中子诱发12C反应的理论计算与分析

利用统一的Hauser–Feshbach理论和角动量相关的激子模型,以中子诱发¹²C反应为例,研究了轻核反应的反应机制和轻核反应的特点,并进行了理论计算,同时与实验数据进行了比较.     

F+H2化学反应中的Feshbach共振之研究

化学反应共振态研究是化学动力学研究的重要前沿课题,对于理解基元化学反应的机理有重要的意义.本文中介绍了最近在这一研究方向的重大进展.通过对F＋H2化学反应的全量子态分辨的分子束反应散射实验研究,观测到了F＋H2中反应中明显的反应共振现象.通过高精度的全量子散射动力学研究,发现这一共振现象是由两个Feshbach共振态所引起的,而且这两个Feshbach共振态之间在前向散射有明显的量子干涉效应.这项研究工作使得我们对这一重要基元反应中的化学反应共振态研究向前迈进了一大步。     

97MeV~(16)O轰击~(51)V靶出射α粒子的测量与理论分析

用不同种类的粒子望远镜测量了97MeV~(16)O+~(51)V反应中出射的α粒子.得到了单举α粒子能谱、角分布、E-θ平面内的d~2θ/dEDΩ截面等高图及速度平面内的d~2θ/PCdEdΩ截面等高图.区分了复合核蒸发的α粒子与直接机制发射的α粒子.用激子模型对预平衡发射的α粒子贡献进行估计,得到的预平衡发射和复合核蒸发分别在α粒子总产额中所占的比份.     

中子与90,91,92,94Zr反应的理论计算和分析

根据中子与天然Zr及其同位素反应的总截面,去弹截面和弹性散射角分布的实验数据,得到一组普适的光学模型势参数;应用得到的光学模型势参数,光学模型,Hauser-Feshbach理论,预平衡反应的激子模型和扭曲波玻恩近似理论,系统计算和分析了中子与^90,91,92,94Zr反应的非弹散射角分布和双微分截面,理论结果与实验很好的一致.     

金等离子体中Au48+-Au52+平衡分布的统计热力学研究

基于全相对论多组态Dirac Fock理论 ,采用“多功能相对论原子结构程序 (GRASP2 )” ,考虑量子电动力学 (QED)效应和Breit修正 ,涉及实验谱中Au等离子体M带的几类重要跃迁 ,计算了Au4 8 —Au52 离子的能级结构和能级简并度 .用统计热力学方法计算各离子的配分函数 ,由配分函数计算等离子体内这五种离子的电离与复合平衡常数 ,根据同时反应的平衡理论研究电离与复合达到平衡时等离子体内各离子的相对分布基于全相对论多组态Dirac Fock理论 ,采用“多功能相对论原子结构程序 (GRASP2 )” ,考虑量子电动力学 (QED)效应和Breit修正 ,涉及实验谱中Au等离子体M带的几类重要跃迁 ,计算了Au4 8 —Au52 离子的能级结构和能级简并度 .用统计热力学方法计算各离子的配分函数 ,由配分函数计算等离子体内这五种离子的电离与复合平衡常数 ,根据同时反应的平衡理论研究电离与复合达到平衡时等离子体内各离子的相对分布     

中子与裂变核238U反应双微分截面的理论分析

根据中子与原子核U及其同位素反应的总截面,去弹性散射截面和弹性散射角分布的实验数据,获得了入射中子能量0.1—20MeV的一组普适中子与U及其同位素反应的光学模型势参数.应用光学模型,核裂变理论,耦合道理论,扭曲波玻恩近似理论,宽度涨落修正的Hauser-Feshbach理论和预平衡反应的激子模型,计算和分析了中子与²³⁸U反应的所有截面、角分布、能谱和双微分截面.理论计算与实验数据进行了分析比较     

激子模型中的激子态寿命和激子态跃迁率与发射率

通过预平衡核反应理论中的激子模型理论 ,给出了激子模型中的激子态寿命和激子态发射率与跃迁率的表达式。并通过计算给出了轻核 (9Be、1 6O) ,中等核 (56Fe、64Zn) ,重核 (1 75Lu、1 84W)等核的各激子态寿命以及它们的发射率和跃迁率。结果表明对于轻核 ,前几个激子态在反应中占主要的部分 ,使用时可以作无返回近似。这反映了在轻核反应中 ,预平衡反应占主要的地位。     

预平衡裂变

实验证明,重离子熔合裂变与入射道的质量不对称有关,说明对于某些反应体系,熔合形成的复合体系,裂变时未忘记其形成的历史.在实验观察的基础上,我们提出了预平衡裂变模型,解释了重离子垒下熔合裂变碎片角分布各向异性异常.     

中子与116,118,120,122,124Sn反应非弹性散射的理论计算和分析

应用现有的中子与原子核Sn及其同位素反应的总截面,去弹性散射截面,弹性散射截面和弹性散射角分布的实验数据,获得一组普适的中子与Sn及其同位素反应的光学模型势参数.利用这组光学模型势参数,扭曲波玻恩近似理论,核反应Hauser-Feshbach理论和预平衡反应的激子模型,计算和分析了中子与^{116,118,120,122,124}Sn反应分离能级的非弹性散射截面和角分布.理论计算与实验数据进行了分析比较.     

中能中子与208Pb反应的理论计算和分析

在中子与原子核Pb及其同位素反应的总截面,去弹性散射截面,弹性散射截面和弹性散射角分布的实验数据基础上,获得了入射中子能量从1—300MeV的一组普适中子与Pb及其同位素反应的光学模型势参数.应用光学模型,扭曲波玻恩近似理论,宽度涨落修正的Hauser-Feshbach理论,预平衡反应的激子模型和核内级联模型,计算和分析了中子与²⁴⁸Pb反应的所有截面、角分布和能谱.理论计算与实验数据进行了分析比较     

Model Calculation of n 6Li Reactions Below 20 MeV

Based on the unified Hauser-Feshbach and exciton model for light nuclei, the calculations of reaction cross sections and the double-differential cross sections for n 6Li are performed. Since all of the first-particle emissions are from the compound nucleus to the discrete levels, the angular momentum coupling effect in pre-equilibrium mechanism must be taken into account. The fitting of the measured data indicates that the three-body break-up process needs to be involved, and the pre-equilibrium reaction mechanism dominates the reaction processes. In light nucleus reactions the recoil effect must be taken into account.``     

Model Calculation of n+6Li Reactions Below 20 MeV

Based on the unified Hauser-Feshbach and exciton model for light nuclei, the calculations of reaction cross sections and the double-differential cross sections for n+⁶Li are performed. Since all of the first-particle emissions are from the compound nucleus to the discrete levels, the angular momentum coupling effect in pre-equilibrium mechanism must be taken into account. The fitting of the measured data indicates that the three-body break-up process needs to be involved, and the pre-equilibrium reaction mechanism dominates the reaction processes. In light nucleus reactions the recoil effect must be taken into account.     

The Angular Distribution of Light Particle Projectile Based on a Semi-Classical Model

A semi-classical model of multi-step direct and compound nuclear reactions is proposed to describe the angular distributions of the light particle projectiles from reaction processes induced by a nucleon with energies of several tens of MeV. The exact closed solution to the time-dependent master equation of the exciton model is applied. Based on the Fermi gas model, the scattering kernal between two-nucleon collieion includes the influences of the Fermi motion and the Pauli exclusion principle, which give the significant improvement to rise of the backward distributions. The energyangular correlation for the first few steps of the collision process (muli-step direct process) yields the further improvement of the angular distribution. The pick-up mechanism is employed to describe the composite particle emission. Thh reasonable physical picture reproduces the experimental data of the energy spectra of the composite particles satisfactorily. The angular distribution of the emitted composite particle is determined by an angular factor in terms of the momentum conservation of the nucleons forming the composite cluster. The generalized master equation is employed for the multi-step compound process. Thus a classical approach has been establised to calculate the double differential cross sections for all kinds of particles emitted in multi-step nucler reaction processes.     

Stuty on the Process of the Fast Neutron Radiative Capture for 40Ca and 208Pb

The pre-equilibrium correction for the nuclear reactions induced by fast neutron is considered as follows: the nuclear reaction processes with the exciton number equal to or larger than five can still be described by the statistical theory of the nuclear reaction. The particle emission processes, in which less than five excitons are involved, are calculated by means of the exciton model and the γ emission of one exciton state is calculated by the direct capture mechanism. For the three-exciton state, only the semidirect capture mechanism, which plays the main role in the researched energy region, is taken into account. The interference effect between the direct and semidirect capture is also considered. The radiative capture cross sections for ⁴⁰Ca and ²⁰⁸Pb in the neutron incident energy region from 3 MeV to 20 MeV are calculated and a better coincidence with the experimental values is obtained. At the same time, the contribution to the (n, γ) reaction cross sections of the γ emissions before and after statistical equilibriums as well as the characteristics of the direct capture, semidirect capture and their interferece terms are discussed.     

Theoretical Model Calculation for d+8Li Reaction

Based on the theoretical models for light nuclei, the calculations of reaction cross sections and the angular distributions for d+⁸Li reaction are performed. Since all of the particle emissions are from the compound nucleus to the discrete levels, the angular momentum coupling effect in pre-equilibrium mechanism is taken into account. The three-body break-up process and the recoil effect are involved. The theoretical calculated results are compared to existing experimental data.     

The statistical theory of multi-step compound and direct reactions

The theory of nuclear reactions is extended so as to include a statistical treatment of multi-step processes. Two types are distinguished, the multi-step compound and the multi-step direct. The wave functions for the system are grouped according to their complexity. The multi-step direct process involves explicitly those states which are open, while the multi-step compound involves those which are bound. In addition to the random phase assumption which is applied differently to the multi-step direct and to the multi-step compound cross-sections, it is assumed that the residual interaction will have non-vanishing matrix elements between states whose complexities differ by at most one unit. This is referred to as the chaining hypothesis. Explicit expressions for the double differential cross-section giving the angular distribution and energy spectrum are obtained for both reaction types. The statistical multi-step compound cross-sections are symmetric about 90°. The classical statistical theory of nuclear reactions is a special limiting case. The cross-section for the statistical multi-step direct reaction consists of a set of convolutions of single-step direct cross-sections. For the many step case it is possible to derive a diffusion equation in momentum space. Application is made to the reaction ¹⁸¹Ta(p, n)¹⁸¹W using the statistical multi-step compound formalism.     

Analysis of Neutron Double-Differential Cross Sections for n + 12C Reaction Below 30 MeV

Based on the light nucleus reaction model (Nucl. Sci. Eng. l33 (1999) 218), four aspects (neutron incident energy region, reaction channel analysis, the renewed level schemes and the optical model parameters) of n 12 C reaction are improved to calculate total outgoing neutron double-differential cross sections with modified LUNF code below 30 MeV. The calculated results agree fairly well with the experimental data at En = 14.1 MeV and 18 MeV. The analysis shows that the pre-equilibrium mechanism, which is exactly considered the conservation of energy, momentum and parity, dominates the whole reaction process. The contribution of the neutron emission from 5He to total energyangular spectra is also considered properly. This modified LUNF code will be a useful tool to set up the file of neutron double-differential cross sections below 30 MeV in the neutron evaluation nuclear data library.     

Comment on application of statistical multi-step direct emission theory to a neutron-emitting reaction

The statistical multi-step direct emission theory formulated by Feshbach, Kerman and Koonin (FKK) was applied to the analysis of 14 MeV (n, n') scattering on ⁹³Nb and Ag. It is concluded that the FKK theory as it stands disagrees with experimental data, because of anomalously large multi-step cross sections due to the resonance-like behaviour of non-DWBA matrix elements which are entered in the calculation.     

Analysis of Neutron Double-Differential Cross Sections for n ＋ ^12 C Reaction Below 30 MeV

Based on the light nucleus reaction model （Nucl. Sci. Eng. 133 （1999） 218）, four aspects （neutron incident energy region, reaction channel analysis, the renewed level schemes and the optical model parameters） of n＋ ^12 C reaction are improved to calculate total outgoing neutron double-dilferential cross sections with modified LUNF code below 30 MeV. The calculated results agree fairly well with the experimental data at En = 14.1 MeV and 18 MeV. The analysis shows that the pre-equilibrium mechanism, which is exactly considered the conservation of energy, momentum and parity, dominates the whole reaction process. The contribution of the neutron emission from 5He to total energy- angular spectra is also considered properly. This modified LUNF code will be a useful tool to set up the file of neutron double-differential crass sections below 30 Me V in the neutron evaluation nuclear data library.     

Analysis of Neutron Double-Differential Cross Section of n+14N at 14.2 MeV

By using a new reaction model for light nuclei, the double-differential cross section of n 14N reactions at En = 14.2 MeV has been analyzed. In the case of n 14N reactions, the reaction mechanism is very complex, there are over one hundred opened partial reaction channels even at incident energy En = 14.2 MeV. In this paper the opened reaction channels are listed in detail. With LUNF code the model calculation is performed to analyze the doubledifferential cross sections of total outgoing neutron. The calculated results agree fairly with the experimental data. The results indicate that the pre-equilibrium mechanism dominates the whole reaction processes, and the recoil effect in light nuclear reactions is essentially important. 5He emission has been considered, but it is only a small contribution to the double-differential cross section at incident energy En = 14.2 MeV.