Ar16+与Zr作用产生的X射线谱

本工作对超导离子源(SECRAL)上的10～20 kV/q Ar16 和Ar17 入射到金属Zr表面进行实验研究.实验结果表明,高电荷态Ar16 在金属表面存在着多电子激发过程.Ar空心原子的K层发射X射线强度随入射离子的动能减少,靶原子Zr的L壳层发射X射线强度随入射离子动能的增加而增强.Ar17 单离子的Kα-X射线产额比Ar16 单离子的Kα-X射线产额大5个数量级.     

含表面层的厚靶D-T中子产额计算

本文给出一种含表面层氚钛靶D-T反应中子产额的计算方法,并开发了相应的数值计算程序。以氧化层为TiO2为例,定量计算了D核在TiO2中的深度分布统计及透过率、200keV的D核穿过不同厚度TiO2的能量分布函数、D核入射含不同厚度氧化层TiT1.0的D-T中子产额和不同能量D核入射固定厚度氧化层TiT1.0的D-T中子产额。结果显示,中子产额随氧化层厚度的增加而减小,对于200keV的D核入射到含TiO2氧化层的TiT1.0厚靶,当TiO2厚度为0.1μm时,损失约10%的中子产额,厚度为0.2μm时,损失约20%的中子产额。本方法可推广到其他类型表面层(如污染层、保护层等)的中子产额计算,可用于中子发生器用靶的结构设计和中子产额评估。     

低能高电荷态氮离子与Cu表面相互作用致K-X射线发射的研究

利用中国科学院近代物理研究所的电子回旋共振离子源引出的高电荷态氮离子(N^q+,q=3,5,6),在4.5～120 keV动能范围内系统测量了N^q+(q=3,5,6)与Cu表面碰撞产生的K-X射线能谱;基于原子壳层电离理论计算了K-X射线产额值及其电离截面值。结果表明:N^q+(q=3,5)离子进入靶表面后与Cu原子发生紧密碰撞,在直接库伦电离的作用下发射K-X射线,并且随着入射离子动能的增加,K-X射线产额和K壳层电离截面均单调增大;对于N⁶⁺离子,K-X射线产额与其入射动能没有明显的依赖关系,且在Cu表面形成的"空心原子"级联退激产生的K-X射线产额约占其总K-X射线产额的97%。另外,N⁵⁺离子入射时的K-X射线产额明显高于N³⁺离子入射时的产额。本工作对离子与表面相互作用中形成的"空心原子"的退激过程具有重要的参考价值,并为理解"空心原子"的退激过程和彗星X射线机理的研究提供了基础数据。     

氘离子轰击钼靶的二次电子发射系数测量

设计了一套二次电子测量装置,采用间接测量法,利用中子发生器产生的氘离子束,对氘离子束轰击下钼靶的二次电子发射系数进行了实验研究,获得了钼的二次电子发射系数随不同入射氘离子能量的变化趋势,氘离子束能量为170 keV时的二次电子发射系数最大,约为2.33.     

利用MCNP对阵列电离室电极片电子发射产额的影响研究

本文通过对7种材质(C、Al、Fe、Cu、Ag、W、Pb)电极片在1.25 Me V入射光子束作用下的电子发射产额进行模拟计算,研究了厚度和材质对阵列电离室电极片电子发射产额的影响关系。研究结果表明:阵列电离室电极片在辐射光子作用下的电子发射产额在电极片厚度较厚时(特别是大于0.01 mm时),随着电极片厚度或电子密度的增大而明显减小。另外,电极片长度的选取需要结合电极的材质适当考虑。     

不同带电情况下介质材料二次电子发射特性研究

空间材料二次电子发射特性是决定航天器表面带电速率和充电平衡电位水平的重要参数。本文利用1~5keV的脉冲电子束开展了聚酰亚胺(kapton)、玻璃盖片和光学太阳反射镜(OSR)材料的二次电子发射系数(δ)测试,并完成了介质材料表面不同充电情况下的二次电子发射特性研究。研究结果表明,在入射电子能量为1~5keV范围内材料二次电子发射系数随入射电子能量上升而下降,同时当二次电子发射系数大于1时,材料表面将累积正电荷,二次电子发射系数下降,当二次电子发射系数小于1时,材料表面将累积负电荷,二次电子发射系数将增加。     

沟道效应及其应用

沟道效应,亦称方向效应。在非晶体中,原子排列是无序的,所以具有一定能量的离子沿不同方向射入靶时,它的背向散射产额各向同性。但在单晶靶中,由于原子是有规则地按晶格点阵排列,从晶轴方向看,密集地排列着一列列原子列,且具有一定的对称性,所以当具有一定能量的离子从不同方向入射到单晶靶上时,由于入射离子与原子列上的原子作用情况不同,其背向散射产额不再是各向同性的了。当离子沿着晶轴方向入射时,只要离子入射方向与晶轴偏离的角度ψ足够小,离子就会在原子列对入射离子形成的屏蔽鞘里近似于成连续曲线前进,而不被散射出来,最后被阻止在晶体中。当上面的ψ角增大到某一角度ψ_c时,入射离子的背散射产额就会突然增加,如图1所示。这种入射离子在单晶靶上的背散射产额随入射角度而变化的现象,称沟道效应。原子列形成的通道称沟道。ψ_c称为临界角。     

氮化镓材料中质子输运过程的模拟

对入射能量为50keV~3MeV的质子在氮化镓(GaN)材料中的输运过程进行了Monte-Carlo模拟,得到垂直入射质子在GaN材料中的输运情况、质子与晶格原子作用产生的空位分布特征。模拟结果表明：在厚度固定（3.5μm）的GaN材料中,当入射能量小于300keV时,质子束横向扩展明显,入射离子全部被阻止在材料中;随着入射能量增大,质子束横向扩展减弱,离子穿透几率增大,当能量大于500keV时,入射离子几乎全部从材料中穿出。     

能量从10 keV到300 MeV的电子和光子在物质中若干物理参数计算程序

本工作提供一组计算程序,可计算电子的能量损失(碰撞损失、轫致辐射损失、阻止本领)及射程、电子轫致辐射光子能谱及轫致辐射光子数目分布、电子和光子的反应几率(电子轫致辐射几率、电子同电子碰撞几率;光子康普顿散射几率、核场中对生成几率)。物质组成可分为单一元素、化合物或混合物。能量范围从10 keV到300 MeV分为75个间隔。以数据表的形式给出上述诸量。结果满意。     

固体衬底上超薄膜表面的低能电子背散射系数计算研究

应用单次碰撞直接Monte Carlo方法模拟了keV级低能电子在超薄膜/固体衬底结构材料中的输运过程,计算研究了不同能量低能电子在几十nm厚超薄膜表面的背散射系数。给出了几种常用超薄膜/固体衬底结构的电子背散射系数r随薄膜厚度D的变化曲线,及r-D曲线线性段背散射常数C随电子入射能量的变化关系。结果表明,在低能电子入射下,r-D曲线初始段存在单一的线性区,背散射常数C随入射电子能量减小而增加,并最终达到一极大值,该规律可作为薄膜厚度测量中选择电子能量的重要参考依据。     

Comparison of secondary electron emission in helium ion microscope with gallium ion and electron microscopes

To evaluate secondary electron (SE) image characteristics in helium ion microscope, Si surfaces with a rod and step structures is scanned by 30 keV He and Ga ion beams and 1 keV electron beam. The topographic sensitivity of He ions is in principle higher than that for scanning electron microscope (SEM) because of the stronger dependency of SE yield versus incident angle for He ions. As shrinking to sub nm patterns, the pseudo-images constructed from line profile of SE intensity by the electron beam lose their sharpness, however, the images for the He and Ga ion beams keep clearness due to darkening the bottom corners of the pattern. Here, the sputter erosion for Ga ions must be considered. Furthermore, trajectories of emitted SEs are simulated for a rectangular Al surface scanned by the beams to study voltage contrast, where positive and negative voltages are applied to the small area of the sample. Both less high energy component in the energy distribution of SEs and dominant contribution of direct SE excitation by a projectile He ion keep a high voltage contrast down to a sub nm sized area positively biased against the zero-potential surroundings.     

Investigation of the projectile atomic number influence to the total sputtering yield in the keV energy region

The non-monotonous dependence of the total sputtering yield on the projectile atomic number, which is unexpected in the frame of the Sigmund linear cascade theory, is investigated using Monte Carlo simulations (program SRIM 2003). This effect is studied on the example of aluminum sputtering by six different projectiles (N, Ne, Al, Ar, Kr and Xe) at normal incidence. The incident projectile energy is 2 keV. Investigation consists of the analyses of ASI distributions of sputtered atoms as well as of nuclear energy loss depth distributions of projectiles with fixed number of ejected atoms. The results show that the non-monotonous behavior of Y(Z) is due to the ability of projectiles somewhat lighter than aluminum to efficiently eject large number of atoms by formation of collision cascades in the subsurface region which are directed towards the surface. On the other hand, ions that are heavier or significantly lighter than aluminum cannot form this type of cascades - the heavier ions cannot transfer a lot of energy to recoils in a primary knock-on collision that will move towards the surface, while significantly lighter ions transfer the energy too deep into the target.     

A Monte Carlo computer code for evaluating energy loss of 10 keV to 10 MeV ions in amorphous silicon materials

The basic concepts of a computer simulation code for determining the energy loss of ions in the 10 keV to 10 MeV energy range in amorphous silicon materials were presented and discussed. Data obtained were found in good agreement with those obtained by using a SRIM programme. Electronic and nuclear energy losses were evaluated. Variation of the energy loss as a function of the incident ion energy were studied. This new computer code is a good tool for evaluating stopping powers of various materials for light and heavy ions.     

Evaluation of influence of surface barrier in solid on ion reflection by bipartition model of ion transport

The influence of surface barrier of solid upon ion reflection was studied in a few papers of other authors by using the Monte-Carlo simulation. Based on the bipartition model of ion transport, a new analytical theory has been developed instead of the MC simulation, due to important implication of the effect for fusion research. In the present paper we have calculated the number reflection coefficients of H , D , He and T normally incident on C, Al and Cu for ion energy from several eV to one hundred keV respectively. Our computational results accorded with the MC simulation. The results have shown that the effect of surface barrier on ion reflection becomes evident when the energy of incident ions is lower than one keV. In particular, for the ion energy from several eV to one hundred eV, the discrepancies of number reflection coefficients can increase up to 0.1-0.3, showing this influence to be very important.     

Effect of the laser focus position on characteristics of X-ray and ion emission from gold plasmas generated by a sub-nanosecond laser

X-ray and ion emission from gold plasma produced by a sub-nanosecond Nd:glass laser has been studies as a function of distance of the target from the best focus position. Thermal ion (kinetic energy <19 keV) signals and soft X-ray flux (photon energy >0.7 keV) measurements decrease as the target is moved closer to the best focus position in spite of an increase in laser intensity. We observe simultaneously a strong correlation between the onset of this drop in the flux of soft X-ray and the growth of harder X-ray (photon energy 3–5 keV), alongside a growth in fast ion (energy >67 keV) numbers. This is indicative of the onset of non-linear processes at the higher irradiances (～10¹⁴ W/cm²) associated with the best focus position. Our results show that when using laser plasmas as X-ray or ion sources, X-ray and ion emission in a desired spectral range can be optimized by adjusting the focusing on the target.     

Stopping power measurements of 20–180 keV 1H and 4He in indium phosphide using thick target backscattering

The stopping power of InP for 20–180 keV ¹H and ⁴He was determined by measuring the absolute backscattering yield of thick InP targets. Several nonideal aspects of surface barrier detectors were observed which should be accounted for when making low to medium energy backscattering yield measurements. These are: non-Gaussian detector resolution, nonlinear detector energy calibration, and ion reflection in the detector Au window. The present ¹H in InP stopping cross section measurements are in good agreement with those recently obtained by Khodyrev et al. using a relative, bulk yield, single spectrum approach. The ⁴He in InP stopping cross section measurements were found to be velocity proportional below 30 keV/amu. The estimated standard error of the measurements is ±7% at 20 keV and ± 4% at 180 keV.     

正电子致电离截面实验中束流强度的在线测量

本文采用HPGe探测器实时收集了正电子碰撞厚Ti靶伴随产生的湮灭光子,并结合HPGe探测器对放置在碰撞点处²²Na标准源产生的511 keV湮灭光子的探测效率刻度值,得到了8~9.5 keV正电子引起Ti原子内壳层电离截面实验中正电子束流强度的实时测量结果。结果表明,在实验测量的38 h内,基于²²Na标准源慢正电子束流装置产生的正电子束流强度不稳定,随时间的变化有着不同程度的衰减,且存在小幅度波动现象。因此,低能正电子致原子内壳层电离截面实验中应采用在线法获取慢正电子束流强度。     

Total bremsstrahlung spectral photon distributions in metallic targets in the photon energy range of 5-10 keV by Tl beta particles

Total bremsstrahlung spectral photon distributions produced by beta particles of the ²⁰⁴Tl beta emitter in thick targets of Al, Ti, Sn and Pb targets were evaluated at photon energies from 5 keV to 10 keV. Experimental measurements were compared with the theoretical total bremsstrahlung spectral photon distributions obtained from Elwert corrected (non-relativistic) Bethe-Heitler theory and modified Elwert factor (relativistic) Bethe-Heitler theories for ordinary bremsstrahlung, and the modified Elwert factor (relativistic) Bethe-Heitler theory which includes polarization bremsstrahlung in the stripped atom approximation. The experimental results show better agreement with the modified Elwert factor (relativistic) Bethe-Heitler theory which includes the contribution of polarization bremsstrahlung. The contributions of polarization bremsstrahlung decrease with increased photon energy, particularly for medium and high Z elements. Hence its contribution cannot be neglected while studying the total bremsstrahlung spectral photon distributions in thick targets, produced by continuous beta particles in the studied energy region.     

Comparison of experimental and simulated low energy alkali ion scattering

Computer simulations performed using the MARLOWE code are compared with experimental results obtained for 1 keV Li⁺ ion scattering from W(001). Trajectory analysis using distributions of impact points, deflections and maximum penetration depths allow characterization of the contributing scattering mechanisms. Computer simulation is found to reproduce many of the observed features in the energy distributions and in the incident angle dependence, but there are also important differences. In agreement with experiment, a sharp, surface sensitive binary collision peak is found at certain angles but at others this peak may broaden, shift to lower energy and grow in intensity due to subsurface scattering mechanisms. Simulations overestimate the relative scattering intensity near critical edges and the importance of focused scattering from second layer atoms.     

Forward and Backward Photoemission Yields from Metals at Various X-Ray Angles of Incidence

X-ray generated photoemission from thin metal foils backed by graphite was measured with radiation incident from the front and back sides at several angles. Irradiation was provided by a 100-kV x-ray tube with three different filters to harden the spectrum. The total 2? photoelectron emission current from a surface was measured; a biased grid retarded the low-energy secondary electrons, which added only 10-30% to the current at zero grid bias. Investigated metals were: Mg, Al, Ti, Fe, Cu, Ag, Ta, Au, and Pb; also the total emission from just the graphite support was measured. The front-to-back ratio of emission currents at normal incidence ranged from about 1.9 for Al and Mg down to about 1.1 for Ta. The photoelectron yield was found to be Ge ?a Se electrons/photon, where ?a and Se are the energy-dependent photon absorption cross section and computed electron mean path length in the emitter, and Ge is a constant assumed independent of photon energy in the range studied (but does depend on radiation angle of incidence). For the photon energy range of 20-70 keV, the measured emission current densities corresponded to the following average values for ?e: 0.37 ± 0.06 for C, 0.30 ± 0.03 for Al, 0.21 ± 0.02 for Cu and Ag, and 0.18 ± 0.02 for Ta.