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The heaviest currently known nuclei, which have up to 118 protons, have been produced in 48Ca induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing 48Ca with the radioactive target 249Bk, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short α decay chains were observed. The long chains were attributed to the decay of 294Ts. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to 293Ts. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts.

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion., 5 pages, 4 figures; Accepted for publication in Physical Review Letters

The formation of a compound nucleus in different projectile and target combinations is a powerful method for investigating the fusion process. Recently, the dominance of quasi-fission over fusion-fission has been inferred for 34 S+208 Pb in comparison to 36 S+206 Pb; both reactions lead to the compound nucleus 242 Cf* .The mass and angle distributions of the fission fragments from these reactions were studied in order to further investigate the presence of quasi-fission.

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16 O, 32 S+ 208 Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with Δ Z = 1 and Δ Z = 2 were extracted, indicating surprisingly large absolute probabilities at sub-barrier energies. A comparison of 2 p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to ~ 15MeV for the 16 O and ~ 25MeV for 32 S-induced reactions demonstrate the population of highly excited states in the residual nuclei, indicating substantial dissipation of kinetic energy. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries., accepted for publication in Physics Letters B

The reaction ${}^{16}$O $+$ ${}^{208}$Pb is a benchmark in nuclear reaction studies as it involves two doubly magic nuclei. New measurements of back-scattered projectile-like fragments at sub-barrier energies show that the probability of two-proton ($2p$) transfer is much larger than that of $\ensuremath{\alpha}$-particle transfer. At energies around the fusion barrier the probability for $2p$ transfer is $\ensuremath{\sim}10%$, similar to that for one-proton transfer. The $2p$ transfer probabilities are enhanced by up to an order of magnitude compared to calculations based on an independent particle picture as simulated by the fully microscopic time-dependent Hartree-Fock model (TDHF). Since beyond mean-field correlations like nucleon pairing are not included in the TDHF model, the enhancement indicates strong pairing correlations between the transferred protons. $2p$ transfer leads to excitation energies (most likely in the target-like nucleus) up to $\ensuremath{\sim}13$ MeV, indicating that it may represent an effective doorway for the dissipation of energy and thus provide a microscopic mechanism toward understanding the inhibition of fusion and energies both above and below the barrier.

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16 O,32 S+208 Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ∆Z = 1 and ∆Z = 2 were extracted, indicating surprisingly large absolute probabilities at subbarrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments ~15 MeV and ~25 MeV for the 16 O and 32 S-induced reactions, respectively, indicate the population of highly excited states in the residual nuclei. A comparison with expected optimum Q -values suggests large losses in kinetic energy of the projectile-like fragments. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.

Below-barrier no-capture breakup measurements of the weakly bound $^{9}\mathrm{Be}$ nucleus, incident on targets ranging in atomic number from 62 to 83, have been carried out using a large-area high-resolution back-angle detector array. It is shown that the three-body reconstructed reaction $Q$-value and relative energy of the breakup fragments together reveal the full dynamics of the breakup mechanism, identifying all physical processes that lead to the breakup of the projectile-like nucleus. Contrasting with the simple expectation of direct breakup into the most energetically favored clusters, the data show that breakup following $n$-transfer dominates the total breakup yield. Breakup from long-lived states in the projectile-like nucleus, which on the reaction time scale may be considered stable, has been isolated from the prompt breakup yield. It has been shown that the prompt breakup probability essentially depends on the surface separation of the interacting nuclei. The measured prompt breakup probability functions for each target have been used together with a classical trajectory model to predict the above-barrier suppression of complete fusion. The suppression factor, expressed as the fraction of incomplete fusion, is nearly independent of target charge.

The sub-barrier excitation function of the octupole vibrational state at 2.615 MeV in {sup 208}Pb is analyzed within the coupled-channels framework. It is shown that the position of the minimum in the excitation function, which is due to the destructive interference of the Coulomb and nuclear scattering amplitudes, is very sensitive to the nuclear potential for both the ground state and the octupole state. A different nuclear potential for the 3{sup -} state may arise from changes in the matter distribution of {sup 208}Pb due to the particle-hole excitations. The present analysis places a strong limit on the 3{sup -} nuclear potential diffuseness, giving a difference of only {Delta}a{sub 0}=0.011+-0.004 fm between the ground state and 3{sup -} state diffuseness.

High precision measurements for back-angle quasi-elastic scattering of $^{16}\mathrm{O}$ from various target nuclei have been made, and are compared with coupled channels calculations to determine the diffuseness of the real part of the Woods-Saxon nuclear potential. The extracted diffuseness parameters are in the range of $0.60$ to $0.69$ fm, and agree well with previous results using a $^{32}\mathrm{S}$ projectile. The measured quasi-elastic energy spectra close to the barrier show both discrete peaks and a broad continuous structure with $Q$-values down to $\ensuremath{-}25$ MeV. These suggest the inadequacy of coupled channels calculations that include couplings only to low energy, discrete states of the colliding nuclei.

The formation of new, superheavy elements [1] requires the fusion of two massive nuclei. After contact of the two colliding nuclei, the dinuclear system is pictured as moving in deformation space over a potential energy surface (PES). The system may reach compact (mononuclear) shapes inside the fission barrier, and thus have the possibility of forming a heavy element in its ground state. Alternatively, before reaching a compact shape, the system may reseparate into two heavy fragments, intermediate in mass between the projectile and target nuclei. This is called quasifission [2,3]. The probability of fusion (forming a new nucleus with a compact near-equilibrium shape) is determined by many variables. Those believed to play a major role include the mass-asymmetry of the two colliding nuclei, the charge of the heavy element being formed, and the neutron and proton shell structure encountered during the fusion process [4]. The detailed role and significance of shell structure is the most complex and open question. Shell effects after contact can be categorized as dynamic or static. Nonequilibrium dynamic effects, associated with the diabatic motion of the nucleons at avoided crossings of their molecular orbitals, may effectively result in timedependent shell structures [5] that can influence the early collective motion of the dinuclear system. After thermal equilibration, the well-known static shell structures affect the PES, and also the inertia and the strength of energy dissipation (and thus the magnitude of shape fluctuations), which together determine the motion over the PES. In spontaneous and low energy fission of heavy nuclei, the role of (static) shell structures is well known. Fission mass yields and total kinetic energy distributions, and the existence and properties of fission isomers, can be understood as resulting from local structure in the PES caused by energetically favorable shell structures in the nascent fragments, both for spherical and deformed shapes [6]. It is expected (and observed) that as the excitation energy Ex increases, these shell effects are exponentially attenuated. This strong dependence on Ex may be a key to isolating their role in quasifission dynamics. However, there are uncertainties regarding the effective energy scale of the attenuation [7], which is critical to predict the influence of shell structure on quasifission, where Ex is often >20 MeV. In collisions of heavy nuclei, a further complication often arises, resulting from nuclear structure effects in the entrance channel. The heavy reaction partner may have a large static deformation. This is caused by unfavored shell effects for a spherical shape, compared with a gain in energy (shell stability) for a deformed shape. This deformation affects the entry point into the PES. For nuclear collisions involving prolate nuclei, when the deformation axis is aligned with the projectile nucleus, the dinucleus is very elongated at contact, whereas it is more compact if antialigned. In experiments, by choice of the bombarding energy, only the aligned orientation (low beam energy) or all orientations (higher energy) can be selected [8–10]. Measurements have consistently shown that the lower energies are associated with the largest deviation of fission properties from the expectations for fusion-fission [10 –14], as well as showing suppression of the yield of heavy elements following capture [15,16]. Both observables should be associated with an increased probability of quasifission [10,11].

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16O,32S+208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ΔZ = 1 and ΔZ = 2 were extracted, indicating surprisingly large absolute transfer probabilities at sub-barrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to ~ 15MeV for the 16 O and ~ 25MeV for 32S-induced reactions show the population of highly excited states in the residual nuclei, and indicate substantial dissipation of kinetic energy. A new phenomenological framework explores the effect of energy dissipation through these highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes on the probability for fusion in the reaction 16o+208Pb. Preliminary results show that the suppression of fusion at energies above the barrier in this reaction can be explained within the new framework.

Shell effects can play a prominent role in fission fragment mass distributions. For lighter systems in the region of A~180-200, mass distributions were generally expected to be symmetric. However, a recent experiment showed that fission of 180 Hg following electron capture of 180 Tl leads to an asymmetric mass split. Recent calculations by various groups indicate that the mechanism of asymmetric fission could be very different in this mass region compared to the actinide region. To investigate the role of shell effects in this mass region, we have measured the fission fragment mass distribution for the 13 C+182 W,176 Yb reactions forming the compound nuclei 195 Hg and 189 Os respectively, at lab bombarding energies of 60, 63 and 66 MeV using the CUBE detector setup located at the ANU Heavy Ion Accelerator Facility. The experimental data were fitted with single and double Gaussian distributions. The results indicate an asymmetric mass split for 195 Hg, whereas for 189 Os, the mass distribution is well fitted with a single Gaussian distribution.

The quasi-fission mechanism hinders fusion of heavy systems because of a mass flow between the reactants, leading to a re-separation of more symmetric fragments in the exit channel. A good understanding of the competition between fusion and quasi-fission mechanisms is expected to be of great help to optimize the formation and study of heavy and superheavy nuclei. Quantum microscopic models, such as the time-dependent Hartree-Fock approach, allow for a treatment of all degrees of freedom associated to the dynamics of each nucleon. This provides a description of the complex reaction mechanisms, such as quasi-fission, with no parameter adjusted on reaction mechanisms. In particular, the role of the deformation and orientation of a heavy target, as well as the entrance channel magicity and isospin are investigated with theoretical and experimental approaches., Invited talk to NSRT12. To be published in Eur. Phys. J. Web of Conf