Effects of irradiation parameters on metal surface erosion by pulsed ion beams

Potential application of high-power ion beams of submicrosecond and microsecond durations with the initial particle energy from 50 to 1000 keV and power density 10⁷–10⁹ W/cm² to ensure metal surface erosion are investigated. Evaporation is treated as a major erosion mechanism and the erosion coefficient is taken as an efficiency indicator. Dependences of the erosion coefficients of several metals on beam parameters obtained via calculations using a technique based on the solution of thermal conductivity equation with phase transitions are presented. The ion species, their initial energy, current pulse duration and power density are used as the beam parameters controlling the result of irradiation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–54, August, 2007.     

Model of metal surface erosion under irradiation by high-power pulsed ion beams

The mechanisms of erosion of metal surfaces under the action of submicrosecond (10⁻⁹−10⁻⁶ s) ion beams in the power density range of P = 10⁶−10⁹ W/cm² with a particle energy of 1–2000 keV are considered. It is shown that the collective processes associated with the radiation heating of the surface are of great importance. A model for the erosion is proposed. In accordance with this model, the flow of atoms of the target leaving the surface being irradiated consists of two independent components caused by collisional sputtering and evaporation, respectively. The influence of the irradiation parameters on the erosion coefficient and the ratio between the sputtering and evaporation factors is analyzed.     

Anomalous thermal field emission

The field emissivity of ZrO₂/W(100) nanoheterostructures made by applying a thin (≈10 nm) ZrO₂ layer on the surface of a needle-like W(100) microcrystal is studied. At a nanoheterostructure temperature of ≈2000 K, electron emission is found to start at a low extracting (Laplace) field (below 50 V/μm). Under the conditions of steady electron emission, the emission current density from the surface of the heterostructure may reach anomalously high values (～10⁸ A/cm²). A phenomenological model of anomalous thermal field emission of electrons from the surface of the conductor (metal)-thin insulator heterostructure is suggested.     

飞秒激光热反射系统测量金属薄膜中的热波

金属材料中的热波现象可以利用包含电子弛豫时间影响的双曲两步模型进行理论分析.通过飞秒激光热反射实验系统对金属薄膜材料进行了测量.利用偏振分光棱镜将飞秒激光分成抽运光和探测光,其中较强的抽运光用于加热金属薄膜而较弱的探测光用于探测薄膜表面反射率随时间的变化,两束光之间的光程差通过步进电机进行精确控制.利用金属薄膜反射率和电子温度的正比例关系就可以得到电子温度随时间的变化规律.实验发现在加热激光脉冲过后的电子温度下降区间会出现另一个较弱的电子温度波峰,并利用相同厚度的两块金属薄膜样品重复测量对实验结果进行了验证.理论上这一现象可以解释为金属薄膜中热波在背面反射的结果,并且实验结果和双曲两步模型给出的热波理论计算结果相符合.根据实验结果计算出热波传递速度约为5×105m/s,对应的电子弛豫时间为60fs.     

Surface strain-hardening of iron alloys with an intense pulsed electron beam

Results are presented from study of surface strain-hardening and measurements of the structure of carbon (St. 45, U7A, 40Kh) and alloy (R6M5, Kh6VF) steels subjected to surface fusion by pulsed electron beams with the following parameters: electron energy 20–250 keV, pulse duration 5·10^–8–3·10^–4 sec, power density 10⁵–10⁹ W/cm³. It is shown that the microhardness of the surface of most alloys increases by a factor of 1.2–1.7 on quenched specimens and by a factor of 2.5–3.5 on unquenched specimens, depending on the regime. Microhardness increases in the surface layer due to quenching from the liquid state. An increase in electron energy from 40 to 250 keV with a pulse duration of 6·-10^–8 sec leads to a severalfold increase in the thickness of the strengthened layers and to a shift of the microhardness peak from the surface to a depth of 70 m. Here, microhardness reaches 2000 kgf/mm². This is due to an increase in the mean free path of the electrons in the metal and displacement of the energy-release maximum of the bundle deeper into the specimen.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 6, pp. 38–43, June, 1985.     

Electron beam effects in auger analysis of physisorbed xenon

Adsorbed xenon on evaporated films of nickel and platinum has been analysed by Auger electron spectroscopy. The primary electron beam is shown to cause some surface heating resulting in a displacement of the isotherms. This temperature effect is the same for both metals and is due to the limiting thermal conductivity of the glass substrate. A further effect, the electron induced desorption (EID) of xenon is evidenced by a distortion of the isotherms at low equilibrium gas pressures. This effect is more clearly observed on platinum because the coverage is higher than on nickel. The EID cross-section for xenon on both metals is found to be 1 × 10^?17cm². The attenuation of the metal Auger peaks by the xenon overlayer is found to be less for platinum than for nickel. This difference is attributed to a lower packing density of xenon on the platinum surface.     

Electron-stimulated processes in boron nitride-based ceramics

The low-energy electron radiation resistance of boron nitride-based BN + Si₃N₄ and BN + SiO₂ ceramics proposed as a construction material for ion space engines was studied. It was shown that a reduced silicon phase is formed on the BN + Si₃N₄ ceramics surface in the high-temperature region (～900 K), which results from material thermal destruction. The BN + SiO₂ ceramics exhibits high thermal stability and is not prone to destruction due to electron-stimulated oxygen desorption (the cross section of this process does not exceed 10^?20 cm²). It is preferable to use such ceramics as a construction material. Based on the results obtained, some models were proposed that explain variations in the erosion rate of ceramic units of ion engines under electron and ion irradiation.     

Control over the space-time structure of electron beams by high-intensity femtosecond laser radiation

The space-velocity distribution of electrons propagating in vacuum can be deformed by the ponderomotive potential produced by high-intensity femtosecond laser pulses, which makes it possible to subsequently separate such electrons from the initial beam. It is shown that optical modification of electron beams with kinetic energies on the order of 100 eV by femtosecond laser radiation with an intensity from 10¹⁴ to 10¹⁸ W/cm² makes it possible to form electron beams with a duration on the order of 50–100 fs. Examples of optical control over the shape of electron beams, based on deflection, reflection, focusing, and splitting of electron beams, are considered.     

The effect of screening in an erosion flare on the recoil momentum and on the measurement of the reflection coefficient

Experimental investigations have been carried out to measure the transmission coefficient of laser light through an erosion flare, the coefficient of reflection from the irradiated solid surface, and the recoil momentum of insulating and metal targets in the range of pulsed laser intensities from 10⁶ to 10⁷ W/cm² with variation in the pressure of the surrounding medium from 10⁻² to 10⁵ Pa. An analysis is made of how the screening properties of the erosion flare affect the recoil momentum, and it is found necessary to take into account absorption of the radiation in the flare in determining the coefficient of reflection of the radiation from the surface. Zh. Tekh. Fiz. 67, 58–62 (March 1997)     

Pulsed ion beam parameters to ensure maximum coefficients of metal surface erosion

Erosion of metal surface under pulsed ion beams is investigated using a two-phase evaporation model. The coefficients of erosion due to evaporation are calculated for aluminum, iron and copper under irradiation by submicrosecond ion beams of carbon with the initial particle energy within the range 10–1000 keV. The ion energy and ion beam current density values of the most efficient erosive impact are determined. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 31–36, May, 2007.     

Electromagnetic effects on a molecule at a metal surface: I. Effects of nonlocality and finite molecular size

The electromagnetic effects on a molecule near a metal surface are considered with the view to understanding the surface-enhanced-Raman-scattering (SERS) effect. The image enhancement effect is calculated including the nonlocal response of the metal and finite molecular size. The effect is much reduced (× 10^?5) from that for a point molecule above a local metal but can still give a gain ≈ 10³. The power emitted by a dipole above a smooth surface is also calculated. For an Ag surface the power emitted in the form of photons, surface plasmons, and electron-hole excitations are found to be in the ratio 1 : 3 : 10⁶. The numerical results are calculated using the semi-classical infinite-barrier model of the metal surface with a Lindhard dielectric function modified to take into account finite electron lifetime and core polarization.     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

金刚石薄膜中二次电子输运的蒙特卡罗模拟

本文通过编写的二维MATLAB蒙特卡罗程序, 对倍增的二次电子在金刚石薄膜中的输运特性进行了初步模拟. 研究表明: 二次电子的迁移率对温度和外加电场的大小很敏感, 在杂质浓度比较低时(<10¹⁷/cm^-3)受杂质浓度的影响不大. 模拟得到的 二次电子的饱和速度为1.88×10⁷ cm/s, 无外加电场时的迁移率为3732 cm² /V.s. 同时, 通过对二次电子束团在金刚石薄膜中的整体输运特性的模拟, 证明了束团电荷密度在应用要求的范围内时, 空间电荷力的影响可以忽略不计.     

Formation of silver nanoparticles during deposition onto viscous-fluid epoxy resin

A new method is proposed for synthesizing metallic nanoparticles in a polymer matrix. These nanoparticles are synthesized during thermal vacuum evaporation of a metal (4.8 × 10⁻⁶ g/cm²) onto the surface of viscousfluid epoxy resin (at a viscosity of 20–120 Pa s) having room temperature, which is well below the glass transition temperature of the polymer. As a result, epoxy resin layers containing silver nanoparticles in their volume form; these nanoparticles are studied by transmission electron microscopy and optical absorption spectroscopy. Various types of disperse structures formed by metallic nanoparticles in the polymer are detected. The morphology of the composite material is found to be controlled by the polymer viscosity and the metal deposition time.     

Propagation of high-power partially coherent fibre laser beams in a real environment

The propagation performance of high-power partially coherent fibre laser beams in a real environment is investigated and the theoretical model of a high-power fibre laser propagating in a real environment is established. The influence of a collimating system and thermal blooming is considered together with atmospheric turbulence and mechanical jitter. The laser energy concentration of partially coherent beams in the far field is calculated and analysed based on the theoretical model. It is shown that the propagation performance of partially coherent beams depends on the collimating system, atmospheric turbulence, mechanical jitter and thermal blooming. The propagation performance of partially coherent beams and fully coherent beams is studied and the results show that partially coherent beams are less sensitive to the influence of thermal blooming, which results in that the energy degeneration for partially coherent beams is only 50% of that for fully coherent beams. Both partially coherent beams and fully coherent beams become less sensitive to thermal blooming when the average structural constant of the refraction index fluctuations increases to 1.7 × 10^-14 m^-2/3. The investigation presents a reference for applications of a high-power fibre laser system.     

Collimation of atomic beams by resonant laser radiation pressure

The application of the resonant light pressure created by an axially symmetrical light field for collimating atomic beams has been considered. As an example, consideration is given to the possibility of collimating an atomic beam by the light field produced by the reflection of a plane wave from the internal surface of a metal cone. It has been shown that the radiation pressure can reduce the atomic-beam transverse velocities to the value of the order of 100 cm/s which corresponds to effective temperature of about 10^–3 K. A method for producing collimated beams of cold atoms based on simultaneous deceleration and collimation of atomic beams by resonant laser radiation pressure is proposed.     

Surfactant sputtering

Sputter erosion of materials is among the most important techniques for fabricating advanced thin film coatings. Sputter processes are also of considerable relevance for surface polishing down to an atomic scale, nano-structuring of surfaces as dot and ripple patterns and micro-machining of materials using focused ion beams or reactive ion etching. We present a new, versatile sputter technique utilizing the steady state coverage of a substrate surface with up to 10¹⁶ cm⁻² of foreign or self atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These surfactant atoms (surface active agents) strongly modify the substrate sputter yield on atomic to macroscopic length scales. Depending on the surfactant–substrate combination, the novel technique allows enhanced smoothing of surfaces, the generation of novel surface patterns and nanostructures and the controlled shaping of surfaces on the nanometer scale. We present selected examples of surface morphology evolution, smoothing of surfaces and shaping of surfaces to demonstrate the capabilities of the new surfactant sputtering technique.     

Measurements of neutron radiation and induced radioactivity for the new medical linear accelerator,the Varian TrueBeam

Contemporary linear accelerators applied in radiotherapy generate X-ray and electron beams with energies up to 20 MeV. Such high-energy therapeutic beams induce undesirable photonuclear (γ,n) and electronuclear (e,e'n) reactions in which neutrons and radioisotopes are produced. The originated neutron can also induce reactions such as simple capture, (n,γ), reactions that produce radioisotopes. In this work measurements of the non-therapeutic neutrons and the induced gamma radiation were carried out in the vicinity of a new medical accelerator, namely the Varian TrueBeam. The TrueBeam is a new generation Varian medical linac making it possible to generate the X-ray beams with a dose rate higher than in the case of the previous models by Varian. This work was performed for the X-ray beams with nominal potentials of 10 MV (flattening filter free), 15 MV and 20 MV, and for a 22 MeV electron beam. The neutron measurements were performed by means of a helium chamber and the induced activity method. The identification of radioisotopes produced during emission of the therapeutic beams was based on measurements of the energy spectra of gammas emitted in decays of the produced nuclei. The gamma energy spectra were measured with the use of the high-purity germanium detector. The correlation between the neutron field and the mode and nominal potential was observed. The strongest neutron fluence of 3.1 × 10⁶ cm⁻² Gy⁻¹ and 2.0 × 10⁶ cm⁻² Gy⁻¹ for the thermal and resonance energies, respectively, was measured during emission of the 20 MV X-ray beam. The thermal and resonance neutron fluence measured for the 15 MV X-rays was somewhat less, at 1.1 × 10⁶ cm⁻² Gy⁻¹ for thermal neutrons and 6.7 × 10⁵ cm⁻² Gy⁻¹ for resonance neutrons. The thermal and resonance neutron fluences were smallest for the 10 MV FFF beam and the 22 MeV electron beam and were around two orders of magnitude smaller than those of the 20 MV X-ray beam. This work has shown that the neutron reactions are dominant because of relatively high cross sections for many elements used in the accelerator construction. The detailed analysis of the measured spectra made it possible to identify 11 radioisotopes induced during TrueBeam delivery. In this work the following radioisotopes were identified: ⁵⁶Mn, ¹²²Sb, ¹²⁴Sb, ¹³¹Ba, ⁸²Br, ⁵⁷Ni, ⁵⁷Co, ⁵¹Cr, ¹⁸⁷W, ²⁴Na and ³⁸Cl.     

B+注入HgCdTe快速热退火的研究

借助二次离子质谱和俄歇电子能谱对B⁺注入HgCdTe有无ZnS膜包封在快速热退火条件下表层的Hg损失进行了深入的分析-快速热退火温度为300，350和400℃，退火时间为10s-ZnS膜厚度为160nm-结果表明，300℃，10s快速热退火后，有ZnS膜包封的HgCdTe表层没有Hg损失-对B⁺注入HgCdTe N⁺-P结快速热退火工艺流程进行了优化-结果表明，快速热退火放在光刻金属电极之后进行，可以改善结的特性- 关键词：     

Electron beam effects on oxygen exposed aluminum surfaces

The damaging effects of electron beams during the acquisition of electron spectra have long been an obstacle in surface analysis. In order to understand the physico-chemical processes which take place under electron irradiation in an AlO system, we have carried out an experiment in which artifices, such as heating, charging, and gas contamination, were absent. We have observed with Auger Electron Spectra increases of the oxidation extent and the oxygen concentration on an oxygen exposed (111) textured polycrystalline surface under electron irradiation (5 keV, 9 × 10^?5 A/cm²). These increases were not observed on a clean surface, and were very feeble on a (100) single crystal surface. The increase of oxygen concentration was independent of residual gas pressure (3 × 10^?9 to 6 × 10^?10 Torr) and its composition; and therefore cannot be explained by gas contamination during the experimental period (about 70 min). We attribute the increase of oxidation degree to the transition of chemisorbed oxygen atoms into oxide through direct momentum transfer from the incident electrons. We suggest that the increase of oxygen within the irradiated area is due to the surface diffusion of chemisorbed oxygen atoms from outside the irradiated area. These oxygen atoms are excited by the electrons scattered from the vacuum chamber walls and gain energy through Franck-Condon type mechanism. The absence of chemisorbed oxygen atoms on (100) surface explains the near absence of these increases on this surface.