Role of Barrier Modification and Nuclear Structure Effects in Sub-Barrier Fusion Dynamics of Various Heavy Ion Fusion Reactions

The role of barrier modifications and the relevant nuclear structure effects in the fusion of the \( {}_8{}^{16}O+{}_{62}{}^{144,148,150,152,154}Sm \) and \( {}_3{}^{6,7}Li+{}_{62}{}^{152}Sm \) systems is analyzed within the context of the energy-dependent Woods-Saxon potential model (EDWSP model) and the coupled channel model. For the \( {}_8{}^{16}O+{}_{62}{}^{144,148,150,152,154}Sm \) reactions, where the colliding pairs are stable against breakup, the collective excitations and/or static deformations are sufficient to account for the observed fusion enhancement. In contrast, the model calculations overpredict the complete fusion data at above - barrier energies for the \( {}_3{}^{6,7}Li+{}_{62}{}^{152}Sm \) systems, where the importance of projectile breakup effects has been pointed out. Due to the low threshold of the alpha-breakup channel, the weakly bound projectiles \( \left({}_3{}^{6,7}Li\right) \) break up into charged fragments before reaching the fusion barrier and consequently the complete fusion cross section is suppressed by 28% (25%) in the \( {}_3{}^6Li+{}_{62}{}^{152}Sm\;\left({}_3{}^7Li+{}_{62}{}^{152}Sm\right) \) reaction with respect to predictions of coupled channel calculations. However, the EDWSP model based calculations can minimize the suppression factor by as much as of 13% (8%) in the \( {}_3{}^6Li+{}_{62}{}^{152}Sm\;\left({}_3{}^7Li+{}_{62}{}^{152}Sm\right) \) reaction with reference to the predictions made by the coupled channel calculations. Therefore, the complete fusion data of the \( {}_3{}^6Li+{}_{62}{}^{152}Sm\;\left({}_3{}^7Li+{}_{62}{}^{152}Sm\right) \) reaction at above - barrier energies is reduced by 15% (17%) with respect to the expectations of the EDWSP model. The extracted suppression factors for the studied reactions are due to the modifications of the barrier profile as a consequence of the energy - dependence in nucleus-nucleus potential, and thus greater barrier modifications occur for more weakly bound system, which in turn, confirms the breakup of projectile in the incoming channel.     

Neutron and Proton Diffusion in Fusion Reactions for the Synthesis of Superheavy Nuclei

The restriction of the one dimensional （1D） master equation （ME） with the mass number of the projectile-like fragment as a variable is studied, and a two-dimensional （2D） master equation with the neutron and proton numbers as independent variables is set up, and solved numerically. Our study showed that the 2D ME can describe the fusion process well in all projectile target combinations. Therefore the possible channels to synthesize super-heavy nuclei can be studied correctly in wider possibilities. The available condition for employing 1D ME is pointed out.     

重离子反应中质子与中子转移对超重元素合成的影响

研究了采用类弹碎片质量数为变量的一维(1D)主方程(ME)描述重离子熔合反应时的受限制条件, 建立了以类弹碎片中子和质子数为独立变量的二维(2D) ME, 并采用分步差分格式法数值解(2D) ME得到了熔合几率。 新的方法适用于研究任何弹靶组合系统, 给出了与实验符合更好的结果, 为寻求合成超重核的反应道提供了更为宽阔的选择空间。 同时也研究了弹靶的不同同位旋组合对复合核形成截面的影响。 The limitation of the one dimensional master equation with the mass number of project like fragment as the variable is discussed. A two dimensional master equation with neutron and proton numbers of the project like fragment as variables is set up, and solved numerically by means of a two step difference scheme to obtain fusion probability. The new equation is suitable for studying fusion reactions with any projectile target combination, and can better fit the experimental data, so that it can provide a large probability to choose the reaction system for synthesizing a certain superheavy nucleus. The influence of projectile target combinations with different isospins on the cross sections of compound nuclei is studied.     

Entrance Channel Mass Asymmetry Effects in Sub-Barrier Fusion Dynamics by Using Energy Dependent Woods-Saxon Potential

The present article highlights the inconsistency of static Woods-Saxon potential and the applicability of energy dependentWoods-Saxon potential to explore the fusion dynamics of ₂₂⁴⁸Ti+₂₈^58,60,64Ni, ₂₂⁴⁶Ti+₂₈⁶⁴Ni, ₂₂⁵⁰Ti+₂₈⁶⁰Ni, and ₉¹⁹F+₄₁⁹³Nb reactions leading to formation of different Sn-isotopes via different entrance channels. Theoretical calculations based upon one-dimensional Wong formula obtained by using static Woods-Saxon potential unable to provide proper explanation for sub-barrier fusion enhancement of these projectile-target combinations. However, the predictions of one- dimensional Wong formula based upon energy dependent Woods-Saxon potential model (EDWSP model) accurately describe the observed fusion dynamics of these systems wherein the significantly larger value of diffuseness parameter ranging from a = 0.85 fm to a = 0.97 fm is required to address the experimental data in whole range of energy. Therefore, the energy dependence in nucleus-nucleus potential simulates the influence of the nuclear structure degrees of freedom of the colliding pairs.     

同质异能靶核的中子核反应计算

基于光学模型和带宽度涨落修正的Hauser-Feshbach理论编写了程序OMHFISO, 该程序适用于计算靶核处于同质异能态并且入射粒子能量小于5MeV的核反应, 入射粒子和出射粒子均可以是n,p,d,t,³He和⁴He 6种.迄今还很少有人计算过同质异能靶核的全套核数据.利用程序UNF, OMHFISO以及DWUCK4计算了不稳定同位素⁹⁰Rb和^{91,93,96,97,98}Y的同质异能靶核在入射粒子能量低于20.0MeV的全套中子反应数据, 并且得到了合理的结果.     

An Electron Beam Initiated Fusion Neutron Generator

An experimental scheme is proposed which seems to satisfy all the requirements for use of a high energy electron beam to initiate a thermonuclear plasma. One-dimensional expansion is utilized to obtain confinement times longer than the pulse length of the electron beams. A magnetic field is used to limit the radial heat conductivity, and this magnetic field also serves as a guiding field for the electron beams when they are in the vicinity of the target. Two opposing electron beams are employed and the forces produced by these counterstreaming currents in the overlap region of the beams are sufficient to stop the beams within the target. Estimates made of all the critical factors indicate that beams achievable with current technology can be focused and stopped in T-D targets 6 cm long with densities as low as 1021 cm-3. With a containing magnetic field of 750 kilogauss the containment time of the plasma is sufficiently long so that beam pulse lengths up to 8 × 10-9 sec can be used. Furthermore a positive fusion energy yield relative to the energy delivered to the target is predicted.     

熔合反应中动力学势垒的研究

用改进的量子分子动力学模型研究了与入射能量相关的重离子熔合势垒. 随着 入射能的降低可以观察到动力学势垒的最低值, 这个最低动力学势垒与绝热势垒 非常接近；动力学势垒随着入射能的增加而升高, 最终接近于静态势垒(非绝热势垒). 基于动力学势垒的研究, 对于重离子熔合反应的额外推动(extra-push)给出了微观理解,对势垒贯穿给出了一种新的解释. 为进一步理解动力学势垒, 还研究了颈部的形成和体系的动力学形变, 分析了动力学势垒降低的原因.     

中子引起氘核破裂反应的实验研究

对25 MeV中子引起氘核破裂反应的中子-中子准自由散射和17.36 MeV中子引起氘核破裂反应的中子-中子末态相互作用进行了细致的研究。首先,以小于5%的不确定度,精确测量了25MeV中子-中子准自由散射出射的中子三重微分截面。实验数据用基于现实核子-核子势(CD-Bonn,Argonne!18,Nijm I和II)的理论计算配合Monte-Carlo模拟进行了分析。实验结果比基于CD-Bonn势的理论预言高(16.0±4.6)%,进一步证实了目前的理论在中子-中子准自由散射方面还无法准确描述实验数据。其次,通过运动学非完全测量,精确测量了17.36 MeV中子引起氘核破裂反应在0°角附近出射的质子能谱,用基于现实核子-核子势(CD-Bonn、Bonn-B和Nijm I)的理论计算以及Monte-Carlo模拟分析了所测得的质子能谱,确定了中子-中子散射长度ann=(-16.8±0.6)fm。     

Photoinduced Gas-Phase Electron Transfer Reactions

The electron transfer quenching process, when a reactive excited state is singlet or triplet, for gas-phase systems (benzophenone and anthraquinone with amines and pyridine as well as carbazole with halomethanes) was systematically investigated using time-resolved fluorescence. Bimolecular rate constants were obtained. Variable-temperature measurements were performed for eight donor-acceptor pairs. It was found that under solvent-free conditions various quenchers differing in photochemical reactivity led to change in quenching rates by almost three orders of magnitude. Positive and negative temperature dependences for the electron transfer rate constants were observed. The data were analyzed in terms of the Marcus-Jortner theory.     

中子与9Be和6,7Li反应的次级中子发射谱实验研究

测量了8.17MeV与10.27MeV中子与⁹Be和^6,7Li作用的次级中子双微分截面. 对于10.27MeV, 为了消除从D(d,np)破裂反应来的源破裂中子对双微分截面测量结果的影响, 采用了常规多探测器快中子飞行时间谱仪和非常规多探测器快中子飞行时间谱仪相结合的办法. 用Monte-Carlo方法对实验测量得到的飞行时间谱进行了详细的模拟, 通过测量谱与模拟谱的比较, 得到了实验测量的次级中子双微分截面. 实验测量结果以n-p(常规谱仪)和n-C(非常规谱仪)弹性散射微分截面作为归一. 测量结果与评价数据以及其他测量数据进行了比较. 用一个基于Hauser-Feshbach和激子模型的轻核核反应理论模型对^6,7Li的次级中子双微分截面进行了计算, 理论计算结果与实验结果符合得较好.     

弱束缚弹核引起的熔合反应

⁶Li, ⁹Be为弱束缚核. 通过弱束缚弹核⁶Li, ⁹Be轰击靶²⁰⁸Pb,²⁰⁸Be的实验熔合激发函数与理论预言的比较, 讨论了弱束缚弹核破裂对熔合过程 的影响. 比较结果显示, 弱束缚弹核与重靶核的完全熔合截面在垒上能区 明显压低. 由部分熔合截面与完全熔合截面之和得到总熔合截面. 研究结果 表明, 破裂对总熔合截面几乎没有影响. 由此可见, 弱束缚核的部分熔合 可能发生在强吸收区域附近. 最后还给出了全熔合截面与部分熔合截面 之间的关系.     

近垒重离子熔合反应中的中子转移效应(英文)

近势垒及其以下能区重离子熔合反应中的中子转移耦合道效应是一个复杂且有争议的问题。简要介绍了近年来在中国原子能科学研究院的HI-13串列加速器上,基于静电偏转板装置完成的32S+90,94,96Zr,18O+74Ge和18O+58Ni等几个典型体系的熔合反应研究情况,并结合耦合道理论对实验数据进行了分析。选取反应体系时同时关注了正Q值的中子拾取和削裂道。另外,基于完全耦合道理论计算,提出了一种能够定量提取熔合反应中中子转移效应的自洽方法。这些研究进一步证实了垒下重离子熔合反应中的中子转移效应,同时指出了其复杂性。需要进一步的实验和理论研究来澄清相关核反应机制。The effect of neutron transfers on near-and sub-Coulomb-barrier fusion of heavy-ions is still a complicated and controversial problem. This paper reviews the recent experimental results of the fusion excitation functions of several typical systems, which have been measured by using an electrostatic deflector setup at the HI-13 tandem accelerator of CIAE. Both the neutron pickup and stripping effects were studied. Moreover, a self-consistent method to reliably isolate the transfer effect quantitatively based on the coupledchannels calculation is proposed. These studies give a further support for the neutron transfer effect on sub-barrier fusion of heavy-ions and its complexity. Further experimental and theoretical studies are needed for clarifying the relevant reaction mechanisms.     

一种用于DT聚变诊断的新型中子谱仪的设计

介绍了一种基于反冲质子法和磁分析技术的新型DT聚变中子谱仪, 能够以高信噪比对9—17MeV的中子能谱进行精确测量, 适用于稳态及脉冲条件下的等离子体温度（T_i）和燃料面密度（<ρR）的诊断, 对14 MeV的初级DT中子具有约4%的能量分辨率和约10-8的探测效率。谱仪的磁分析系统使用高性能钕铁硼二极永磁铁建造, 焦平面上使用CR-39固体径迹探测器测量反冲质子的位置分布。使用239Pu α源对磁分析系统进行了实验研究,并建立了配套的粒子输运模拟程序。 结合实验和程序模拟结果,使用蒙特卡罗软件模拟分析了谱仪整体性能。 谱仪结构紧凑、性能良好且适用于稳态及脉冲辐射场,可望在未来的聚变研究中得到有效应用。 Design of a novel spectrometer for measurements of neutrons in the energy range 9—17 MeV has been proposed. Based on proton recoil method and magnetic analyzing technigue, the spectrometer can be used to determine DT fuel areal density(<ρR) and ion temperature under steady state and pulsed radiation field. The detection efficiency is about 10-8 for measuring 14 MeV neutrons with an energy resolution of about 4%. A neodymium iron boron permanent dipole was used for the magnetic analyzing system and CR-39 track detectors were used in the focal plane. The system has been tested with 239Pu α suorce and a corresponding particle transport simulation program has been worked out. Performance of the spectrometer was investigated with Monte Carlo simulation, experiment data and the transport simulation. Due to its good performance, compact structure,and availability for both steady state and pulsed radiation field, the spectrometer can be effectively used for fusion research in the future.     

重核融合反应动力学位垒的微观研究

在简要评述重核融合过程中几种主要理论模型的基础上,提出了微观输运动力学模型,即改进的量子分子动力学模型.在这个模型的框架内,我们研究了重核融合位垒的动力学行为.我们发现,随入射能量的减少,可以得到最低的动力学位垒,它趋近于绝热静态位垒.而随入射能量的增加,动力学位垒增加,最后趋近于非绝热静态位垒,这给出了位垒分布的两个边缘.在微观输运动力学模型基础上,我们还研究了在融合路径上,动力学位垒与融合体系微观构型的关系.考虑到融合过程不同时刻的单粒子位势与双中心壳模型位势的相似性,我们可以很好的研究融合过程中,在构型空间里单粒子态及相关量的时间演化行为.