Possible Way to Synthesize Superheavy Element Z=117

Within the framework of the dinuclear system model, the production of superheavy element Z = 117 in possible projectile-target combinations is analysed systematically. The calculated results show that the production cross sections are strongly dependent on the reaction systems. Optimal combinations, corresponding excitation energies and evaporation channels are proposed, such as the isotopes^248.249 Bk in ^48 Ca induced reactions in 3n evaporation channels and the reactions ^45Sc＋246.248Cm in 3n and 4n channels, and the system ^51 V＋ 244pu in 3n channel.     

Nuclear Potential and Fusion Cross Sections for Synthesizing Super-Heavy Elements in Di-nuclear Systems

A double foMing method with simplified Skyrme-type nucleon nucleon interaction is used to calculate the nuclear interaction potential between two nuclei. The calculation is performed in tip-to-tip orientation of the two nuclei if they are deformed. Based on this method, the potential energy surfaces~ the fusion probabilities and the evaporation residue cross sections for some cold fusion reactions leading to super-heavy elements within di-nuclear system model are evaluated. It is indicated that after the improvement, the exponential decreasing systematics of the fusion probability with increasing charge number of projectile on the Pb based target become better and the evaporation residue cross sections are in better agreement with the experimental data.     

Fusion near the Coulomb barrier for the synthesis of heavy and superheavy elements: A theoretical approach

The compound nucleus formation is considered as a two-step process of touching and subsequent tunneling of the projectile into the target. The deep minima in the potential energy curve are due to shell effects in the experimental binding energies and give possible target-projectile combinations for the synthesis of heavy and superheavy elements. The asymmetric channels thus obtained are in remarkable agreement with the known experimental channels. In our model, the colliding partners are first shown to be captured in the pocket behind the outer (touching) barrier and the composite system so formed finally tunnels through the inner (fusion) barrier to form the resulting compound nucleus. These calculations reveal the importance of the fusion barrier, which occur only for the asymmetric target-projectile combinations. The calculated fusion cross-sections show a reasonable comparison with the observed one-neutron evaporation residue cross-sections. An estimate of the excitation energy carried by the compound nucleus is also obtained from our model calculations.     

Origin of Unexpected Isotopic Trends in Synthesis of Superheavy Nuclei

We investigate the dependence of the yield of superheavy nuclei with Z = 110, 112, 114 on the neutron excess of the projectile nucleus with a two-parameter Smoluchowski equation. It is confirmed that in some cases, the cold fusion reactions with less neutron excess are more favourable than those with more neutron excess. In order to probe the origin of these unexpected isotopic trends, we also investigate the probabilities of capture, fusion and survival in the cold fusion reactions in detail It is found that the maximal ER cross sections of the superheavy nuclei exponentially increase as a function of Bf - Sn with Bf being the fission barrier and Sn being the neutron separation energy. Although the probabilities of capture and fusion have some influences, the unexpected isotopic trends mainly due to the dependence of the ER cross sections on the Bf - Sn value. Therefore, the reactions with larger Bf - Sn values should be more favourable for synthesis of superheavy nuclei.     

Entrance channel mass asymmetry dependence of compound nucleus formation

The fusion reactions ⁴⁸Ca + ¹⁵⁴Sm and ¹⁶O + ¹⁸⁶W leading to the same compound nucleus ²⁰²Pb are studied within the framework of an improved isospin dependent quantum molecular dynamics model. The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies, Coulomb barriers and fusion cross sections. The calculated fusion cross sections agree quantitatively with the experimental data. We conclude that the compound nucleus formation is favorable for the system with larger mass asymmetry.     

Shell Correction and Pairing Energies in the Dinuclear System Model

We investigate the dependences of the potential energy surfaces （PES） and the fusion probabilities for some cold fusion reactions leading to super-heavy elements on the nuclear shell effect and pairing energy. It is found that the shell effect plays an important role in the fusion of the super-heavy element while pairing energy＇s contribution is insignificant. The fusion probabilities and evaporation residue cross sections as functions of the Ge-isotope projectile bombarding ^208Pb are also investigated. It is found that evaporation residue cross sections do not always increase with the increasing neutron number of Ge-isotope.     

Boundary Conditions of Wigner-Seitz Cell in Inner Crust of Neutron Stars with Relativistic Mean Field Approach

The microscopic structure of the Wigner-Seitz （W-S） cell in the inner crust of neutron stars is investigated with the relativistic mean field （RMF） approach. The W-S cell is composed of a cluster of neutrons and protons localized in a region around the centre and surrounded by a neutron gas of approximately uniform density. In order to generate the density of the W-S cell, appropriate boundary conditions in the calculation of the single-particle wavefunctions are necessary. We emphasize on the choice of the boundary conditions in the RMF approach. Three kinds of boundary conditions are suggested. The properties of the W-S cell with the three kinds of boundary conditions are investigated. The neutron density distributions in the RMF and Hartree-Fock-Bogoliubov （HFB） models are compared. It is found that the neutron gas densities of the W-S cell in the RMF model is higher than those obtained in the HFB model.     

Nuclear Hexadecapole Deformation Effects on the Production of Super-Heavy Elements

We investigate the effects of the nuclear hexadecapole deformations on the interaction potentials between nuclei, the driving potentials and the fusion probabilities for some cold fusion reactions leading to super-heavy elements. It is found that nuclear hexadecapole deformations change significantly the structure of the driving potentials and the fusion probabilities for some reaction channels.     

Calculation of Interaction Potentials between Spherical and Deformed Nuclei

The interaction potential for spherical-deformed reaction partners is calculated. The shape, separation and orientation dependence of the interaction potential and fusion cross section of the system ^32S＋^154Sm are investigated within the double-folding model of the deformed nuclei. The effective nucleon-nucleon interaction is taken to be the M3Y-Reid potential. The density is considered for three terms of the expansion using the truncated multipole expansion method, which is a deformed Fermi shape With quadrupole and hexadecapole for the density distribution of ^154Sm. It is found for the interaction potential that the height and the position of barrier strongly depend on the deformations, the orientation angle of the deformed nucleus, and hence produce great effects on fusion cross section. The integrated fusion cross section is in good agreement with the experimental data.     

Modified Woods-Saxon Potential for Heavy-Ion Fusion Reaction

A modified Woods-Saxon （MWS） potential is proposed for describing nucleus-nucleus interaction based on the Skyrme energy-density functional approach. Fusion barriers for a large number of fusion reactions from light to heavy systems can be described well with this potential. The suitable incident energies for fusion reactions leading to superheavy nuclei are also explored. It seems to us that the MWS potential is useful for selecting the suitable incident energies for fusion reactions for producing super-heavy nuclei.     

Associated Production of Scalars and New Gauge Bosons from a Little Higgs Model at the LHC

The littlest Higgs model with T-parity （LHT model） predicts the existence of the T-odd scalars （Φ^±, Φ^0, and Φ^P）. We consider the production of these new particles associated with T-odd gauge bosons at the Large Hadron Collider （LHC）. It is found that the partonic process qq^1 →Φ^＋ BH can generate a number of the characteristic signal events with a charged lepton and large missing energy at the LHC.     

The Average Lifetime of Giant Composite Systems Formed in Strongly Damped Collisions

The dynamic, adiabatic and diabat ic entrance potentials in strongly damped reactions of ^238 U＋^238 U, ^232 Th ＋ ^250Cf are calculated and compared. The feature of the dynamical potential implies that it is possible for the composite systems to stick together for a period of time. By means of the improved quantum molecular dynamics model the time evolution of the density and charge distributions of giant composite systems and their fragments for reactions ^238U＋^238U, ^232Th＋^250Cf are investigated, from which the lifetimes of giant composite systems at different energies are obtained. The longest average lifetime of ^238U＋^238U is found when the incident energy is about Ec.m =1080 MeW, which is about 1200 fm/c.     

Possible Mechanisms of Ternary Fission in the 197Au+197Au System at 15 A MeV

Ternary fission in ^197Au＋^197Au collisions at 15 A MeV is investigated by using the improved quantum molecular dynamical （ImQMD） model. The experimental mass distributions for each of the three fragments are reproduced for the first time without any freely adjusting parameters. The mechanisms of ternary fission in central and semicentral collisions are dynamically studied. In direct prolate ternary fission, two necks are found to be formed almost simultaneously and rupture sequentially in a very short time interval. Direct oblate ternary fission is a very rare fission event, in which three necks are formed and rupture simultaneously, forming three equally sized fragments along space-symmetric directions in the reaction plane. In sequential ternary fission a binary division is followed by another binary fission event after hundreds of fm/c.     

Isotopic dependence of fusion probabilities for neutron-deficient and -rich colliding nuclei

Isotopic dependence of the fusion dynamics is studied by analyzing the collision of a large number of isotopes of Ca and Ni with 0.6 N/Z 2. This study, which results from the Skyrme energy density formalism, reveals that the addition of neutrons favors fusion of reacting partners, whereas the reverse happens with the removal of neutrons. The fusion barrier heights and positions follow a non-linear second-order dependence on ( -1 ), whereas fusion cross-sections can be parameterized by a straight line.-1     

Coulomb Potentials between Spherical and Deformed Nuclei

Coulomb potentials for spherical-deformed reaction as well as the conventional formulas. Our results approaches have quite different behaviours in the partners are calculated in terms of the double folding model show that the Coulomb potentials calculated with different internal region of the potential. Because fusion process is sensitive to the barrier height and the internal part of the potential, the fusion excitation function, especially the fusion barrier distribution, should provide a strict test of the interaction potentials. Therefore, we calculate the fusion excitation function and barrier distribution for the ^16O ＋^154Sm system with different versions of the Coulomb potentials, in comparison with the experimental results. It is found that the fusion excitation function and barrier distribution of ^16O＋^154Sm are obviously different for the different versions of the Coulomb potentials. By means of this comparison, we may conclude that the double folding model with the accurate approximate form can provide rather reasonable Coulomb potentials.     

Isotope Dependence of Superheavy Nucleus Formation Cross Section

The dynamical process in the superheavy nucleus synthesis is studied on the basis of the two-dimensional Smoluchowski equation. Special attention is paid to the isotope dependence of the cross section for the superheavy nucleus formation by means of making a comparison among the reaction systems of ^54Re＋204pb, ^56Re ＋206Pb, and ^58Fe＋^208Pb. It is found by this comparison that the formation cross section is very sensitive to the conditional saddle-point height and the neutron separation energy of the compound nucleus. Reaction systems with lower height of conditional saddle-point and smaller neutron separation energy are more favourable for the synthesis of the superheavy nucleus.     

Impact Parameter Dependence of the Double Neutron/Proton Ratio of Nucleon Emissions in Isotopic Reaction Systems

Within the transport model IBUU04, we investigate the double neutron/proton ratio of free nucleons taken from two reaction systems using two Sn isotopes at a beam energy of 50 MeV/nucleon and with impact parameters 2fm, 4fm and 8fm, respectively. It is found that the double neutron/proton ratio from peripheral collisions is more sensitive to the density dependence of the symmetry energy than those from mid-central and central collisions.     

Entrance channel potentials in the synthesis of the heaviest nuclei

Entrance channel potentials in nucleus-nucleus collisions, relevant for the synthesis of superheavy elements, are systematically studied within a semi-microscopic approach, where microscopic nuclear densities of the colliding spherical or deformed nuclei are used in semi-classical expressions of the energy-density functional. From experimental data on fusion windows evidence is found that the existence of pockets in the entrance channel potentials is crucial for fusion. Criteria for the choice of the best collision systems for the synthesis of superheavy elements are discussed. Received: 14 May 2002 / Accepted: 6 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: v.denisov@gsi.de, denisov@kinr.kiev.ua RID="b" ID="b"e-mail: w.nrnbrg@gsi.de Communicated by P. Schuck     

Experimental signature of entrance channel effect in heavy mass region via evaporation residue cross section and spin distribution measurements

Evaporation residue (ER) cross sections and gamma multiplicity distributions have been measured for ¹⁶O + ¹⁸⁴W and ¹⁹F + ¹⁸¹Ta systems in the excitation energy range of 50–90 MeV, leading to the same compound nucleus ²⁰⁰Pb^∗. Comparison of experimental results of both the systems shows that ER cross sections and moments of gamma multiplicity distribution of ¹⁶O + ¹⁸⁴W system are significantly higher than those of ¹⁹F + ¹⁸¹Ta system at higher excitation energies. Present measurements directly shows the experimental signature of entrance channel effect even with the systems which are not very different with respect to their entrance channel mass asymmetry. It is further demonstrated that the reduction in the ER cross section and moments of spin distribution for ¹⁹F + ¹⁸¹Ta system is mainly due to the suppression of fusion of higher l values.