Molecular dynamics study of kinetic electron emission induced by slow sodium ions incident on gold surfaces

Electron excitation and emission phenomena, due to Na⁺ ion impact on Au (1 0 0) surfaces, are studied at incident projectile energies below the threshold for kinetic electron emission. The trajectories and velocities of the projectile and the target atoms are simulated with molecular dynamics. This information are used to calculate the energy loss by electronic stopping as a series of discrete events, localized in space and time, that are treated as sources of excitation energy. The diffusion of the energy deposited by the projectile into the solid is converted into electron yield as proposed by Duvenbeck and coworkers [14]. The results show similar trends to available experimental data.     

The formula for the secondary electron yield at high incident electron energy from silver and copper

Based on main physical processes of secondary electron emission from metals, the relation that the multiplication of the real efficient secondary electron yield at high incident electron energy from metals, energy exponent and incident energy of primary electron is equal to constant C was deduced; based on the relation between the real efficient secondary electron yield at high incident electron energy from metals and secondary electron yield, the relation that the multiplication of the secondary electron yield at high incident electron energy, energy exponent and incident energy of primary electron is equal to constant D was deduced. The energy exponent n of primary electron in the energy range 10-100 keV hitting on silver and copper and the constant D are computed with the ESTAR program and experimental results in scanning electron microscope (SEM), respectively, therefore, the formulae for the incident energy dependence of secondary electron yield in the energy range 10-100 keV from gold and aluminum were deduced, the formulae were proved to be true by experimental results in SEM, the results were discussed and a conclusion was drawn that the formulae from silver and copper were successfully deduced.     

Numerical Analysis of Carbon Isotope Separation by Plasma Chemical Reactions in Carbon Monoxide Glow Discharge

The separation of carbon isotopes in CO glow discharge has been studied, in which the formation of stable products enriched in ¹³C is analyzed by the numerical simulation of kinetic model. Vibrational kinetics and vibrationally induced chemical reaction of CO molecules are considered in the kinetic model as well as electron impact reactions and isotope scrambling reactions of isotopically enriched products. The reaction yield and final isotope enrichment of the stable products are derived as a function of mean electron energy. When mean electron energy is 2.1 eV in the case of Maxwellian electron energy distribution and 3.3 eV in the case of Druyvesteyn one, the kinetic model can reproduce experimental isotope enrichment of precipitated carbon atoms in discharge reactor. The calculation suggests optimum mean electron energy of the plasma for isotope separation as 1.0 eV and, in this case, it is expected that ¹³C enrichment coefficient for precipitated carbon atoms is about 10 and its reaction yield is about 0.5%.     

Fast ion collisions with C60 in vapour phase and collective excitation: Comparison with other gaseous targets

The single and double ionization of a free C₆₀ molecule in collisions with fast heavy (F and Si) ions is investigated using a recoil ion time-of-flight mass spectrometer. The projectile charge state (q_p) dependence has also been investigated. A linear q_p-dependence has been explained in terms of a plasmon excitation model. In addition, continuum electron spectroscopy has been used to detect the electron emission from fullerenes. The measured electron angular distribution for the fullerene target is compared with that for a gaseous target at a fixed electron energy. The ratio of forward-to-backward cross section for C₆₀ is quite different from that for Ne.     

Monte Carlo calculations of low-energy electron dose-point-kernels in water using different stopping power approximations

The spatial distribution of absorbed energy in irradiated matter can be conveniently described by dose-point-kernel (DPK) distributions that characterize the average energy deposition around single charged-particle tracks during their slowing down process. In the present work, electron DPKs in liquid water in the energy range from 100 eV to 10 keV are presented based on Monte Carlo simulation of electron transport in the continuous-slowing-down-approximation (csda). Elastic collisions are individually simulated using the screened Rutherford formula, whereas the energy loss from inelastic interactions is determined from stopping power (SP) theory. Along with the standard Bethe SP formula we examine different empirical expressions of general-use which are meant to improve the performance of the Bethe formula at low electron energies. Comparison is also made with a recent Bethe-type parametric expression obtained from a dielectric optical data model of liquid water. Our findings indicate that for electron energies below ∼1 keV the discrepancies between the DPKs calculated by the general-purpose SP formulae become apparent. Moreover, the results obtained by the empirical expressions compare rather poorly with those from the dielectric model over the entire energy range examined.     

SND-S1型液体闪烁体光输出的能量响应

采用不同能量的γ射线标准源及D-D、D-T核反应的单能中子源分别测定了新型液体闪烁体SND-S1的光输出随粒子能量的变化,采用Monte-Carlo程序Penelope模拟计算了¹³⁷Csγ射线的能量分布谱,最大康普顿电子能量的计算值和实验值相差2.7%。将实验结果与文献值作了比较,能量低于3MeV时,符合较好;能量高于6MeV时,略有差别。结果表明,液体闪烁体的光输出与电子能量呈线性正比关系,而与中子能量呈非线性关系。     

Energy transmission by relativistic electron beams

The feasibility of energy transfer over long distances by high-energy electron beams propagating inside an autonomous, flexible vacuum ion duct, is examined. Since the electrons do not lose energy in the ion duct, itself, the energy transfer efficiency is determined by the efficiencies of a linear resonance electron accelerator at the energy source and of an accelerator-recovery system at the delivery site. Unfortunately, existing linear resonance accelerators do not have a sufficiently high efficiency. Thus, ways of raising the efficiency from the present level of 60–70% to the required ≥90% are discussed. If a high efficiency is achieved in the resonance electron accelerator, then it is possible to design high-power, efficient (≥95%) systems for transfer of energy over long distances, with efficiencies that are independent of the transmission distance or the energy flux. Translated from Atomnaya énergiya, Vol. 105, No. 4, pp. 225–232, October, 2008.     

Controlling Energy Flow and Loss in p<Superscript>6</Superscript>Li Fusion Plasma

The proton-Lithium-6 (p⁶Li) fusion reaction is significant because it produces energy through charged particles. By selecting this reaction, the problems of tritium processes and 14 MeV neutron fluxes will be reduced. One of the main concerns for p⁶Li plasma is the control of energy flow and loss that occur in fusion reactor. The calculations of energy balance are essential for investigating the energy flow and loss in p⁶Li plasma. It has a fundamental role for describing the material conditions in this plasma. Energy production must compete with inevitable losses in plasma. The losses perform a principal role in determining the operating temperature of thermonuclear plasma. Some losses of energy can be minimized by the suitable selection of designing parameters while some are intrinsic in reactant system. Calculations of energy flow and loss suggest an operating point at 800 keV for p⁶Li plasma. The effect of electron temperature on ion–electron energy transfer and the bremsstrahlung losses is reviewed. It is indicated that the bremsstrahlung radiation losses resulting from large mean electron energies are a serious difficulty for p⁶Li fusion reactor. It would be highly desirable to reduce the electron temperature below their normal equilibrium values. If the ion–electron energy transfer be reduced from the classical value, the electron temperature and thus bremsstrahlung radiation losses would be reduced substantially and as a result the performance of a p⁶Li fusion reactor would be improved significantly. Meanwhile, the bremsstrahlung radiation losses can be minimized with suitable mixture for p⁶Li plasma in a fusion reactor.     

西门子加速器射束特点的Geant4模拟研究

利用Geant4程序包编程构建SIEMENS直线加速器机头结构,模拟产生6MV-X射线的电子打靶及后续粒子输运过程,通过与测量数据对比,在保证模型构建合理的基础上,获取常用射野平面粒子出射信息并分析所得相空间文件,得到光子和电子的平均能量、能谱分布、粒子注量分布、能量注量分布及角分布等信息。结果表明：出射光子平均能量要高于电子;光子和电子能谱呈连续分布;出射电子粒子注量与光子相差两个数量级以上;射野内光子的粒子注量和能量注量分布较均匀,电子则波动很大;射野外光子粒子注量和能量注量均迅速下降,电子的变化趋势不太明显;出射光子角分布主要集中在与中心轴成10°范围内,电子角分布范围则较大。     

Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide’s measurements of the Cu⁻ population sputtered from a clean Cu-surface, at emission energies below 100 eV.     

Energy-spread measurement of triple-pulse electron beams based on the magnetic dispersion principle

The method of energy dispersion in magnetic field is used to analyze the energy spread of the triple-pulse electron beam generated by the Dragon-II linear induction accelerator.A sector magnet is applied for energy analysis of the electron beam,with a bending radius of 300 mm and a deflection angle of 90°.For each pulse,the time-resolved and integral images of the electron position at the output port of beam-bending line are recorded by a streak camera and a CCD camera,respectively.Experimental results demonstrate an energy spread of less than ±2.0%for the electron pulses.The cavity voltage waveforms obtained by different detectors are also analyzed for comparison.     

Post-collision interaction after electron impact measured by (e,2e) coincidence technique

Auger electron line-shapes after electron impact inner-shell ionization of Argon at 350 eV primary electron energy have been studied by coincidence spectrometry. Emitted Auger electrons are detected in coincidence with the scattered electrons and the energy of the very slow PCI inducer ejected electron was calculated from energy conservation. The background, caused by outer-shell electrons was measured and then removed from the coincident spectrum. The effect of PCI is studied on the background-free diagram Auger spectrum. A systematic deviation was found from the line-shape given by the semi-classical approach.     

Monte Carlo calculation of electron Rutherford backscattering spectra and high-energy reflection electron energy loss spectra

The electron Rutherford backscattering spectra and high-energy reflection electron energy loss spectra have been calculated by a Monte Carlo method for bulk solids and overlayer/substrate systems. The simulation model is mainly based on the use of Mott cross section for elastic scattering and the use of Penn’s dielectric functional approach to the electron inelastic scattering inside the solid. Moreover, it has further considered the recoil energy loss of energetic electrons and the thermal vibration of atoms with an isotropic distribution of the velocity direction. The calculated energy loss spectra for Al/Pt and Al/Mo agree with the experimental spectra quite well. The signals owing to different kinds of atoms can be separated by taking the scatter for the maximum-scattering angle event along an electron trajectory as the Rutherford backscattering atom, enabling a theoretical estimation of the peak intensity ratio. Furthermore, the simulation has also indicated that the multiple scattering is the dominant process to the quasi-elastic and energy loss of the electrons and is responsible mainly to the difference on the peak intensity between the linear model and experiment.     

螺线管磁场旋转束流法测量Mini-LIA电子束能量

介绍了螺线管磁场旋转束流法的能量测量原理和在Mini-uA上开展的单脉冲电子束能量测量实验.通过测量束流在螺线管磁场中的旋转角度,得到Mini-LIA出口处电子束能量,与加速器总的加速电压符合得很好.分析了旋束法的测量误差和应用特点,给出了旋束法测量的适用范围.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

Optical and structural properties of ZnO thin films; effects of high energy electron irradiation and annealing

High energy electron irradiation (HEEI) effects on the as-grown and annealed ZnO thin films grown by electrochemical deposition were investigated. Both samples were exposed to the sequential electron irradiations of 6, 12 and 15 MeV energies at a fluence of 1 × 10¹² e⁻/cm². The results of X-ray diffraction suggest that a highly strong crystallographic structure can be produced by annealing process. Photoluminescence (PL) studies show that the EI produces violet emission which results from the zinc interstitial. Recombination lifetime (RL) values of the both films reveal that the high quality crystals are obtained. The decreasing trends of RL values with increasing electron energy have been explained by the formation of crystal defects due to the HEEI.     

Threshold energy surface and frenkel pair resistivity for Cu

In-situ electrical resistivity damage-rate measurements in the high voltage electron microscope have been used to study electron irradiation induced defect production in copper single crystals at $\text{T < 10 K}$. Analysis of the directional and energy dependence yields a threshold energy surface that is significantly different from those of previous investigations: two pockets of low threshold energy centered at 〈100〉 and 〈110〉 surrounded by regions of much higher threshold energy. The corresponding damage function exhibits a plateau at 0.6 Frenkel pairs. The present results imply a Frenkel pair resistivity for Cu of (2.75 ^+0.6_?0.2) × 10^?4 Ω cm.     

电子能量损失谱仪用中高能电子枪的研制

研制了一台电子能量损失谱仪用的中高能电子枪。其产生的电子与原子、分子发生碰撞,通过谱仪收集、分析散射电子的动量和能量,可以获得靶的电子结构和碰撞动力学信息。该电子枪结构简单,由热阴极、栅极、阳极、聚焦极和偏转板组成;电子能量可调范围大(1-3 keV),操作简单。为了获得最优的束流条件,利用SIMION电子光学软件模拟了电子发射源大小和初始发散角对靶点处的束斑大小和束流发散角的影响。在电子能量为1.5 keV条件下,实验检验给出在离电子枪出口27 mm处可获得束径约为0.95 mm、束流发散角约0.93°和束流强度6.27mA的电子束,满足电子能量损失谱仪的使用要求。