飞秒激光-物质相互作用产生K_α X射线(英文)北大核心CSCD

用无色散X射线谱仪分别在靶前后测量了飞秒激光辐照铜箔产生的KαX射线,获得了能量转换效率。入射激光脉冲宽度33 fs,能量在50 mJ^5 J,强度1017~1019W/cm2。靶后发射的KαX射线强度随入射激光能量的增加而增加,其单色性较靶前好。采用100μm厚靶,其能量转换率为2.2×10-5。     

超短超强激光产生正电子的蒙特卡罗模拟

通过理论分析,建立了超短超强激光与固体靶作用产生正电子的蒙特卡罗模拟模型及Geant4模拟程序。模拟研究了靶材料、靶厚度及超热电子温度等对正电子产额的影响,结果表明:对铝、铜、锡、钽、金、铅6种靶材料,金靶的正电子产额最高,是优秀的正电子产生靶;不同超热电子温度下存在不同的最佳靶厚度,在最佳靶厚度以下,正电子产额随靶厚度增长而增大,靶厚度取3 mm较为合适;超热电子温度越高,正电子产额也越高,提高激光强度是增加正电子产额的有效途径。模拟研究给出了正电子角分布及其能谱,结果显示,正电子发射明显前倾,从大于90方向范围发射的正电子数量极少,且超热电子温度越高前倾特点越明显,能量呈类麦克斯韦分布,靶背法线方向出射的正电子的温度随超热电子温度升高而升高。     

慢正电子的应用

表面分析慢正电子近年来大量应用于表面分析,尤其研究接近表面几十到几千埃层内结构状态。当慢正电子入射到金属,受到声子和等离激元(Plasmon)散射很快损失能量,其中一些慢正电子同金属晶体中的电子发生湮没,大部分慢正电子扩散到表面不断热化,接近表面遇到表面偶极层.     

飞秒激光-物质相互作用产生Kα X射线

 用无色散X射线谱仪分别在靶前后测量了飞秒激光辐照铜箔产生的Kα X射线,获得了能量转换效率。入射激光脉冲宽度33 fs,能量在50 mJ～5 J,强度10₁₇～10₁₉ W/cm₂。靶后发射的Kα X射线强度随入射激光能量的增加而增加,其单色性较靶前好。采用100 μm厚靶,其能量转换率为2.2×10_-5。     

A1/GaAs界面微结构的慢正电子束研究

基于正电子扩散方程,对于线形全吸收型等特殊的界面模型,求得了正电子湮没辐射多普勒展宽S参数与单能慢正电子入射能量E之间函数关系S(E)的解析表达式.测量不同膜厚、不同退火条件下A1/GaAs的S(E),用所得解析式拟合实验数据,发现A1/GaAs界面系统与线形全吸收型界面模型符合得很好.通过拟合,得出了界面S参数S₁及其与膜厚和退火温度的关系,由此对界面微结构及其动力学特性作了讨论.     

偏滤器等离子体中杂质对钨壁材料的侵蚀模拟研究

偏滤器是托卡马克中与等离子体直接接触的部件,为了保证装置的寿命,需要尽可能地减小等离子体对偏滤器靶板的侵蚀.本文用粒子模拟的方法研究了不同等离子体温度情况下碳和铍两种杂质离子对钨偏滤器侵蚀速率的影响.模拟首先得到稳定的鞘层结构、入射到靶板的离子流和能流密度,并通过统计获得了入射离子的能量和角度分布,最终根据这些物理参量,采用经验公式计算出钨靶板的侵蚀速率.研究表明,在等离子体温度不太高的情况下,钨靶板的热侵蚀几乎不起作用,而由于杂质离子对钨的物理溅射阈值较低,并且会通过鞘层加速获得能量,因此其对钨壁材料的物理溅射是导致靶板侵蚀的主要原因,另外靶板材料的侵蚀速率随着等离子体温度升高以及杂质含量增大而急剧增大.     

主脉冲参数和入射条件变化对等离子体状态的影响

用一维非平衡辐射流体力学程序，计算和分析了主脉冲激光斜入射与预脉冲垂直入射耦合作用锗靶介质的等离子体状态和激光增益区。研究表明，主脉冲以４０°斜入射与其垂直入射相比，在相同靶面功率密度下，电子、离子温度、等离子体烧蚀深度下降约１０％。在相同的靶面总能量下，４０°斜入射电子温度下降约１５％～２０％。采用主激光斜入射时间延迟技术，这种下降差别还会更小，能量吸收效率会更高，激光增益区更大。     

主脉冲参数和入射条件变化对等离子体状态的影响

 用一维非平衡辐射流体力学程序,计算和分析了主脉冲激光斜入射与预脉冲垂直入射耦合作用锗靶介质的等离子体状态和激光增益区。研究表明,主脉冲以40°斜入射与其垂直入射相比,在相同靶面功率密度下,电子、离子温度、等离子体烧蚀深度下降约10%。在相同的靶面总能量下,40°斜入射电子温度下降约15%～20%。采用主激光斜入射时间延迟技术,这种下降差别还会更小,能量吸收效率会更高,激光增益区更大。     

北京慢正电子强束流运行性能测试

慢正电子强束流采用高能脉冲电子束流轰击金属钽靶, 以产生正负电子对的方式提供正电子, 作为慢正电子束流方法学研究和薄膜材料缺陷研究的束流基础. 本文是在该装置实现运行后, 对慢正电子束流的强度、能散、形貌等运行性能的测试工作的介绍, 以及慢正电子湮没多普勒测量系统的调试和标准样品的测量结果.     

正电子冲击下氦电离三重微分截面的理论计算

我们发展了一种正电子碰撞原子电离的畸变波Born近似方法， 在这个方法中，正负电子偶素通道通过一个ab initio的光学势附加到入射粒子和靶的相互作用势上，且通道对电离作用被第一次被考虑在正电子碰撞原子电离的过程中. 应用这个方法计算了在50 eV入射能量范围氦的电离的三重微分截面，计算结果和实验数据很好的符合.     

Slow positrons and thermal Ps production using thin foils

A one-dimensional diffusion model for positrons implanted in Cu and Al single crystal thin foils was solved. The fraction of thermalized positrons reaching the surface was obtained for various film thicknesses as a function of the incident positron energy in a transmission-mode geometry. The results indicate that for foil thicknesses of the order of the diffusion length (1000 Å) the reemission fraction is roughly half the fraction of a semi-infinite crystal. For thicker foils the annihilation of the positrons in the bulk and smearing effects due to a higher implantation energy effectively reduce the surface reemission. In this paper it is shown that thin foils can also be used to efficiently produce low-energy (from thermal to a few eV) Ps beams, physically separated from the primary positron beam.     

Simulation study of indirect positron generation by an ultra-short laser

Positron generation by an ultra-short ultra-intense laser in an indirect manner has been studied with two-dimensional particle-in-cell (PIC) and Monte Carlo (MC) simulations. In this generation scheme, positrons are produced with energetic electrons accelerated by an ultra-shot laser pulse propagating through an underdense plasma. The dependence of the positron beam properties on the plasma length and secondary target (converter) thickness was investigated in detail. The simulation results reveal that the positron yield is strongly correlated with the total energy of laser-accelerated electrons; both the temperature and divergence of the positron beam are sensitive to the plasma length; and the positron beam has a pulse duration comparable to the incident electron beam. In addition, it is indicated that even with the optimal converter thickness, only a small fraction (11.4%) of positrons can escape out and most of the detected positrons originate from the back edge of the converter.     

Low-temperature measurements of the fraction of re-emitted positronium from a Cu(111)+S surface

A positron beam has been utilized to measure the positronium (Ps) fraction re-emitted from a Cu(111)+S sample from 40 K to 350 K for incident energies ranging from 0.5 to 5 keV. Our results at 525 eV incident energy are compared with two recent theories from 40 K to 850 K. The Ps fraction shows only a slight positive temperature dependence below room temperature and we conclude that the positron trapping rate and probability of Ps formation at the surface are largely independent of sample temperature for Cu(111)+S. In addition we present evidence that positrons in the bulk material are not localized in shallow traps at low temperatures for well-annealed high-purity single crystals of Cu and Al.     

不同入射角度下强流脉冲电子束能量沉积剖面和束流传输系数模拟计算

 强流脉冲电子束在材料中的能量沉积剖面、能量沉积系数和束流传输系数受其入射角的影响很大，理论计算了0.5~2.0MeV的电子束以不同的入射角在Al材料中的能量沉积剖面和能量沉积系数，并且还计算了0.4~1.4MeV电子束以不同入射角穿透不同厚度C靶的束流传输系数。计算结果表明，随着入射角的增大，靶材表面层单位质量中沉积的能量增大，电子在靶材料中穿透深度减小，能量沉积系数减小，相应的束流传输系数也减小；能量为0.5~2.0MeV的电子束当入射角在60°~70°时在材料表面层单位质量中沉积的能量较大。     

Positron impact ionisation of He ion

Triple differential cross-sections (TDCS) of a hydrogenic (He⁺) ion has been studied by positron impact using coplaner geometry for both symmetric and asymmetric kinematics in the intermediate and medium high incident energy region. TDCS has also been studied of He⁺ ion by electron impact for symmetric kinematics taking account of the electron exchange effect. The final state wavefunction is chosen as the correlated 3-body Coulomb wavefunction satisfying the exact asymptotic boundary condition. The long range Coulomb interaction in the initial channel between the ionic target and the projectile has also been taken into account properly. For positron impact, the collision is found to be almost recoilless at lower incident energies, in contrast to the strong recoil peak noted in the case of electron impact ionisation. For electron impact, the exchange effect is found to be significantly high for equal energy sharing in the final channel. Received 10 July 1999 and Received in final form 7 December 1999     

Analysis of positron diffusion data

In this paper we present and discuss experimental methods to determine the positron diffusion coefficient from slow positron beam measurements. We also evaluate the use of the annihilation line Doppler-broadening technique in positron diffusion measurements, as compared on the more commonly used method of positronium fraction. The effects of incomplete positron thermalization and uncertainties of the positron implantation profile at low-positron incident energies to the measured data are discussed. We apply the presented methods to the model case of A1(110) system in the temperature range from 20 to 500 K. This data shows that Doppler-broadening and positronium fraction measurements give consistent results for the positron diffusion coefficient in A1(110), where D₊(300 K) = 1.7(2) cm²/s with the temperature dependence D₊ T ^–0.62(3).     

Intense source of slow positrons from pulsed electron accelerators

A pulsed LINAC is used for pair production in a tantalum target of 2.5 radiation lengths in an energy range from 80 to 260 MeV. Several well-annealed tungsten vanes are placed immediately behind the target and thermalize a small fraction of the fast positrons. The slow positrons are extracted from the target region and magnetically guided over a distance of 17 m to the detector at the end of an S-shaped solenoid. Two Nal detectors with well-known detection efficiency are used to register the 511 keV annihilation-rays. To reduce pile-up effects 50 mm of Pb were placed in front of the detectors. At an average electron current of 1 A we could detect about 10⁷ slow positrons per second. The positron yield is proportional to the electron current, and shows an increase with the electron energy for our target. The positron energy distribution has a FWHM of 1.8 eV.     

Elastic scattering of positrons by xenon atoms

Summary Partial and total cross-sections of positron scattering by Xe atoms are calculated at twenty values of incident energy ranging from 0.01 Ry to 4.00 Ry. The calculations are carried out via model potential (describing the positron target interaction) and a polarization potential introduced previously by Lane and Geltmann (1967). The comparison between our results and those obtained by other authors shows significant agreement and supports our simple model scattering process. The author of this paper has agreed to not receive the proofs for correction.     

Experiments at the Giessen source TEPOS with low energy positrons

The TEPOS facility at the Giessen LINAC delivers intense positron beams in the energy range between some eV and 6 keV; with postacceleration up to 80 keV. Results for remoderation and positron storage will be discussed. Further the energy-loss of positrons in thin aluminium foils at incident energies of 6–20 keV was measured. Cross sections for K- and L-shell ionization of thin silver and gold targets by positron and electron impact were determined at projectile energies of 30–70 keV. The experimental results are presented in detail; they are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and take into account the deceleration or acceleration of the incident projectile in the nuclear field of the target atom.     

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.