Narrow Results

The fusion-fission excitation functions for the decay of compound nucleus ²¹⁵Fr*, formed in ¹¹B+²⁰⁴Pb and ¹⁸O+¹⁹⁷Au reaction channels, are studied on the Dynamical Cluster-decay Model (DCM), showing entrance channel independence, in agreement with experiments, not invoking any quasi-fission (qf) process in either of the two channels.

The dynamical cluster-decay model (DCM), with deformations and orientation degrees of freedom of the incoming nuclei and of outging fragments included, is used to study the excitation functions of the "equatorial" compact hot fusion reaction ²⁴⁴Pu+⁴⁸Ca→²⁹²114^* (compact orientation angle θ_c=90° for ²⁴⁴Pu). Considering the higher multipole deformations up to hexadecapole deformation β₄ and configurations with "compact" orientation θ_c, the model is shown to give a good description of the measured individual light-particle decay channels σ_xn, here x=3, 4 and 5, and other decay channels, the fusion-fission σ_ff and quasi-fission σ_qf (equivalently, capture σ_cap) with in one parameter fitting, the neck length ΔR. The quasi-fission is also considered as a cold process with an elongated "polar" configuration. The xn-channel cross-sections for collisions between nuclei with static deformations at their respective compact orientations are shown to be much more than for the case of the nuclei taken to be spherical, signifying the increase in fusion threshold for an intermediate hot fusion reaction to be associated with the static deformation of the target nucleus and its orientation at the point of collision in its path toward the (spherical) compound nucleus. The shell effects in both the potential and kinetic energy (the mass parameters) terms of the Hamiltonian are shown to be important. The free parameter ΔR of the model is shown to depend strongly on limiting angular momentum, which in turn depends on the use of sticking or non-sticking moment of inertia for angular momentum effects. For the sticking moment of inertia, the evaporation residue (neutron emission) is shown to occur almost promptly (largest ΔR), followed by the competing (hot/cold) quasi-fission and ending finally with fusion-fission of hot compound nucleus. Different ΔR's (equivalently, relative separations) for the three processes means to predict that the processes ER, ff and qf happen in different time-scales, in agreement with the indications of experiments.

The decay of compound nucleus ²⁰²Pb^*, formed in entrance channel reaction ⁴⁸Ca+¹⁵⁴Sm at different incident energies, is studied by using the dynamical cluster-decay model (DCM) where all decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated results show an excellent agreement with experimental data for the fusion-evaporation residue cross-section σ_ER together with the fusion-fission cross-section σ_FF (taken as a sum of the energetically favored symmetric and near symmetric A=65–75 plus complementary fragments), and the competing, non-compound-nucleus quasi-fission cross-section σ_QF where the entrance channel is considered not to loose its identity (and hence with preformation factor P₀=1). The interesting feature of this study is that the three decay processes (ER, FF and QF) are quite comparable at low energies, ER being the most dominant, whereas at higher energies FF becomes most probable followed by ER and QF. The prediction of two fission windows, the symmetric fission (SF) and the near symmetric fission (nSF) whose contribution is more at lower incident energies, suggests the presence of a fine structure effect in the fusion-fission of ²⁰²Pb^*. This result is attributed to the shell effects (magic shells) playing effective role in the fragment preformation yields for ⁴⁸Ca+¹⁵⁴Sm reaction at lower excitation energies, giving rise to "shoulders", to an otherwise Gaussian FF mass distribution, responsible for the QF process. As a further verification of this result, absence of "shoulders" (hence, the QF component) in the decay of ¹⁹²Pb^* due to ⁴⁸Ca+¹⁴⁴Sm reaction is also shown to be given by the calculations, in agreement with experiments. The only parameter of the model is the neck-length ΔR which shows that the ER occurs first, having the largest values of ΔR, and the FF and QF processes occur almost simultaneously at lower incident energies but the FF takes over QF at higher incident energies. In other words, the three processes occur in different time scales, QF competing with FF at lower incident energies.

A combined dynamical model using concept of dinuclear systems (DNS) and one-dimensional (1D) Langevin equations was applied to investigate the effect of entrance channel on dynamics of heavy ions collision. The ${}^{30}$Si+${}^{170}$Er, ${}^{16}$O+${}^{184}$W and ${}^{19}$F+${}^{181}$Ta reactions which formed the compound nucleus ${}^{200}$Pb have been considered to study this effect. We studied these reactions dynamically and calculated the ratio of evaporation residue cross-section to fusion cross-section $({\sigma }_{ER}/{\sigma }_{\mathrm{\text{Fus}}})$ as a tool for investigation of entrance channel effect. Results of combined model are compared with available experimental data and results of 1D Langevin equations. Obtained results based on combined model are in better agreement with experimental data in comparison with results of Langevin equations. We concluded for ${}^{30}$Si+${}^{170}$Er and ${}^{19}$F+${}^{181}$Ta reactions the results of combined model that support the quasi-fission process are different relative to Langevin dynamical approach, whereas for ${}^{16}$O+${}^{184}$W system the two models give similar results.

The pathway toward the production of superheavy elements crosses with the one of an in depth study of the fission and quasifission processes. Quasifission affects the probability of forming a compound nucleus after capture and fission plays a decisive role during the cooling of the compound nucleus. To search for optimal reactions to produce superheavy elements in fusion reactions and to provide an estimate of their production cross section measurements of the capture and fusion cross sections along with the survival probabilities are a mandatory task. In this presentation an overview of the impact of fission and quasifission over the synthesis of superheavy elements is discussed along with the need to perform measurements over a wide range of masses and energies.

The overlap in the mass symmetric region of the reaction products from fusion-fission and quasi-fission complicates the assignment of symmetric events to complete fusion on the basis of the mass distribution alone. Additional observables, besides mass distribution, should be used. The method proposed here relies on the fact that fusion-fission and quasifission are characterized by a different timescale. Within this framework we performed a detailed study to find out if timescales can be probed via angular momentum as measured via γ rays multiplicity. The proof of principle was carried out by measuring the γ rays in coincidence with two fragments in the reaction ³²S + ¹⁹⁷Au at beam energy near the Coulomb barrier. The experiment has been performed at the Tandem ALTO facility at IPN Orsay using a detection setup consisting of ORGAM and PARIS γ detector arrays coupled with the CORSET time-of-flight spectrometer. Preliminary results on the sensitivity of this method to distinguish reaction channels with different dynamics are discusses.

The concept of a nuclear molecule or a dinuclear system assumes two touching nuclei which carry out motion in the internuclear distance and exchange nucleons by transfer. The dinuclear model can be applied to nuclear structure, to fusion reactions leading to superheavy nuclei and to multi-nucleon transfer reactions.

The nuclear fission process involves a drastic rearrangement of the nuclear matter of the fissioning system as it undergoes shape changes from a single, compact nucleus to two separated fission fragments. Understanding the dynamics of this process has been the focus of much research for decades, both theoretical and experimental, using a number of different approaches. Early on, Kramers [1] proposed that the fission rate is not well described by the statistical counting of transition states as proposed by Bohr and Wheeler [2] but that the friction or viscosity of the nuclear matter can play a substantial role in slowing down the rate by large factors. Aside from the theoretical estimates of fission time scales, a number of different approaches have been used to obtain experimental information on this quantity. Although the divergence of results obtained by the different experimental approaches have been known for years, these discrepancies have not yet been resolved and they have recently received renewed attention [3]. In this paper I will recount some of the experimental approaches to estimate the fission time scales and discuss their respective strengths and weaknesses without coming close to provide a resolution of the discrepancies.