An extension of the generalized exciton model and calculations of (p,p') and (p,α) angular distributions

We present a model for calculating the energy-angle double differential cross section of the nucleon emission from preequilibrium states in nucleon-induced reactions. It is an extension of the generalized exciton model of Mantzouranis et al. for treating the kinematics of the NN collision properly. Its final expression is compact and easy to use. Numerical calculations for various reactions were carried out and some improvements over the previous results were obtained. The angular distribution was well reproduced except at backward angles. The validity of the “fast” particle assumption used in the previous models was also examined.We then applied the present model to composite-particle emission by employing the technique for composite-particle emission spectra we developed previously. Here we assumed that the angular distribution of the composite particle is obtained by folding that of the “leading” particle over the internal motion of the composite particle. The numerical calculation for α-emission was performed and a good fit to the experimental data was obtained for various targets and incident energies without any adjustable parameters.     

Theoretical Calculations and Analysis of n+208Pb Reaction for Intermediate Energy Neutron

Based on experimental data of total, nonelastic, elastic cross section and elastic scattering angular distributions for n+Pb reactions, a set of neutron optical model potential parameters is obtained in the region of incident neutron energy from 1—300 MeV. The cross sections, angular distributions and energy spectra are calculated and analyzed by optical model, distorted wave Born approximation theory, Hauser-Feshbach theory, exciton model and cascade mechanism inside nuclear. The results indicate that the cross sections can be given for n+ ²⁰⁸Pb reactions which are all in good agreement with experimental data.     

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.     

中能中子与58Ni反应的理论计算

在中子与⁵⁸Ni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基础上, 获得了入射中子能量从0.825—150MeV的一组普适的中子与⁵⁸Ni反应的光学模型势参数. 利用光学模型、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的中能核反应计算程序MEND, 计算了中子与⁵⁸Ni反应的所有截面、角分布和能谱, 并将理论计算结果与实验数据和评价数据进行了分析比较.     

晕核6He（25 MeV/u）的单、双中子转移截面测量

实验测量了25 MeV/u 的6He+9Be反应中的单、双中子转移微分截面. 利用反应耦合道模型初步分析了实验数据, 计算大体重现截面值. 转移反应对末态有相当的选择性, 并且双中子转移截面主要来自到基态的一步过程. 后角度截面的上升可能与连续态的耦合有关. Differential cross sections for quasi elastic scattering at forward angles and for 1n and 2n transfer reaction at backward angles induced by 6He at 25 MeV/u from 9Be target were measured. The experimental data were analyzed in the framework of the CRC model. The raise at backward angle of the 1n and 2n transfer cross section can not reproduced by the current calculation.     

Calculation and Analysis of Neutron-Induced Reactions on 58Ni Below 150MeV

Based on the experimental data of the total, the nonelastic and the elastic cross sections and the elastic scattering angular distributions for n+⁵⁸Ni reactions, a set of parameters for a neutron optical model potential is obtained in the incident neutron energy region from 0.8—150MeV. Then reaction cross sections, angular distributions, energy spectra, gamma-ray production cross sections and gamma-ray production energy spectra are calculated in terms of the optical model, the distorted wave Born approximation theory, the Hauser-Feshbach theory, the exciton model and the cascade mechanism inside nuclear. The results are analyzed and compared with the existing experimental data and other evaluated data from ENBF/B-6. They are in agreement with each other within error bars.     

中子与52Cr反应截面的理论分析

根据中子与天然核Cr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从1MeV—250MeV的一组普适中子与Cr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,和预平衡反应的激子模型,计算和分析了中子与⁵²Cr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.     

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     

Theoretical Analysis of Double Differential Cross Sections for n+ 238U Reaction

Based on the new measurements of total, nonelastic, elastic cross section and elastic scattering angular distributions for n+U reactions, a set of neutron optical model potential parameters is obtained in the region of incident neutron energy from 0.1 to 20 MeV. The cross sections, angular distributions, energy spectra and double differential cross sections are calculated and analyzed by optical model, nuclear fission theory, distorted wave Born approximation theory, coupled channel theory, the unified Hauser-Feshbach theory, as well as exciton model. The results indicate that our theoretical model can reasonably analysis n+ ²³⁸U reaction data with neutron energy lower than 20 MeV.     

Classical Description of Two-Cluster Transfer Reactions in a Coulomb Field

The theory of two-cluster transfer reactions is reconsidered on the basis of a simple semiclassical approximation. The obtained expression for the differential cross section is in a closed form so as to be easily calculated. A theoretical study with calculations for the angular distribution for different incoming and outgoing energies, and also for different angular momentum transfer is introduced. Comparing our theoretical calculations with experimental data, satisfactory agreement is obtained especially in the backward direction.     

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.     

中子与63,65Cu反应的理论计算

根据中子与天然Cu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, 得到中子的光学模型势参数; 应用得到的光学模型势参数, 光学模型, 统一的Hauser-Feshbach和激子模型理论, 以及扭曲波玻恩近似理论, 系统计算和分析了中子与^63,65Cu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.     

SYSTEMATICAL ANALYSES OF PROTON-NUCLEI SCATTERING WITH RELATIVISTIC MICROSCOPIC OPTICAL POTENTIAL

Relativistic microscopic optical potential of a nucleon above the fermi sea based on Walecka's model is used to systematically analyze the proton elastic scattering from nuclei at energies below 300MeV.It is shown that the experimental data of differential cross sections,analyzing powers and spin rotation functions are reproduced satisfactorily except for large angles.This simple model might be used in the nuclear transport theory and heavy ion collision to take account of both nuclear medium and relativistic effects.     

质子与~(52)Cr反应数据理论计算与分析

在质子与48Ti,51V和52Cr反应的去弹截面、弹性散射角分布实验数据的基础上,获得了一组入射质子能量在150 MeV以下的质子与52Cr反应的光学势参数。应用光学模型、扭曲波玻恩近似理论、核内级联模型、蒸发模型、带宽度涨落修正的Hauser-Feshbach理论以及激子模型(含改进的Iwamoto–Harada模型)计算得到了质子与52Cr反应的所有截面、角分布、能谱和双微分截面。对理论计算结果与实验数据以及TENDL中的数据进行了比较分析,结果显示,理论计算结果与实验数据符合较好,且反应道截面优于TENDL的结果。     

A Semiclassical Theory of Multistep Nuclear Reaction Processes

The master equation theory of precompound and compound nuclear reactions has been generalized with inclusion of the conservation of angular momentum and parity. This improved semiclassical theory has been extended for application as an evaluation tool of the calculations of nucleon induced reaction cross sections and double differential cross sections. For structural materials at incident neutron energies below 20 MeV, it is demonstrated that the constructed model contains the Hauser-Feshbach, Weisskopf-Ewing as well as the exciton models as limiting cases. The unified treatment ofpre-equilibrium processes includes a number of interesting features, such as the exciton state densities with the exact Pauli exclusion correction which are renormalized to the back-shifted Fermi-gas formula, the introduction of formation factors of composite particle in calculations of pick-up type composite particle emission and the double differential cross sections (DDCS) for all kinds of particles in terms of the leading particle model.     

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.     

113Cd在0.01～20MeV能区中子诱导核反应的光学模型计算

基于核反应的光学模型和相应的实验数据,采用计算机自动调节势参数的方法,得到了中子入射¹¹³Cd能量为0.01～20Mev区间的一组光学势参数,并对实验数据少的核的光学参数调节方法进行了探讨。根据该参数对¹¹³Cd计算了中子诱导核反应弹性散射角分布和各种截面。理论计算值与实验数据符合很好。     

Angular distribution calculation based on the exciton model taking account of the influence of the Fermi motion and the Pauli principle

An exact closed form solution to the time-integrated master equation of the exciton model is applied to the calculation of the angular distribution for both the preequilibrium and equilibrium decays of the neutron-induced reaction. The distribution probability of two-nucleon collision fromΩ toΩ′ based on the Fermi gas model and the influence of the Fermi motion and the Pauli principle on the shape of the angular distribution are studied in detail. We have concluded that the influence of these effects on the shape of the angular distribution is rather significant for reactions in the energy range of several tens of MeV. As an example to compare with the experiments we have calculated the neutron-induced reaction⁹³Nb(n,n′) atE _n =15 MeV. It seems that the most significant improvement of the present approach is the rise of the backward direction of the double differential cross section for the higher energy emitted neutrons.     

Correlations between nucleon transfer and fusion at near-barrier energies for the systems32S +100,101Ru and33S +90,91,92Zr

Transfer reactions at two near-barrier energies for the systems³²S +^100,101Ru have been studied. Differential cross sections have been extracted for a large variety of channels, with a set-up yielding good mass, nuclear charge and energy resolutions. A steeper decrease of the measured angular distributions at backward angles with respect to the prediction of a semiclassical theory is interpreted as a loss of transfer flux feeding more complex channels and/or fusion. Coupled channels calculations, which use the transfer form factors derived from the experiment, reproduce the fusion cross sections at sub-barrier energies. Multi-nucleon transfer seems to play an essential role for the fusion process, as evidenced by the found strong correlations. A systematic comparison is made with the systems³³S +^90,91,92Zr, which have been the object of a previous experimental study with the same set-up.