Transfer, breakup, and fusion reactions of 6He with 209Bi near the Coulomb barrier

The fusion of ⁶He with a ²⁰⁹Bi target displays a large enhancement at energies near to and below the Coulomb barrier. Recently, a ⁴He group of remarkable intensity, which dominates the total reaction cross-section, has also been observed in the near-barrier interaction of the same system. It is argued that this transfer/breakup channel acts as a doorway state to fusion. Received: 1 May 2001 / Accepted: 4 December 2001     

Semiclassical description of reactions at energies near the Coulomb barrier

The modified semiclassical approximation of Coulomb matrix elements is extended to include effects of distorting nuclear potentials in the scattering wave functions. The applicability and efficiency of the proposed semiclassical method are discussed. The advantages of this approximation are shown for a typical heavy-ion transfer reaction.     

Comparison of the fusion reactions12C+20Ne and16O+16O near the Coulomb barrier

The partial cross sections of heavy residual nuclei produced in the heavy ion fusion of¹²C+²⁰Ne have been measured atE _c.m.=6–15 MeV viaγ-ray spectroscopy with a Ge(Li) detector. Windowless and recirculating gas target systems have been used. The dominant residual nuclei are²⁴Mg,²⁷Al,²⁸Si,³⁰Si,³⁰P and³¹P, which arise from two- and three-body breakups in the exit channels. The observed excitation functions are smooth in their energy dependence and give no indications for the existence of pronounced resonance structures, in contrast to theoretical predictions. The Coulomb excitation of²⁰Ne served as an intrinsic calibration standard in the determination of absolute partial and total fusion cross sections. The same experimental set-up was also used in the reaction studies of¹⁶O+¹⁶O atE _c.m.=7–14 MeV, going through the same compound nucleus³²S at similar excitation energies. The observed energy dependence in the excitation functions is in good agreement with previous work. The total fusion cross section agrees fairly well with two sets of values reported previously, but deviates significantly from other reported absolute cross section values. The relative evaporation distributions of the residual nuclei are similar for both heavy ion reactions. However, the ratio of their total fusion cross sections deviates from model predictions and suggests that compound nucleus formation does depend on the microscopic structure of the colliding nuclei in the entrance channel. From the observed energy dependence of the above ratio, particularly at subcoulomb energies, geometrical effects in the entrance channel (due to deformed and spherical nuclei) appear to be weak. The astrophysical aspects of the data in the context of late stellar nucleosynthesis are discussed.     

Competition between binary reactions and fusion in heavy-ion collisions at the Coulomb barrier

Mass and charge distributions for binary reaction channels have been measured for the reactions⁸⁶Kr with⁷⁶Ge,¹⁰⁴Ru and¹³⁰Te at the Coulomb barrier using chemical separations and-ray spectroscopy. These systems span the region where dynamical hindrance to complete fusion sets in. The binary reactions can be subdivided into two components associated withi) reflection from the outer potential barrier (quasielastic), andii) reseparation after passing the barrier (complex reactions). The sum of complex-reaction channels and evaporation residues from complete fusion can be reproduced by a barrier passing calculation. The fraction of the barrier passing flux leading to reseparation increases from 26±10% for the lightest system to more than 90% for the heaviest system. The data indicate that fusion hindrance is primarily caused by reseparation shortly after passage of the barrier before Swiatecki's conditional saddlepoint is overcome, resulting in partitions close to the entrance channel configuration. In addition, for the heaviest system, a quasifission component representing somewhat less than 20% of the barrier-passing flux was observed. From the missing masses of fragment pairs we can deduce that the reseparating complex-reaction products have kinetic energies well below the fusion barrier and share the excitation energy in a way similar to the sawtooth-like curve known from low-energy fission. The quasielastic, predominantly one- and two-nucleon transfer channels, have strongly varying cross sections for the three systems despite similar effectiveQ-values. A systematics of one-neutron transfer cross sections at the Coulomb barrier is established and shown to differ considerably from the smooth behaviour observed at energies 20–30% above the barrier. The connection to nuclear polarization phenomena and orbit matching is pointed out.Nuclear reactions:⁷⁶Ge,¹⁰⁴Ru,¹³⁰Te(⁸⁶Kr, X).E=3.22 MeV/u, (3.64) 3.84 MeV/u, 3.96 MeV/u; enriched targets; catcher foil technique, chemical separations,-ray spectroscopy; deduced mass and charge distributions for binary reactions; competition with complete fusion     

改进的QMD模型研究不同入射道条件对同位旋分馏强度的影响

通过在同位旋相关量子分子动力学模型(IQMD)中引入同位旋依赖的动量相关作用和同位旋相关的Skyrme势,得到了改进的同位旋依赖的量子分子动力学模型,并用此模型分析和讨论了中能重离子碰撞中的同位旋分馏强度随入射道条件的演化.结果表明在中能重离子碰撞中各种入射道条件下同位旋依赖的动量相关作用和Skyrme势有着明显的同位旋效应,它们减弱了同位旋分馏的过程. 关键词： 同位旋分馏 同位旋相关的动量相关作用 同位旋相关的Skyrme势     

Total and partial capture cross sections in reactions with deformed nuclei at energies near and below the Coulomb barrier

Within the quantum diffusion approach, the capture of a projectile nucleus by a target nucleus is studied at bombarding energies above and below the Coulomb barrier. The effects of deformation of interacting nuclei and neutron transfer between them on the total and partial capture cross sections and the mean angular momentum of the captured system are studied. The results obtained for the ¹⁶O + ¹¹²Cd, ¹⁵²Sm, and ¹⁸⁴W; ¹⁹F +¹⁷⁵Lu; ²⁸Si +^94,100Mo and ¹⁵⁴Sm; ⁴⁰Ca +⁹⁶Zr; ⁴⁸Ca+ ⁹⁰Zr; and ⁶⁴Ni +^58,64Ni, ^92,96Zr, and ¹⁰⁰Mo reactions are in good agreement with available experimental data.     

Fusability and fissionability in 86Kr-induced reactions near and below the fusion barrier

Evaporation-residue excitation functions for the reactions ⁸⁶Kr + ^70,76Ge, ^92,100 Mo, ^99,102,104 Ru have been measured using activation methods and the velocity filter SHIP. The data span the region from well below the fusion barrier up to, and beyond, the energy where limitation by fission competition takes place. The data are shown to be compatible with the concept of complete fusion followed by the statistical decay of the equilibrated compound nucleus. Information on both the fusion probability at and below the fusion threshold and the fissionability of the compound nuclei formed, is extracted. The model dependence of the extracted fission barriers is discussed in detail. In analogy to studies involving lighter projectiles, strong correlations between the low-energy nuclear-structure properties of the nuclei and the subbarrier fusion probability are found. A relative shift of the fusion barrier to higher energies, that increases with the number of valence neutrons in the target nuclei, is observed.     

Asymmetric barrier model for heavy ion fusion and its relation to channel coupling

A new asymmetric parabolic effective fusion barrier model for heavy ion fusion is developed.     

Argon ion reaction studies on tantalum and gold near Coulomb barrier energies

Gold and tantalum targets were bombarded with a ⁴⁰Ar beam at 183 and 209 MeV, close to the Coulomb barrier height. Angular distributions of recoiling reaction products were measured by collecting on A1 catcher foils. The radioactivity was counted after the irradiation by Ge(Li) detectors. One-neutron and one- and two-proton transfer products were prominent. Simple semiclassical transfer theory adequately fits the backward peaking angular distributions at or below the Coulomb barrier but fails at bombarding energies above the barrier. Yields of some products more than two mass numbers removed from reactants were observed in thicker targets at 235 MeV, and their significance is discussed.     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

Total fusion cross sections for 18, 17, 16O + 27Al at energies near the Coulomb barrier

Total fusion cross sections have been measured for ^{18, 17, 16}O + ²⁷Al systems at bombarding energies 27–42 MeV. The evaporation residues were detected in the angular range 4°–25° (lab) using a $\text{2E?E}$ counter telescope. Barrier radii extracted from total fusion and elastic scattering cross sections are found to increase with the projectile mass. The effect of the yrast levels on the isotopic yields in the evaporation cascade is investigated.     

Fusion of 60Ni + 100Mo near and below the Coulomb barrier

Various sub-barrier capture reactions with ^16,18O and ^40,48Ca are treated within the quantum diffusion approach. The role of neutron transfer in these capture reactions is discussed. The quasielastic and capture barrier distributions are analyzed and compared with the recent experimental data.     

Isospin dynamics in heavy ion collisions: From Coulomb barrier to quark gluon plasma

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this report, we present a selection of new reaction observables in dissipative collisions particularly sensitive to the symmetry term of the nuclear Equation of State (Iso-EoS). We will first discuss the Isospin Equilibration Dynamics. At low energies, this manifests via the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation, with the symmetry term acting as a restoring force. At higher beam energies, Iso-EoS effects will be seen in Imbalance Ratio Measurements, in particular from the correlations with the total kinetic energy loss. For fragmentation reactions in central events, we suggest to look at the coupling between isospin distillation and radial flow. In Neck Fragmentation reactions, important Iso-EoS information can be obtained from the correlation between isospin content and alignment. The high density symmetry term can be probed from isospin effects on heavy ion reactions at relativistic energies (few AGeV range). Rather, isospin sensitive observables are proposed from nucleon/cluster emissions, collective flows and meson production. The possibility to shed light on the controversial neutron/proton effective mass splitting in asymmetric matter is also suggested. A large symmetry repulsion at high baryon density will also lead to an “earlier” hadron-deconfinement transition in n-rich matter. A suitable treatment of the isovector interaction in the partonic EoS appears very relevant.     

Double folding model calculation applied to fusion reactions

The interaction potential between a spherical and a deformed nucleus is calculated within the double-folding model for deformed nuclei.We solve the double folding potential numerically by using the truncated multipole expansion method.The shape,separation and orientation dependence of the interaction potential,fusion cross section and barrier distribution of the system 16O+154Sm are investigated by considering the quadrupole and hexadecapole deformations of 154Sm.It is shown that the height and the position of the barrier depend strongly on the deformation and the orientation angles of the deformed nucleus.These are quite important quantities for heavy-ion fusion reactions,and hence produce great effects on the fusion cross section and barrier distribution.     

Double folding model calculation applied to fusion reactions

The interaction potential between a spherical and a deformed nucleus is calculated within the double-folding model for deformed nuclei. We solve the double folding potential numerically by using the truncated multipole expansion method. The shape, separation and orientation dependence of the interaction potential, fusion cross section and barrier distribution of the system ¹⁶O+¹⁵⁴Sm are investigated by considering the quadrupole and hexadecapole deformations of ¹⁵⁴Sm. It is shown that the height and the position of the barrier depend strongly on the deformation and the orientation angles of the deformed nucleus. These are quite important quantities for heavy-ion fusion reactions, and hence produce great effects on the fusion cross section and barrier distribution.