Re-evaluation of neutron displacement cross sections for silicon carbide by a Monte Carlo approach

Neutron displacement cross sections for SiC are re-evaluated by a Monte Carlo approach, with damage energies of primary recoils calculated by the stopping and range of ions in matter (SRIM) code. The validity of the Monte Carlo model is examined by the case of iron, and the results show good agreement with the reference values. Neutron displacement cross sections for SiC at energies up to 100 MeV are calculated, and averaged over the neutron spectra of a fusion DEMO reactor, the high flux test module of the International Fusion Materials Irradiation Facility, and typical fission test reactors. Gas production is also calculated for those neutron irradiation facilities. Finally, the suitability of the displacement cross sections is discussed. The results on comparison among neutron irradiation of different facilities by the current displacement cross sections are similar to those by results of the previous work. Moreover, since neutron displacement cross sections in this study are calculated with damage energies of primary recoils calculated by SRIM, neutron damage evaluated by our displacement cross sections is suitable for correlation with damage by heavy ions calculated by SRIM.     

Cross section database for carbon atoms and ions: Electron-impact ionization, excitation, and charge exchange in collisions with hydrogen atoms

A database has been constructed consisting of the recommended cross sections for electron-impact excitation and ionization of carbon atoms and ions C, C⁺-C⁵⁺, as well as for charge exchange processes between carbon ions C⁺-C⁶⁺ and hydrogen atoms. We have collected a large amount of theoretical and experimental cross section data from the literature, and have critically assessed their accuracy. The recommended cross sections, the best values for use, are expressed in the form of simple analytical functions. These are also presented in graphical form.     

Correlation of Radiation Types with Radiation Effects

The effects produced by different types of nuclear radiation can be compared by making use of the available information on radiation-effects mechanisms. One can separate radiation-effects manifestations into three categories: transient radiation effects, which are due to excited and ionized electrons; displacement radiation effects, which are manifestations of atoms displaced from their normal lattice sites in crystalline solids; and chemical radiation effects, which are due to molecular rearrangements occurring as a second stage to ionizing interactions. The effects produced by different primary radiations can be compared by first identifying the manifestation as belonging to one of these three types. For displacement effects one should then compute the relative cross sections for producing displaced atoms and the average size of the displacement cascade produced by the primary recoil atom. Similarly, for transient and chemical effects one should compute the ionization density produced by the incident radiation. Examples of the pertinent cross sections have been calculated for electrons, gamma rays, protons, and neutrons of various energies.     

核数据处理软件NECP-Atlas中的光子相关数据计算方法研究

NECP-Atlas是西安交通大学自主研制的核数据处理软件,具有丰富的功能,可将评价核数据制作为后续核设计所需的应用核数据库,本文在NECP Atlas中建立了光子相关数据的计算方法,可计算产生中子核反应释放的瞬发光子产生截面、光子与原子的反应截面,裂变产物衰变释放的缓发光子多群产生矩阵,以及光子辐照损伤截面等数据。数值结果显示,如果不考虑缓发光子,钠冷快堆中控制组件、反射层组件的光子功率与参考解的最大偏差可达3258%、2041%,采用NECP Atlas计算的多群缓发光子产生矩阵后两类组件偏差降为093%以下。采用文献结果对Fe的光子辐照损伤截面进行了验证,计算结果与参考解吻合良好。     

Molecular effects in carbon K-shell Auger-electron production by 0.6–2.0 MeV protons and extraction of an atomic cross section

Carbon K-shell Auger-electron production cross sections are reported for 0.6–2.0 MeV protons incident on CH₄ (methane), C₂H₂ (acetylene), C₂H₄ (ethylene), C₂H₆ (ethane), nC₄H₁₀ (normal butane), i-C₄H₁₀ (isobutane), C₆H₆ (b monoxide), and CO₂ (carbon dioxide). A constant-energy mode 45° parallel-plate electrostatic analyzer was used for detection of Auger electrons. The carbon KLL Auger-electron cross sections for all molecules were found to be lower than that found for CH₄ by 9–23%. All carbon KLL Auger-electron data could be brought into agreement when corrected for the chemical shift of the carbon K-shell binding energy in molecules and for intramolecular scattering. KLL Auger-electron production cross sections are compared to first Born and ECPSSR theories and show good agreement with both after the chemical shift of the carbon K-shell binding energy in molecules and the effects of intramolecular scattering are considered.     

A new tool to compare neutron and ion irradiation in materials

The slowing down of neutrons produced in nuclear plants or in spallation sources generates many defects in a material, driving this material far from its equilibrium state. This concentration of defects leads to important structural modifications of solids. To model the impact of radiation in solids, particle accelerators are useful tools. The nature and the energy of ions in accelerators must be chosen to produce similar effects to those occurring in nuclear plants. A new program, DART, based on the binary collision approximation, has been developed to optimize the choice of incident particles. In this program, the anisotropy of the neutron atom interaction is treated with a new formalism. Such a formalism allows us to compute accurate displacement cross sections and recoil spectra for all kinds of particles (ions, neutrons, or electrons). The comparison of these displacement cross sections as well as recoil spectra due to ions, electrons and neutrons permits to define the nature and the energy of ions able to simulate damages in reactors.     

Backscattering of 7Li-ions from oxygen below 6.25 MeV

Differential scattering cross sections for ⁷Li-ion scattering from oxygen between 2.75 and 6.25 MeV at a laboratory detection angle 170° have been determined. Rutherford scattering was observed below 3 MeV. Above 3.50 MeV the cross sections were found to vary from 1.07 to 0.18 of the Rutherford value. No sharp resonances were detected, ⁷Li-induced charged particle thick target reaction yields from carbon were investigated. From the point of view of Li-ion backscattering analysis, these reaction yields will cause significant interference above 5 MeV whenever major amounts of carbon are present in the sample under study.     

Exploring reciprocity as a tool in low-energy electronic stopping

A computer code has been developed to explore reciprocity, i.e. the equivalence of electronic stopping cross sections for the ion-target pair A in B with that for B in A in the regime around and below the Bohr speed. With the aim of establishing reciprocity as a tool for identifying reliable experimental data and interpolating between them, we have studied stopping cross sections involving carbon, aluminium and gold either as projectile ions or as target materials. The case of carbon - where numerous data are available - is used to illustrate in detail various options to establish a body of credible data. Aluminium and gold serve to illustrate the case where these elements mostly are found as target materials in available data.     

Electron-impact cross sections and rate coefficients for excitations of carbon and oxygen ions

Cross sections have been compiled for electron-impact excitation of carbon and oxygen ions (C II–VI and O II–VIII). A selection has been made to recommend “best” values for use. The resulting recommended values are fitted to an analytical formula and the fitting coefficients are given in a table. The cross sections (in the form of collision strengths) and the rate coefficients calculated therefrom are shown graphically. The reliability of the recommended data is roughly estimated.     

Polarisation effects in low-energy positron-molecule scattering

The UK molecular R-matrix method has been adapted to treat positron collisions from polyatomic targets. A simple empirical enhancement factor which corrects for the underestimation of electron-positron polarisation and correlation effects in the calculations performed with the static-plus-polarization model at low scattering energies is presented. Application of this model to positron scattering from carbon dioxide at energies below 8 eV is discussed. Introduction of the enhancement factor improves the integral cross sections significantly and introduces structures in the differential cross sections consistent with other studies. The prospects for a fully ab initio treatment of this problem are discussed.     

Proton elastic scattering cross sections of carbon,nitrogen and silicon for backscattering analysis in the energy range 0.7—2.5 MeV

Differential elastic scattering cross sections of 0.7—2.5 MeV protons for carbon, nitrogen and silicon have been determined at a scattering angle θ = 170°. Results from previous measurements near θ = 170° show large variations, of the order of 10–30% in the nonresonant regions. The purpose of the present investigations is to tabulate cross section data for backscattering analysis at a single scattering angle for many elements. Proton backscattering in the non-Rutherford energy region is more sensitive in the detection of C, N and many other light elements than other ion beam methods when a high background from the matrix is absent. Proton backscattering by using the present scattering cross sections and computer data analysis are outlined. The possible interference from other nuclear reactions is considered.     

Study of the fragmentation of a displacement cascade into subcascades within the Binary Collision Approximation framework

When a material is subjected to irradiation, many primary defects are created at the atomic level by sequences of ballistic collision events to form highly disordered regions defined as displacement cascades. The long term evolution of materials under irradiation is dictated by the number and the spatial distribution of the surviving defects in the displacement cascade. The peculiar power law shape of collision cross sections is responsible for the fractal feature of atomic trajectories set and then the fragmentation of a displacement cascade into smaller subcascades. We present a criterion to describe the fragmentation of a displacement cascade into subcascades and to calculate the volume fraction of subcascades in the overall cascade due to this fragmentation. Such a volume fraction then provides a natural framework to estimate the efficiency of different projectiles to induce phase transformation or amorphisation in solids under irradiation. From this definition of the fragmentation of a displacement cascade, this work gives some new insights to compare different irradiations performed with different particles.     

Development of a hybrid method to improve the sensitivity and uncertainty analysis for homogenized few-group cross sections

In the framework of two-step method of reactor core calculation, few-group homogenized cross sections generated by lattice-physics calculations are key input parameters for the three-dimensional full-core calculation. Conventional method for few-group cross-sections sensitivity and uncertainty (S&U) analysis related to the nuclear data was performed based on the effective self-shielding cross sections instead of the continuous-energy cross sections, which means resonance self-shielding effect (implicit effect) is neglected. Furthermore, the multi-group covariance data is generated from the continuous-energy cross sections. Therefore, in order to perform S&U analysis with respect to the continuous-energy cross sections for both accuracy and consistency, a hybrid method is proposed in this paper. The subgroup-parameter sensitivity-coefficients are calculated based on the direct perturbation (DP) method. The sensitivity-coefficients of the effective self-shielding cross sections and the responses (k_eff and few-group homogenized cross sections) are calculated based on the generalized perturbation theory (GPT). A boiling water reactor (BWR) pin-cell problem under different power conditions is calculated and analyzed. The numerical results reveal that the proposed hybrid method improves the sensitivity-coefficients of eigenvalue and few-group homogenized cross sections. The temperature effects on the sensitivity-coefficients are demonstrated and the uncertainties are analyzed.     

K-shell ionization cross sections of Cl and Lα, Lβ X-ray production cross sections of Ba by 6-30 keV electron impact

The absolute K-shell ionization cross sections of Cl and L_α, L_β X-ray production cross sections of Ba by 6-30 keV electron impact have been measured. The target was prepared by evaporating the thin film of compound BaCl₂ to the thick pure carbon substrate. The effects of multiple scattering of electrons penetrating the target films, electrons reflected from the thick pure carbon substrates and Bremsstrahlung photons produced when incident electrons impacted on the targets are corrected by using Monte Carlo method. For Ba L-shell X-ray characteristic peaks, the spectra were fitted by using spectrum-fitting program ALLFIT to extract more accurately the L_α and L_β peak counts. The experimental results, reported here for the first time in the energy region of 6-30 keV, were compared with some theoretical results developed recently.     

结构材料核的原子位移截面计算

中子辐照引起的材料损伤是裂变反应堆设计的重要考虑因素。对于晶体结构材料,其辐照损伤主要来自晶格原子的位移。结构材料核与中子发生带电粒子反应的截面、原子位移(DPA)截面、KERMA因子是计算辐照损伤的基础。为比较不同程序计算的DPA截面的差异和基于不同评价核数据库的DPA截面的差异,采用核模型计算程序UNF及核数据处理程序NJOY计算了27 Al、48 Ti、90Zr、Cr、Fe、Ni、Cu等结构材料核的DPA截面,将二者计算结果进行了比较分析;比较分析了基于不同评价核数据库的采用NJOY计算的DPA截面;比较分析了NJOY与蒙特卡罗程序计算的DPA截面。结果表明,UNF与NJOY的结果存在一定的差别,不同评价库的结果也是有差别的,蒙特卡罗程序采用不同模型计算时结果也存在一定的差别。     

The 11B(p,α0)8Be nuclear reaction and 11B(p,p)11B backscattering cross sections for analytical purposes

The ¹¹B(p,α₀)⁸Be nuclear reaction and the ¹¹B(p,p)¹¹B backscattering cross sections were measured at a laboratory scattering angle of 165° in the energy range from 1700 to 2700 keV with an absolute accuracy of about 7%. The cross section values were derived by using the simultaneously measured RBS spectra of the protons backscattered from a thin gold layer. The measured cross section values are compared with X-ray photoelectron spectroscopy (XPS) and heavy ion elastic recoil detection analysis (HERDA) results of amorphous hydrogenated boron/carbon (a-B:C:H) layers. The cross section data are presented in graphical and tabular form.     

Incorporation of the Goudsmit–Saunderson electron transport theory in the Geant4 Monte Carlo code

The present work describes the implementation of the multiple scattering formalism for charged particle transport based on the Goudsmit–Saunderson angular distribution and satisfying the Lewis spatial displacement theories. Furthermore, the most realistic total cross sections and transport cross sections for single elastic scattering of electrons and positrons by neutral atoms were carried out using the ELSEPA code. The extended model has been incorporated in the Geant4 Monte Carlo code system (currently used in many fields of application for transport of particles and covering a large range of energies from 0.1 keV to 100 TeV). Additionally, techniques are developed that allow the distributions to be scaled to account for energy loss. This new representation allows on-the-fly sampling of Goudsmit–Saunderson angular distributions in a screened Rutherford approximation suitable for Class II condensed history Monte Carlo codes. The reliability of the model is demonstrated through a comparison of simulation results with experimental data. Good agreement is found for electrons with kinetic energies down to a few keV.     

Cross section adjustment method based on random sampling technique

A cross section adjustment method based on the random sampling technique is proposed. In the proposed method, correlations among cross sections and core parameters are used instead of sensitivity coefficients of cross sections, which are necessary in the conventional method. The correlations are statistically estimated by the random sampling technique. The proposed method is theoretically consistent with the conventional method and provides comparable adjusted cross sections when sufficient number of random sampling is taken into account. The proposed method would be suitable for practical light water reactor (LWR) core analysis since estimation of sensitivity coefficients, which requires considerable computational cost in typical LWR problems, is not necessary. Through a benchmark problem in simple pin-cell geometry, adjusted cross sections by the present and the conventional cross section adjustment method are compared. The adjusted cross sections by the present method well reproduce the conventional ones, thus the feasibility of the present method is confirmed.     

溅射输运理论中的经典散射截面

利用溅射理论中常用的Lindhard标度相互作用势碰撞散射分析了Sigmund单幂散射截面与双幂散射截面之间的区别。计算表明，对于Born-Mayer相互作用势碰撞散射，双幂截面可以给出比单幂截面好得多的描述。特别对于Urbassek Kr-c相互作用势，只有利用双幂截面求解输运方程才能计算出反冲密度函数比较好的近似值。