm{Ca}+^{58,64}\mathrm{Ni}$ systems. Moreover, we highlighted the effect of the neutron pair-transfer channel on the fusion cross sections in $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$.

Experimental lifetimes and branching ratios for the -decay of even-even actinide nuclei can be fitted with wavefunctions at the nuclear surface which are practically nucleus independent. This suggests some common preformation process across the entire mass region. Our analysis is achieved without the introduction of a `knocking frequency' and within reasonably well defined limits, the amplitudes that we calculate can be directly related to the -particle preformation factor.

How can we explain that some Popular education militants are also referring to the Information Society and thus seem to join this plan, carried to a great extent by merchants and the authorities ? Which are the stakes at work in this “meeting” ? Popular education, in addition to a long and plural history, is not homogeneous. However, Popular education is marked by a common philosophy aiming at developing social, cultural and political people's emancipation. In the mean time, political and economic authorities need to get the support of social actors to carry out the Information Society. Within this framework, associations would be the relay of the development of this society ; the necessary social mediator of this plan. Meanwhile, Popular education movements are seeking ways to appropriate this concept in order to make it able to serve the interests of Popular education. But they also question the specific purposes of this model. Indeed, the reference to the Information Society allows the militants of Popular education to update their traditional matters, and also to come out of the crisis they are facing. Lastly, if this meeting seems, at first sight, to generate consensus, the inherent conflicts in the confrontation of the values and identities do not therefore disappear and question the real stakes at work.

Precise fusion and quasi-elastic scattering excitation functions have been measured for the systems 40 Ca + 90,96 Zr at energies spanning the Coulomb barrier. From both the fusion and the quasi-elastic scattering data representations of the barrier distributions have been extracted. The barrier distributions of the two reactions are qualitatively very different. While the lighter system is well described by coupled-channels calculations including multi-phonon excitations, this coupling scheme fails completely for the heavier system, possibly due to multi-neutron transfer.

This paper has three parts. In the first, it is thrown light about the economics of innovation departing from Schumpeter up to now. The main message is that innovation is a key issue to get economic wealth. But this is not without creating ups and downs in society using market economics and being liberal democracy. In the second, the global village's wealth at the long run is illuminated giving a world's legitimacy to the concern for innovation. In the third, a new knowledge policy is proposed. It is a way to capitalise on a wider spectrum of knowledge than today through market economics. Indeed, the new policy looks at the full set of knowledge a society from a given civilisation has at a certain moment of time. This seems to be more relevant when markets are open and civilisations encounter more than in the past. The new knowledge policy is a way to get new and more shared growth than in the past.

$\alpha-$decay through a deformed potential barrier produces significant mixing of angular momenta when mapped from the nuclear interior to the outside. Using experimental branching ratios and either semi-classical or coupled-channels transmission matrices, we have found that there is a set of internal amplitudes which are essentially constant for all even--even actinide nuclei. These same amplitudes also give good results for the known anisotropic $\alpha-$particle emission of the favored decays of odd nuclei in the same mass region. PACS numbers: 23.60.+e, 24.10.Eq, 27.90.+b

The rotational structure of ${}^{246}$Fm has been investigated using in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques. The experiment was performed using the JUROGAMII germanium detector array coupled to the gas-filled recoil ion transport unit (RITU) and the gamma recoil electron alpha tagging (GREAT) focal plane detection system. Nuclei of ${}^{246}$Fm were produced using a 186 MeV beam of ${}^{40}$Ar impinging on a ${}^{208}$Pb target. The JUROGAMII array was fully instrumented with Tracking Numerical Treatment 2 Dubna (TNT2D) digital acquisition cards. The use of digital electronics and a rotating target allowed for unprecedented beam intensities of up to 71 particle-nanoamperes for prompt $\ensuremath{\gamma}$-ray spectroscopy at a level of approximately 11 nb. With all these major experimental advances a rotational band is observed in ${}^{246}$Fm.

International audience; In the context of the creation of heavy evaporation residues, the interplay between capture, quasifission and true compound-nucleus fission is discussed. An approximate factorisation of the cross section into a compound nucleus term and an entrance-channel term is derived, with each of these terms being clearly defined.

Using the random matrix model, we discuss the effect of couplings to non-collective states on the penetrability of a one dimensional potential barrier. We show that these non-collective excitations hinder the penetrability and thus smear the barrier distribution at energies above the barrier, while they do not affect significantly the penetrability at deep subbarrier energies. The energy dependence of the Q-value distribution obtained with this model is also discussed., 4 pages, 2 figures. A talk given at the 10th international conference on nucleus-nucleus collisions (NN2009), Aug. 16-21, 2009, Beijing, China

An isomeric state in {sup 255}Lr with a half-life of t{sub 1/2} = 1.4(1) ms and E{sub x}>720-keV has been observed for the first time using the GABRIELA setup at the focal plane of the VASSILISSA separator. Based on its K-forbiddeness, the configuration of the state is most probably formed by coupling the valence proton to a two quasiparticle neutron excitation. Possible three quasiparticle configurations are discussed.

The 'experimental barrier distribution' provides a parameter-free representation of experimental heavy-ion capture cross sections that highlights the effects of entrance-channel couplings. Its relation to the s-wave transmission is discussed, and in particular it is shown how the full capture cross section can be generated from an l=0 coupled-channels calculation. Furthermore, it is shown how this transmission can be simply exploited in calculations of quasifission and evaporation-residue cross sections. The system {sup 48}Ca+{sup 154}Sm is studied in detail. A calculation of the compound-nucleus spin distribution reveals a possible energy dependence of barrier weights due to polarization arising from target and projectile quadrupole phonon states; this effect also gives rise to an entrance-channel 'extra-push'.

Near-barrier fusion excitation functions for the $^{14}\mathrm{C}$ and $^{18}\mathrm{O}+^{208}\mathrm{Pb}$ reactions have been analyzed in the framework of the barrier-passing model using different forms of the nuclear potential and the phenomenology of a fluctuating barrier. The best-fit fusion potentials were used to estimate cluster decay probabilities from the corresponding ground states of Ra and Th (i.e., for the inverse decay process). The analysis supports the ``\ensuremath{\alpha}-decay-like'' scenario for carbon and oxygen emission from these nuclei.

A precise fusion excitation function has been measured for 40 Ca+ 94 Zr at near- and sub-barrier energies, and the fusion barrier distribution has been extracted. Comparing with the existing data for 40,48 Ca+ 90,96 Zr shows that couplings to inelastic excitations determine the fusion cross sections and the shape of the barrier distributions near the barrier. At lower energies, the two systems possessing neutron-transfer channels with large positive Q-values ( 40 Ca+ 94,96 Zr) have remarkably similar cross sections, and both show a large enhancement with respect to the other systems and to coupled-channels calculations including up to three quadrupole and octupole phonons in the Zr targets.

Mass-energy and angular distributions of fission fragments for the $^{48}\mathrm{Ca}+^{144,154}\mathrm{Sm}\ensuremath{\rightarrow}^{192,202}\mathrm{Pb}$, $^{40}\mathrm{Ca}+^{154}\mathrm{Sm}\ensuremath{\rightarrow}^{194}\mathrm{Pb}$ reactions have been measured. Fusion suppression and the presence of quasifission at energies near and below the Coulomb barrier have been observed for the reactions with the deformed target $^{154}\mathrm{Sm}$. In the case of the spherical $^{144}\mathrm{Sm}$ target no evidence of quasifission has been found. Quasifission cross sections have been extracted from total fission-like events by analysis of their mass and angular distributions.

Large-angle quasielastic scattering has been studied in a high- Z 1 Z 2 nuclear reaction of the type leading to superheavy-element production by cold fusion. We show that despite the presence of strongly dissipative channels, and the complete absence of fusion, the notion of an external barrier distribution, determined by strong coupling to collective excitations of target and projectile, is still valid. Furthermore, our method allows us to deduce some properties of the deep-inelastic processes in this system.

In view of its large deformation, which should give rise to strong barrier structures, the 20 Ne projectile has been used to study the influence of neutron-transfer channels on the distribution of barrier heights. A previous experiment with a combination of projectiles 20,22 Ne suggested that the role of α-particle transfer channels is small. By an appropriate choice of targets (90 Zr and 92 Zr ), having very similar intrinsic structures (excited states and deformation parameters) but giving rise to different Q-values for neutron transfers, we highlight the role of these channels.

Excited states in {sup 249}Fm were populated via the {alpha} decay of {sup 253}No and the subsequent decay was observed with the GABRIELA detection system installed at the focal plane of the VASSILISSA recoil separator. The energies, spins, and parities of these states could be established through combined {alpha},{gamma}, and conversion-electron spectroscopy. The first members of the ground-state rotational band were identified. Their excitation energies as well as the observation of a cross-over E2 transition confirm the assignment of 7/2{sup +}624 for the ground state of {sup 249}Fm. Two excited states were also observed and their decay properties suggest that they correspond to the particle excitation 9/2{sup -}734 and hole excitation 5/2{sup +}622. The analysis suggests that the 279-keV transition de-exciting the 9/2{sup -} state has anomalous E1 conversion coefficients.

A new experiment to determine the thermonuclear cross section of the C-12(alpha,gamma)O-16 reaction has been performed in regular kinematics using an intense alpha-particle beam of up to 340 mu A from the Stuttgart DYNAMITRON. For the first time, a 4 pi-germanium-detector setup has been used to measure the angular distribution of the gamma rays at all angles simultaneously. It consisted of an array of nine EUROGAM high-purity Ge detectors in close geometry, actively shielded individually with bismuth germanate crystals. The C-12 targets were isotopically enriched by magnetic separation during implantation. The depth profiles of the implanted carbon in the C-12 targets were determined by Rutherford backscattering for purposes of cross-section normalization and absolute determination of the E1 and E2 S factors. Angular distributions of the gamma decay to the O-16 ground state were measured in the energy range E-c.m.=1.30-2.78 MeV and in the angular range (lab.) 30 degrees-130 degrees. From these distributions, astrophysical E1 and E2 S-factor functions vs energy were calculated, both of which are indispensable to the modeling of this reaction and the extrapolation toward lower energies. The separation of the E1 and E2 capture channels was done both by taking the phase value phi(12) as a free parameter and by fixing it using the results of elastic alpha-particle scattering on C-12 in the same energy range.

Fusion-evaporation cross sections were measured in the two systems {sup 48}Ca+{sup 90,96}Zr in an energy range from well below to well above the Coulomb barrier. The sub-barrier fusion of {sup 48}Ca+{sup 90}Zr is reproduced by coupled-channels calculations including the lowest quadrupole and octupole vibrations of {sup 90}Zr, and using a Woods-Saxon potential with a standard diffuseness parameter a = 0.68 fm. However, the fusion cross sections are overestimated above the barrier. The low-energy slope of the excitation function for {sup 48}Ca+{sup 96}Zr is steeper. This implies a larger diffuseness parameter a = 0.85 fm. Fusion cross sections are well fit in the whole energy range, and the effect of the strong octupole vibration in {sup 96}Zr is predominant. The extracted fusion barrier distributions are reasonably well reproduced by calculations for both systems. A comparison with previous data for {sup 40}Ca+{sup 90,96}Zr is made in an attempt to clarify the role of transfer couplings in sub-barrier fusion.

The interest in the study of the fission process of superheavy nuclei mainly deals with the opportunity to obtain information about the cross-section of the compound nucleus (CN) formation at excitation energies E*\approx 15–30 MeV. It allows one to estimate the survival probability of the superheavy composite system after evaporation of 1–3 neutrons, i.e. in "cold" or "warm" fusion reactions. However, in order to solve this problem deeper understanding of the coalescence processes between colliding nuclei, the competition between fusion-fission and quasi-fission processes is needed. The characteristics of both processes, their manifestation in the experimental observables and the relative contribution to the capture cross-section in dependence on the excitation energies, reaction entrance channel etc were investigated for a wide range of target-projectile combinations. Results of the experiments devoted to the study of the fusion-fission and quasi-fission processes in the reactions of the formation of the superheavy nuclei with Z = 102–122 are presented. The heavy ions 26Mg, 48Ca, 50Ti, 58Fe and 64Ni were used as projectiles. The choice of the reactions with 48Ca and actinide-targets was inspired by the experiments on the production of the isotopes 283112, 289114 and 283116 in Dubna using the same reactions. The 50Ti, 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) and the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) using the time-of-flight spectrometer of fission fragments CORSET. The role of the shell effects, the influence of the entrance channel asymmetry and the deformations of colliding nuclei on the mechanism of the fusion-fission and the competitive process of quasi-fission are discussed. The recent results on synthesis of superheavy nuclei and the perspectives of the "hot" fusion reaction for the production of superheavy nuclei are considered.

Symmetric heavy-ion collisions are known to display an `extra-push' effect. That is, the energy at which the s-wave transmission is 0.5 lies significantly higher than the nominal Coulomb barrier. Despite this, however, the capture cross section is still greatly enhanced below the uncoupled barrier. It is shown that this phenomenon can be simply explained in terms of entrance-channel effects which account for long-range Coulomb excitations., 9 pages, 4 ps figures, uses elsart.cls

The mass-energy distributions of the fragments, fusion-fission (OFF) and capture (oCap) cross-sections in the reactions 26Mg+248Cm; 48Ca+2"RPb, 232Th, 238U , 2 4 4 ~ u , 2 4 8 ~ m ; 244Pu, 248Cm were measured at the U400 accelerator of Flerov Laboratory of Nuclear Reactions (JINR, Russia) with use of double-ann time-of-flight spectrometer CORSET. The influence of reaction entrance channel on the competition between fusion-fission and quasi-fission processes was studied. Strong manifestation of the shell effects in mass distributions of fusion-fission and quasi-fission fragments was observed. The multimodal fission phenomena were found for 2 7 4 H ~ and 256N0 nuclei at low excitation energies. % ~ ~ 2 0 8 p b , 232~h

In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section σ cap by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to σ cap. For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate σ cap. Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding σ cap through use of an evaporation-model code that takes account of the FF degree of freedom. Some earlier results on σ cap obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the “extra-push” phenomenon discussed. One may also try to obtain σ cap by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the 86Kr + 208Pb system at iThemba LABS in South Africa will be presented.

International audience; A breakthrough was recently obtained in the analysis of the so-called Hyper-Long-HyperDeformed (HLHD) experiment made at the EUROBALL-IV γ-detector array (EB). The 64Ni + 64Ni ⇒ 128Ba* fusion reaction was studied at Ebeam = 255 and 261 MeV, reaching the highest angular momentum that the compound nuclei can accommodate. To date no discrete HD rotational bands have been identified. However, rotational patterns in the form of ridge-structures in three-dimensional (3D) rotational mapped spectra are identified with dynamic moments of inertia J(2) ranging from 71 to 1112 MeV-1 in 12 different nuclei selected by charged particle- and/or γ-gating. The four nuclei, 118Te, 124Cs, 125Cs and 124Xe found with moment of inertia J(2)≥100 2 MeV-1, are most likely hyperdeformed, the remaining nuclei with smaller values of J(2), are considered to be superdeformed, in qualitative agreement with recent theoretical calculations.

Motivated by the extreme deformation parameters of the projectile, we have measured quasi-elastic scattering for 20 Ne + 118 Sn. In contrast to calculations based on known collective states, the experimental barrier distribution is structureless. A comparison with the system 22 Ne + 118 Sn shows that this smoothing is unlikely to be due to nucleon- or α -transfer channels, and is more likely to be due to coupling to many other weak channels.

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.

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.

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.

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.

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.