Development of a new experimental setup for studying collisions of keV-electrons with thick and thin targets

Development of a new lectron-recoil ion/photon coincidence setup for investigating some of the electron induced collision processes, such as electron bremsstrahlung, electron backscattering, innershell excitation and multiple ionization of target atoms/molecules in bombardment of electrons having energies from 2.0 keV to 30.0 keV with solid and gaseous targets is described. The new features include the use of a compact multipurpose scattering chamber, a time-of-flight spectrometer for detection of multiply charged target ions, a 45°-parallel plate electrostatic analyzer for measuring energy and angle of the ejected electrons, a room temperature high resolution Si-PIN photo diode X-ray detector for counting the collisionally induced photons, a coincidence data acquisition system consisting of a 200 MHz Pentium based 8K-multichannel analyzer and a standard network of a fast/slow coincidence electronics. In particular, the details of design, fabrication and assembly of indigenous components employed in the setup are presented. Selected experiments planned with the setup are mentioned and briefly discussed. A report on performance, optimization, efficiency, time resolution etc. of the time-of-flight (TOF) spectrometer and that of the 45°-parallel plate electrostatic analyzer (PPEA) is presented. Test spectra of electron-recoil ion coincidences, energy distribution of ejected electrons and characteristic plus non-characteristic X-ray spectrum are illustrated to exhibit the satisfactory performance of the developed setup.     

Comparison of the bremsstrahlung absolute probability densities produced by experiment and PENELOPE for low‐energy (keV) electrons incident on solid silver

There has been some recent controversy concerning whether or not polarizational bremsstrahlung (sometimes referred to as ‘atomic bremsstrahlung’) contributes to the total bremsstrahlung spectrum in experiments involving electrons incident on solid targets. We have compared the bremsstrahlung absolute probability density predictions of the widely used Monte Carlo code, PENELOPE, to the results of experiments involving 17.5‐keV and 20‐keV electrons incident on a thick silver target. Comparisons of the results of our experiments to the predictions of the PENELOPE code, which is based on ordinary bremsstrahlung alone, suggest that there were no significant polarizational bremsstrahlung contributions to the total bremsstrahlung spectra in our experiments. Copyright © 2015 John Wiley & Sons, Ltd.     

10-25 keV电子致厚W,Au靶韧致辐射谱的测量

测量了10-25 keV电子碰撞厚W, Au靶产生的韧致辐射谱, 并与Monte Carlo程序PENELOPE模拟的X射线谱进行了比较, 除在3 keV前实验谱略低于理论谱外, 整体上两者符合得很好. 在模拟电子与靶材料相互作用产生韧致辐射时, PENELOPE程序中只包含有普通韧制辐射的截面数据. 我们的实验结果表明, 在电子与固体靶相互作用时, 没有明显的极化韧致辐射产生, PENELOPE程序能够可靠地描述电子与固体厚靶相互作用产生的韧致辐射.     

KT—5C托卡马克射频波加热实验用回路天线的负载阻抗测量

针对ＫＴ－５Ｃ托卡马克装置等离子体的射频离子回旋波加热实验，本文给出一种回路天线负载阻抗的简便测量方法和测量结果。该方法的特点是，只涉及射频电流相对比值的测量，不要求专用设备，因此使物理量的标定、数据记录和处理都变得简单易行。     

Thick-target X-ray bremsstrahlung spectra produced in 6.5 keV and 7.5 keVe--Hf collisions

The absolute doubly differential cross-sections (DDCS) for production of the thick-target X-ray bremsstrahlung spectra in collisions of 6.5 keV and 7.5 keV electrons with thick Hf target are measured. The X-ray photons are counted by a Si(Li) detector placed at 90° to the electron beam direction. The bremsstrahlung spectra are corrected for various ‘solid-state effects’ namely, electron energy-loss, electron back-scattering, and photon-attenuation in the target, in addition to the correction for detector’s efficiency. The DDCS values after correction, are compared with the predictions of a most accurate thin-target bremsstrahlung theory [H K Tseng and R H Pratt,Phys. Rev. A3, 100 (1971); Kisselet al, Atomic Data Nucl. Data Tables 28, 381 (1983)]. Also, a dependence of the absolute DDCS on atomic numberZ of the targets (₄₇Ag,₇₉Au and₇₂Hf) at 7.0 keV and 7.5 keV electron energies has been studied. The agreement between experiment and theory is found to be satisfactory within 27% systematic error of measurements. However, an apparent systematic difference between experiment and theory in the region of low-energy photons has been explained qualitatively by considering the fact that the hexagonal atomic structure of Hf offers possibly a greater magnitude of ‘solid-state effects’ in respect of blocking the low-energy bremsstrahlung photons from coming out of the target surface than does the cubic-face centered structure of Ag and Au target in similar conditions of the experiment.     

利用轫致辐射谱回推超热电子温度

超短超强激光打靶产生的超热电子与固体靶相互作用时会产生轫致辐射X射线。利用蒙特卡罗方法,对电子在固体靶中传输产生的轫致辐射X射线进行了模拟。1 MeV电子束与固体靶作用产生的轫致辐射谱模拟结果表明,轫致辐射谱高能段斜率受靶厚度及靶材料的影响不明显。麦克斯韦分布的电子束及单能电子束与30 m铜靶作用的模拟结果显示,两种电子源产生的轫致辐射谱在电子束能量或温度较高时基本一致。给出了一种利用轫致辐射谱斜率反推超热电子温度的定标方法。模拟了不同温度下超热电子产生的轫致辐射光子的能量角分布及光子数角分布,结果显示辐射光子能量通量和光子数随着电子温度的提高越来越向前倾,并给出了另外一种由轫致辐射能量角分布反推超热电子温度的定标关系。     

Comparison of atomic potentials and bremsstrahlung Gaunt factors in dense aluminum plasmas

We determine and compare high temperature high density atomic potentials for dense aluminum plasmas. We then evaluate bremsstrahlung Gaunt factors from these potentials utilizing various methods. The potentials considered are obtained from density functional theory, from the hypernetted chain/Poisson model and from the Thomas-Fermi model. The bremsstrahlung spectra obtained for these three potentials, with the partial wave expansion method and for incident electrons of about 1 keV, are in qualitative agreement. We indicate in which circumstances and with what precision bremsstrahlung Gaunt factors can also be estimated from much simpler potentials, such as the Debye or ion sphere model, and from much simpler calculations of the spectrum, such as the Born-Elwert approximation or a simple classical mechanics approach. The aluminum plasmas considered have temperatures of 0.5-1 keV and electron densities of 10²⁵, 10²⁴, 10²³cm^-3.     

Energieabhängigkeit der Isochromatenstruktur von Wolfram im Energiebereich von 0,15 bis 6 keV

A systematic investigation is presented of the variation of isochromat structures with threshold voltage (quantum energies). With a double focusing X-ray monochromator highly resolved bremsstrahlung isochromats of tungsten are measured in the quantum energy range from 0.15 to 6 keV. Crystals with lattice constants up to about 50 Å could be used as dispersing devices. The initial parts of the isochromats (primary structure) turned out to be essentially independent of quantum energy. With the high resolution of the monochromator at quantum energies below 0.9 keV a fine structure in the primary structure of the corresponding isochromats could be observed. This fine structure is in qualitative agreement with a theoretical density of states calculated by Mattheiss. The following parts of the isochromats (secondary structure) depend strongly on quantum energy. These structures can be understood by the influence of characteristic energy losses in the target of the primary electrons.     

ECR ion source based low energy ion beam facility

Mass analyzed highly charged ion beams of energy ranging from a few keV to a few MeV plays an important role in various aspects of research in modern physics. In this paper a unique low energy ion beam facility (LEIBF) set up at Nuclear Science Centre (NSC) for providing low and medium energy multiply charged ion beams ranging from a few keV to a few MeV for research in materials sciences, atomic and molecular physics is described. One of the important features of this facility is the availability of relatively large currents of multiply charged positive ions from an electron cyclotron resonance (ECR) source placed entirely on a high voltage platform. All the electronic and vacuum systems related to the ECR source including 10 GHz ultra high frequency (UHF) transmitter, high voltage power supplies for extractor and Einzel lens are placed on a high voltage platform. All the equipments are controlled using a personal computer at ground potential through optical fibers for high voltage isolation. Some of the experimental facilities available are also described.     

The bremsstrahlung induced by 0.3–2 keV electron scattering by Ar atoms

The differential spectra of a bremsstrahlung resulting from a 0.3–2 keV electron scattering by Ar atoms are studied. Photon energies within the ultrasoft X-ray band from 124 to 190.8 eV, which is characterized by the low dynamic polarizability of the Ar atom, are considered. For the entire spectrum of photon energies (124–190.8 eV), the intensity of the bremsstrahlung differential spectra first grows with an increase in the electron energy from 0.3 to 0.7 keV and then decreases as the electron energy increases from 0.7 to 2 keV. The increase in intensity is directly proportional, and the decrease is inversely proportional to the square root of the energy of the scattered electrons. Within the context of a "low-energy" approximation, the increase in the number of photons with the electron energy is due to the contribution of the atomic excitation and ionization channels being available during the bremsstrahlung process.     

超热电子的阻止功率及其在慢化过程中引起的轫致谱

在高功率、长波长激光与等离子体耦合时将产生大量的超热电子，这些超热电子将通过轫致辐时，与热电子碰撞，以及晕区电场加速快离子损失其能量。在某些典型条件下，分析和估计了每种能耗机制损失能量的分额。给出了快电子在通过激光等离子体的慢化过程中发射轫致谱的解析表达式。用调整参数拟合实验结果的方法，能够得到超热电子的各种信息，诸如它们的温度、能量、数同以及能谱。计算谱与实验谱作了比较。     

Monte Carlo calculation of electron spectra produced in water phantoms by 20 keV-1 GeV photons

Abstract

A Monte Carlo calculation for the energy spectra of electrons and positrons produced in infinite and semiinfinite water phantoms by photons ranging in energies from 20 keV to 1 GeV are presented. The dominant processes considered are the photoelectric effect, Auger effect, Compton effect, pair, and triplet production. Bremsstrahlung produced by electrons and positrons with energies greater than 1 MeV is also included. The effect of multiple Compton scattering, not considered in the earlier calculation, for the infinite phantom for photon energies higher than 2 MeV has been incorporated. For a semi-infinite phantom, multiple Compton scattering and backscattering through the top are considered. The results are compared with the earlier calculations for the first-collision spectrum. It is found that the inclusion of multiple Compton scattering significantly increases the average number of electrons/cm³ per photon/cm² at all energies considered whereas bremsstrahlung reduces the number of high energy electrons and produces more low energy electrons in the spectrum.     

A COAXIAL MAGNETRON INJECTION GUN (CMIG) FOR A 2 MW, 170 GHz COAXIAL CAVITY GYROTRON

An electron gun (80 kV / 75 A) for a 170 GHz coaxial cavity gyrotron has been designed, fabricated and tested. Special care has been devoted to the design of the technical part of the electron gun in order to avoid trapping of electrons, which may result in a limitation of the high voltage performance due to built-up of a Penning discharge which may limit the high voltage performance. The performance of the electron gun and electron beam has been found to be well in agreement with the design objective. A stable operation up to I_b = 80 A and U_c = 80 kV has been obtained without any observable beam instabilities. No limitations in high voltage performance due to built-up of a Penning discharge occurred. The influence of the type of electron flow on gyrotron performance has been studied experimentally. The maximum output power has been obtained for an intermediate type of flow.     

Koinzidenzexperimente zum Elementarprozeß der Bremsstrahlungserzeugung

By using a coincidence method, the angular distribution of bremsstrahlung has been measured for definite energy loss and definite angle of the outgoing electron. The incident electron energy was 300 keV. With a magnetic spectrometer, outgoing electrons with an energy of 170 keV and scattering angles of 0°, 5° and 10° were selected. The electrons and photons were both detected in scintillation counters. A gold foil of 250 Å thickness served as the target. The experimental results are compared with theoretical calculations.     

Direct measurement of energetic electrons coupling to an imploding low-adiabat inertial confinement fusion capsule

We have imaged hard x-ray (>100 keV) bremsstrahlung emission from energetic electrons slowing in a plastic ablator shell during indirectly driven implosions at the National Ignition Facility. We measure 570 J in electrons with E>100 keV impinging on the fusion capsule under ignition drive conditions. This translates into an acceptable increase in the adiabat α, defined as the ratio of total deuterium-tritium fuel pressure to Fermi pressure, of 3.5%. The hard x-ray observables are consistent with detailed radiative-hydrodynamics simulations, including the sourcing and transport of these high energy electrons.     

Messung der Energie- und Winkelabhängigkeit der Asymmetrie der äußeren Bremsstrahlung

In order to obtain an azimuthal dependence of the external bremsstrahlung produced by electrons, the electron beam has to be transversely polarized. In first-order Born approximation the differential cross section does not depend on the azimuthal angle φ¹, but the second-order approximation includes the spin vector of the electrons^2,3 and yields, consequently, a φ-dependence. This paper deals with a measurement of the bremsstrahlung asymmetry as function of the photon energy and the emission angle Θ. The transversely polarized electron beam was produced by momentum deflection of longitudinally polarizedβ-decay electrons emitted from a⁹⁰Sr source. With a kinetic energy of 300±10 keV and a degree of transverse polarization of 74% the electrons hit a Pb target. Because of the high γ-background it was necessary to detect the emitted bremsstrahlung in coincidence with the incident electrons. We found effects of about 3% in contrast to considerably larger values (13%) published in an earlier paper. The asymmetry was numerically calculated according to Johnson and Rozics. Since at an electron energy of 300 keV the Born approximation gives not yet good results, theory and measurement agree only in order of magnitude, particularly at highγ-energies.     

On the reconstruction of the energy distribution of electrons accelerated in solar flares

A technique for reconstructing energy spectra of electrons accelerated in solar flares is suggested that is based on the rigorous solution of the inverse problem considering their X-ray bremsstrahlung. Model calculations are made for various spectra, and it is proved that this technique makes it possible to find the electron energy distribution in real flare events. The energy distribution of high-energy electrons accelerated in the solar flare observed on July 26, 2002, is reconstructed. It is shown that the hard X-ray spectrum of the flare may result from the bremsstrahlung of three groups of high-energy electrons.     

X射线在影象增强器中背向散射特性的研究

本文用蒙特卡罗方法模拟了轫致辐射Ｘ射线在影象增强器中的背向散射特性。针对Ｘ线源管电压为３０－１２０ｋＶ的轨致辐射入射，给出了背向散射光子的角分布及能谱分布；同时，还给出了背向散射光子的平均能量和平均出射角与管电压的关系以及吸收光子数和直透光子数与管电压的关系。     

飞秒电子衍射系统的设计

研发的超快电子衍射系统由超快电子枪、样品室、超快读出系统、电源系统,以及真空系统等组成,该超快电子衍射系统具有较高的时间分辩能力和较强的探测能力.光电阴极是蒸镀于MgFB2窗上的35 nm的银膜,该阴极对266 nm的紫外光比较敏感,有较高的量子效率,又具有很好的化学稳定性.用短磁聚焦系统来实现对光电子的聚焦,有两对偏转板,其中的一对在测量时间脉宽时用作扫描板.用双MCP探测器来增强电子图像的强度,其增益在104以上,具有单电子探测能力.系统的总时间脉宽设计为358 fs.     

Highly Efficient Sources of Wideband and Monochromatic X and Gamma Rays

New physical configurations of x- and gamma-ray sources, in which photons are emitted by circulating electrons in multiple passage through a thin target, are discussed. As examples, sources of bremsstrahlung with an additional acceleration of radiating electrons and sources of monochromatic radiation in a crystalline target are examined. Computer modeling confirms the high efficiency of these configurations that allow the hard photon yield to be significantly increased while retaining the compact dimensions of the facility. This is especially important in the design of various autonomous sources.