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Register a new userEffects of entrance channel on fusion probability in hot fusion reactions
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Within the framework of the dinuclear system (DNS) model, the fusion reactions leading to the compound nuclei 274Hs and 286Cn are investigated. The fusion probability as a function of DNS excitation energy is studied. The calculated results are in good agreement with the available experimental data. The obtained results show that the fusion probabilities are obviously enhanced for the reactions located at high place in potential energy surface, although these reactions may have small values of mass asymmetry. It is found that the enhancement is due to the large potential energy of the initial DNS.
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Background: The cross section for forming a heavy evaporation residue in fusion reactions depends on the capture cross section, the fusion probability, PCN, i.e., the probability that the projectile-target system will evolve inside the fission saddle point to form a completely fused system rather than re-separating (quasifission), and the survival of the completely fused system against fission. PCN is the least known of these quantities. Purpose: To measure PCN for the reaction of 101.2 MeV 18O, 147.3 MeV 26Mg, 170.9 MeV 30Si and 195.3 MeV 36S with 197Au. Methods: We measured the fission fragment angular distributions for these reactions and used the formalism of Back to deduce the fusion-fission and quasifission cross sections. From these quantities we deduced PCN for each reaction. Results: The values of PCN for the reaction of 101.2 MeV 18O, 147.3 MeV 26Mg, 170.9 MeV 30Si and 195.3 MeV 36S with 197Au are 0.66, 1.00, 0.06, 0.13, respectively. Conclusions: The new measured values of PCN agree roughly with the semi-empirical system- atic dependence of PCN upon fissility for excited nuclei.
To explain the experimental facts that the fusion cross sections of proton-halo nucleus on heavy target nucleus is not enhanced as expected, the shielding supposition has been proposed. Namely, the proton-halo nucleus is polarized with the valence proton being shielded by the core. In this paper, within the frame of the Improved Quantum Molecular Dynamics model, the fusion reactions by $^{17}$F on $^{208}$Pb around Coulomb barrier have been simulated. The existence of shielding effect is verified by microscopic dynamics analysis and its influence on the effective interaction potential is also investigated.
The evaporation residue yields from compound nuclei $^{220}$Th formed in the $^{16}$O+$^{204}$Pb, $^{40}$Ar+$^{180}$Hf, $^{82}$Se+$^{138}$Ba, $^{124}$Sn+$^{96}$Zr reactions are analyzed to study the entrance channel effects by comparison of the capture, fusion and evaporation residue cross sections calculated by the combined dinuclear system (DNS) and advanced statistical models. The difference between evaporation residue (ER) cross sections can be related to the stages of compound nucleus formation or/and at its surviving against fission. The sensitivity of the both stages in the evolution of DNS up to the evaporation residue formation to the angular momentum of DNS is studied. The difference between fusion excitation functions are explained by the hindrance to complete fusion due to the larger intrinsic fusion barrier $B^*_{rm fus}$ for the transformation of the DNS into a compound nucleus and the increase of the quasifission contribution due to the decreasing of quasifission barrier $B_{rm qf}$ as a function of the angular momentum. The largest value of the ER residue yields in the very mass asymmetric $^{16}$O+$^{204}$Pb reaction is related to the large fusion probability and to the relatively low threshold of the excitation energy of the compound nucleus. Due to the large threshold of the excitation energy (35 MeV) of the $^{40}$Ar+$^{180}$Hf reaction, it produces less the ER yields than the almost mass symmetric $^{82}$Se+$^{138}$Ba reaction having the lowest threshold value (12 MeV).
We have studied the fission-neutron emission competition in highly excited $^{274}$Hs (Z=108) (where the fission barrier is due to shell effects) formed by a hot fusion reaction. Matching cross bombardments ($^{26}$Mg + $^{248}$Cm and $^{25}$Mg + $^{248}$Cm) were used to identify the properties of first chance fission of $^{274}$Hs. A Harding-Farley analysis of the fission neutrons emitted in the $^{25,26}$Mg + $^{248}$Cm was performed to identify the pre- and post-scission components of the neutron multiplicities in each system. ($Gamma$$_{n}$/$Gamma$$_{t}$) for the first chance fission of $^{274}$Hs (E$^{ast}$ = 63 MeV) is 0.89 $pm$ 0.13, i.e., $sim$ 90 $%$ of the highly excited nuclei survive.The high value of that survival probability is due to dissipative effects during de-excitation. A proper description of the survival probabilities of excited superheavy nuclei formed in hot fusion reactions requires consideration of both dynamic and static (shell-related) effects.
We investigate the reaction path followed by Heavy Ion Collisions with exotic nuclear beams at low energies. We will focus on the interplay between reaction mechanisms, fusion vs. break-up (fast-fission, deep-inelastic), that in exotic systems is expected to be influenced by the symmetry energy term at densities around the normal value. The evolution of the system is described by a Stochastic Mean Field transport equation (SMF), where two parametrizations for the density dependence of symmetry energy (Asysoft and Asystiff) are implemented, allowing one to explore the sensitivity of the results to this ingredient of the nuclear interaction. The method described here, based on the event by event evolution of phase space quadrupole collective modes will nicely allow to extract the fusion probability at relatively early times, when the transport results are reliable. Fusion probabilities for reactions induced by 132Sn on 64,58Ni targets at 10 AMeV are evaluated. We obtain larger fusion cross sections for the more n-rich composite system, and, for a given reaction, in the Asysoft choice. Finally a collective charge equilibration mechanism (the Dynamical Dipole) is revealed in both fusion and break-up events, depending on the stiffness of the symmetry term just below saturation.
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