中能重离子碰撞中集体流的同位旋效应

在同位旋相关的量子分子动力学模型的基础上,利用Skyrme–Hartree–Fock计算所得的中子、质子密度,同时利用费米气体模型得到相应的中子、质子费米面,抽样出稳定的⁵⁸Fe和⁵⁸Ni初始核.仔细研究了55MeV/u ⁵⁸Fe+⁵⁸Fe和55MeV/u⁵⁸Ni+⁵⁸Ni两个反应中集体流的同位旋效应.在不同碰撞参数下对不同类型的碎块,观察到丰中子反应系统⁵⁸Fe+⁵⁸Fe比⁵⁸Ni+⁵⁸Ni有更强的集体流,并能与实验结果定性符合.同时,研究了同位旋相关的对称能与核子–核子碰撞截面对集体流的影响.     

中能重离子碰撞中平衡能的同位旋效应

利用同位旋相关的量子分子动力学模型,计算了⁵⁸Fe+⁵⁸Fe和⁵⁸M+⁵⁸Ni两个反应系统在不同碰撞参数下的平衡能.观察到在不同碰撞参数下丰中子反应系统⁵⁸Fe+⁵⁸Fe比⁵⁸Ni+⁵⁸Ni有更高的平衡能.计算结果能与实验数据定性符合.     

Probing Balance Energy Using Momentum- and Isospin-Dependent Potential

Using the momentum- and isospin-dependent Boltmann-Uehling-Uhlenbeck （BUU） model, we investigate the transverse flow and balance energy in two isotopic colliding systems ^48Ca＋^58Fe and ^48Cr＋^58Ni by adopting different symmetry potentials. By comparing the results between the two colliding systems, we find that the difference between the balance energies of two isotopic systems can be considered as a sensitive probe to the density dependence of symmetry energy.     

Finite Range Effects on Fusion and/or Breakup of ^6He^＋238U and ^11Li＋^208pb Systems

We have studied the effects of the finite range of the interaction between the fragments of the projectile on the fusion and/or breakup of ⁶He+²³⁸U and ¹¹Li+²⁰⁸Pb systems at near barrier energies within the framework of dynamic polarization potential approach. It has been found that at near barrier energies the maximum flux is lost to the breakup channel and at energies well above the Coulomb barrier the fusion coupled with the breakup channel opens up, initially with sharp rise and later becoming saturated at energy nearly twice of theCoulomb barrier. Further, it is found that the breakup cross section increases with the increasing range of the interaction between the fragments of the projectile while the fusion coupled with the breakup channel cross section decreases with the increasing range.     

Learning about the nuclear equation of state using flow

We present results for transverse flow, the balance energy, and elliptic flow for Ar+Sc, ⁵⁸Fe+⁵⁸Fe, ⁵⁸Ni+⁵⁸Ni, Kr+Nb, and Au+Au. We find that a soft equation of state is required to explain these results. We find that a momentum dependent formulation of the nuclear mean field is required. A 30% density dependent reduction of the in-medium nucleon-nucleon cross sections in BUU is required to reproduce the observed balance energies for light systems.     

Inertia effects in the mass transport of heavy ion reactions

The general trend of the multi differential cross section plots for the products obtained from the dissipation collisions of the three systems, forming almost the same composite systems but with different degrees of mass asymmetry in the entrance channel,⁵²Cr (261 MeV)+⁵⁶Fe,⁴⁰Ca (182 MeV)+⁶⁴Ni and¹⁶O (187 MeV)+⁹²Mo, were analysed in the frame of the Diffusion Model, assuming that the intermediate system decays with a statistical life time. A fairly good agreement has been obtained between the calculated multi differential plots and the experimental ones. The half life time of the composite systems seem to depend on the mass asymmetry of the entrance channel.     

质子椭圆流与对称能的密度依赖性

基于同位旋和动量依赖的强子输运IBUU04模型, 研究了¹³²Sn+¹²⁴Sn碰撞系统中的质子椭圆流对对称能的敏感关系. 研究发现入射能量从每核子400MeV到800MeV时质子椭圆流在低横动量端对对称能的敏感性高于高横动量端, 同时发现随着入射能量的增大, 质子椭圆流对对称能的敏感性在降低. 在研究入射能量范围内, 当入射能量为每核子400MeV左右时质子椭圆流对对称能最为敏感.     

Formation and decay of hot nuclei

Formation and decay properties of composite-like nuclei produced by ⁵⁸Ni projectiles bombarding ²³²Th target nuclei are investigated at different energies by means of fission fragment angular correlation measurements. Experimental results are compared to the excition model and excitation energies in the fissioning nuclei are deduced. By comparing the present data with the results on other systems, the influences of entrance channel conditions and composite-like nuclei temperatures are discussed to explain the evolution of the high linear momentum transfer region. The coexistence of both massive transfer mmechanism and head-on collisions followed by preequilibrium is suggested.     

46.7MeV/u12C引起的弹核碎裂及轻带电粒子关联

我们在HIRFL提供的46.7MeV/u的~(12)C离子轰击~(58)Ni、~(115)In和~(197)Au靶所引起的核反应中研究了弹核碎裂与转移反应的竞争,分别提取了对应于弹核碎裂和转移反应的类弹碎片的约化动量宽度,从粒子发射不稳定态的相对布居得到了不同反应系统的核温度参数。 The projectile fragmentation and its competition with the transfer reactions have beenstudied in the reactions induced by 46.7 MeV/u ~(12)C ion on the ~(58)Ni, ~(115)In and ~(197)Au targets. Thereduced momentum distribution widths of the projectile-like fragments for projectile fragmentationand transfer reactions were extracted from the experimental data respectively. The nuclear temperatureperameters were obtained from the relative populations of the unstable particle emission ...     

High-spin states in 58Ni

High-spin states of ⁵⁸Ni were investigated via the study of in-beam γ-ray induced by the compound reaction ⁴⁸Ti(¹²C, 2n)⁵⁸Ni between 26 and 48 MeV. The energies and decay modes of these levels were determined from the analysis of γ-γ coincidence measurements at 35 MeV. The most intense lines in the ⁵⁸Ni γ-ray spectrum correspond to a cascade to the ground state, through levels at 1.454, 2.459, 3.619 and 4.381 MeV, also fed in other reactions, and by two previously unknown levels at 5.125 and 5.662 MeV; the spin assignments based on the present study are (apart from the ground state) 2, 4, 4, 5, 6 and 7 respectively for these levels. The first three were already known and the last three are new. The mixing ratios for the transitions between these levels are also determined. We observe also the same cascade in the reaction ⁵⁶Fe(α, 2n)⁵⁸Ni at an incident energy 18–24 MeV. Comparisons with other reactions, previous studies and recent shell-model calculations are presented.     

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of ²²⁶Th nuclei formed via a complete fusion of ¹⁸O and ²⁰⁸Pb nuclei at laboratory energies of ¹⁸O projectile ions in the range E _lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E _c.m. ? E _B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive⁴He and⁴H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u²³⁸U with²³⁸U and^?197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For²³⁸U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For²³⁸+²³⁸U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In²³⁸U+¹⁹⁷Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.     

Particle evaporation from hot nuclei withA ≦85 mainly prior to scission

We report on measurements of fusion-fission fragments and evaporation residues from the reaction⁵⁸Ni +²⁷Al at 835 MeV incident energy leading to compound nucleiA ≦ 85 with high excitation energies. In the case of fission the mass dependence of the velocity widths of separated fragments indicate that the evaporation of light particles predominantly occurs from the composite nucleus prior to scission. The fraction evaporated from the fully separated fragments after scission can be estimated to approximately 20%.     

Heavy-ion induced fission reactions at near-barrier energies — what have we learnt?

Systematic studies of heavy-ion induced fission reactions at near-barrier energies carried out in the last decade have brought out many interesting aspects of fission process in general. The recent experimental findings which show dependence of fission fragment angular distributions on entrance channel, shape, size and spin of the interacting nuclei and shell closure of the intermediate compound nucleus are summarised in the present paper.     

Mechanism of neon-induced reactions studied between 7.5 and 20 MeV/nucleon: Inclusive cross sections

Light and projectile-like fragments as well as fission fragments have been observed from the ²⁰Ne + ¹⁹⁷Au system at 150, 220, 290 and 400 MeV beam energy. Inclusive cross sections are given, and characteristic parameters like the centroids and the widths of momentum, Z- and A-distributions were derived. For comparison, some results of measurements on the ²²Ne + ¹⁹⁷Au and ^{20, 22}Ne + ⁵⁸Ni systems are included. The qualitative behaviour of the cross sections and the derived parameters show that with increasing bombarding energy the incomplete-fusion mechanism (massive transfer) and the sequential decay of projectile transfer residues (sequential break-up) appear in addition to the complete-fusion and ordinary transfer reactions predominant at the lowest bombarding energies. Other mechanisms do not contribute significantly in the energy range up to 20 $\text{MeV}\text{nucleon}$.     

The ablation stage in the fast abrasion model

In order to verify the effect of the ablation stage on the fast abrasion mechanism, detailed evaporative calculations have been performed on the primary projectile and target-like fragments issued from the reactions⁴⁰Ar+²⁷Al and⁴⁰Ca+²⁷Al. The results account for many of the experimental features observed in the projectile and target-like fragments produced at intermediate energies. However it is pointed out that, in order to get a better agreement between theory and experimental data, the primary projectile and target-like fragments should carry a more substantial excitation energy than predicted by a purely geometric abrasion model.     

40 Ca+58Fe，58Ni反应中的径向膨胀流研究

利用同位旋相关的Boltzmann-Langevin方程研究了^40Ca ^58Fe和^40cA ^58Ni两个反应系统在53,100,1506 200MeV／u入射能量下对心碰撞的径向膨胀流,发现对于丰中子系统^40Ca ^58Fe的径向膨胀流系统性地小于稳定系统^40Ca ^58Ni的径向膨胀,在假定轰击能量与反应体系的压缩密度呈抛物线关系时,能够解释入射能量和径向膨胀流之间呈现的直线关系,提取了出现径向膨胀流的轰击能量阈值,发现对丰中子系统^40Ca ^58Fe得到的能量阈值小于稳定系统^40Ca ^58Ni所得到的能量阈值。     

Discrete γ-rays in the reactions of 143 MeV32S with58Ni

The reactions induced by 143 MeV³²S on⁵⁸Ni have been studied detecting discrete γ-rays in coincidence with projectile-like fragments (PLF). Information on PLF excitation probability and sequential decay of target-like fragments (TLF) has been obtained. For the²⁸Si+⁶²Zn outgoing channel at small energy loss (¦Q¦<20 MeV), both PLF and TLF data indicate that thermal equilibrium is not attained. The hypothesis of an equal excitation energy partition between the two reaction fragments does not describe properly experimental TLF data. A dependence of PLF excitation probability on the outgoing channel is found for the two final channels³²S+⁵⁸Ni and²⁸Si+⁶²Zn. The values of the spin alignment parameterP _zz, derived for PLF and TLF from measurements ofγ-rays anisotropy, are in disagreement with the expectations of the transport theory for dissipative collisions.     

Production mechanism of neutron-rich nuclei around N =126 in the multi-nucleon transfer reaction ~(132)Sn+~(208)Pb

The time-dependent Hartree-Fock approach in three dimensions is employed to study the multi-nucleon transfer reaction~(132) Sn +~(208) Pb at various incident energies above the Coulomb barrier. Probabilities for different transfer channels are calculated by using the particle-number projection method. The results indicate that neutron stripping(transfer from the projectile to the target) and proton pick-up(transfer from the target to the projectile)are favored. De-excitation of the primary fragments is treated by using the state-of-art statistical code GEMINI++.Primary and final production cross sections of the target-like fragments(with Z =77 to Z =87) are investigated. The results reveal that fission decay of heavy nuclei plays an important role in the de-excitation process of nuclei with Z 82. It is also found that the final production cross sections of neutron-rich nuclei depend only slightly on the incident energy, while those of neutron-deficient nuclei depend strongly on the incident energy.