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。     

Messung der Winkelverteilung und des Wirkungsquerschnittes beim Elementarprozeß der Elektron-Elektron-Bremsstrahlung

By using an electron-photon coincidence technique, we have measured the absolute cross section of electron-electron bremsstrahlung in dependence of the photon emission angle for a definite energy and a definite angle of the outgoing electron. An electron beam of 300 keV was used, incident on a carbon target. Within the experimental errors we find agreement with theoretical calculations. Simultaneously with the electron-electron bremsstrahlung the elementary process of electron-nucleus bremsstrahlung of carbon was measured.     

高能电子束韧致辐射特性的理论研究

高能电子束轰击金属靶会产生韧致辐射X射线,为优化韧致辐射X射线品质,需要研究如何获取最佳辐射效率等韧致辐射规律.结合理论分析,并采用MCNP/4C对10,20?MeV电子的韧致辐射规律进行了模拟研究.讨论了不同靶材料产生的韧致辐射效率、角分布、能谱分布、准直锥孔内辐射效率等问题.通过对不同靶材料韧致辐射的模拟研究,给出了不同厚度靶与光子效率、注量分布、出射电子与角分布的关系与规律.由此得到不同靶材料对于10,20?MeV电子在最优韧致辐射效率下的一些边界条件与规律. 关键词： 韧致辐射 最佳效率 角分布 能谱     

Eine experimentelle Anordnung zur Photonenstreuung unterhalb 140 MeV

The facilities for photon scattering experiments at the Mainz electron linear accelerator are described. They include an energy analyzing system, the bremsstrahlung target and a photon spectrometer (NaI-crystals). The question of background elimination is discussed. The scattered photon spectrum from a carbon target is presented.     

电子束参量对轫致辐射照射量角分布的影响

本文研究了电子束半径,发射度及能量等参量对闪光X射线照射量在1m处角分布的影响。求得了使中心照射量最大,束半径与发射度之间应满足的拟合关系,表明了电子束能量越高,发射度越小,那么中心照射量就越大,照射量的空间分布就越理想。对电子细束入射位置和入射方向使用了解析法和随机抽样法求出,两种方法的数值计算所得的照射量分布符合得比较好。     

Photonuclear fission from high energy electrons from ultraintense laser-solid interactions

A new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic, with energies extending well above the threshold for nuclear processes, is studied using a petawatt laser system. In solid target experiments with focused intensities exceeding 10(20) W/cm(2), high energy electron generation, hard bremsstrahlung, and nuclear phenomena have been observed. We report here a quantitative comparison of the high energy electrons and the bremsstrahlung spectrum, as measured by photonuclear reaction yields, including the photoinduced fission of 238U.     

12MeV直线感应加速器X光能谱的实验测量

12MeV直线感应加速器能够产生能量～12MeV、流强2.2kA的电子束, 被传输、聚焦后形成～4mm的束斑, 与轫致辐射靶靶作用来产生高剂量的X光. 首先设计了X光探测器, 并利用多层吸收拟合法来对X光能谱进行了测量, 得到了X光的轫致辐射能谱, X光峰值大约在2MeV,最后对实验结果进行了分析.     

真空磁绝缘传输线损失电子模拟

 真空磁绝缘传输线建立磁绝缘状态的初始阶段,损失电子轰击阳极,发生轫致辐射。针对自限制流同轴圆筒模型,通过粒子模拟获得了损失前沿在能量传输方向的推进速度、电子到达阳极时的能谱和角分布情况,在此基础上采用蒙特卡罗方法模拟得到了轫致辐射所产生的X射线能谱。数值计算结果表明:电磁波损失前沿在能量传输方向的推进速度小于光速;损失前沿电子密度稳定。在自限制流磁绝缘传输线中,损失电子处在较宽的能量范围内,其电子偏移角度较小。建立了对应于同轴圆筒真空磁绝缘传输线的电子/光子输运模型,获得了损失电子轰击阳极产生的X射线能谱。     

Bremsstrahlung of nonrelativistic electrons in metals with allowance made for the polarization channel

We theoretically investigate the bremsstrahlung that appears when nonrelativistic electrons are scattered in a metal target with allowance made for the polarization contribution. We take into account the interference of ordinary and polarization bremsstrahlung, the absorption of radiation in the target material, the energy losses and elastic scattering of an electron by atoms of the medium, and the coherent effects when the radiating electron interacts with the target. We analyze the influence of the target thickness on the process and the contribution of polarization bremsstrahlung to the total yield of bremsstrahlung photons as a function of the problem parameters.     

紫外激光烧蚀Cu产生等离子体辐射特性

通过时间分辨的光谱测量技术,测定了308 nm XeCl紫外激光烧蚀金属Cu在氮气环境中诱导产生等离子体的发射光谱及其强度随时间分布,实验结果表明等离子体辐射光谱线主要由原子光谱线、一价离子光谱线及连续辐射背景光组成,各种光谱线的数目、辐射强度、持续时间不同。结合实验结果对等离子辐射机理进行了探讨,认为电子通过逆韧致辐射获得较高的能量,连续辐射主要来自高能电子的韧致辐射,原子和一价离子的激发主要是通过电子与原子、离子的碰撞传能以及电子与离子的复合产生,并用其定性地解释了所观察的实验现象。     

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.     

Absolute doubly differential cross sections for electron bremsstrahlung from rare gas atoms at 28 and 50 keV

Absolute doubly differential bremsstrahlung cross sections from Xe, Kr, Ar, and Ne have been measured for electron bombarding energies of 28 and 50 keV. Bremsstrahlung photons have been detected at 90 degrees to the incident electron beam at energies ranging from 5 keV up to the kinematic end point. The results are compared with predictions of ordinary bremsstrahlung and of total bremsstrahlung that include polarizational bremsstrahlung from the target atom calculated in the stripping approximation. All previous absolute cross sections have been from thin-film solid targets and have not shown any polarizational bremsstrahlung contribution. The present results, the first from free atoms, provide definitive evidence for the contribution of polarizational bremsstrahlung to the photon spectrum from electron bremsstrahlung.     

Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.     

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.     

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.     

Bremsstrahlung spectrum of nonrelativistic electrons scattered in metallic target: Theory and experiment

This work is devoted to theoretical and experimental investigation of the bremsstrahlung (BrS) spectrum of electrons with an energy of 10–30 eV scattered in a copper plate. Modeling of the photon yield from a target has been carried out taking into account the polarization BrS channel, elastic and inelastic electron scattering by medium atoms, photon absorption in the target material, and coherent effects when electrons interact with a polycrystalline structure of metal. An FEI Quanta 200 scanning electron microscope with a supplement for X-ray microanalysis was used. Good agreement between experimental and theoretical results has been obtained.     

厚靶轫致辐射特性的数值模拟研究

X光机正朝高能强流方向发展.这些装置中普遍采用mm厚度的重金属厚靶,其辐射特性和各种参数间的规律受到关注.本文采用基于蒙特卡罗法的EGS4程序比较全面的研究了高能X光机中所关心的靶厚度的选择、轫致辐射场的角分布和X射线能谱的规律.同时还专门就X光强度和束流发射度、打靶焦斑尺寸三者的依赖关系进行分析,结论表明针对现有的X光机参数,不用担心因追求小焦斑而带来X光照射量的损失。文中还分析了强束流条件下单粒子蒙特卡罗模拟程序的适用性.文中结论和已有实验结果吻合,对实际工作具有意义.     

Bremsstrahlung of the free electrons arising in an irradiated specimen

A model of the bremsstrahlung produced by photoelectrons, Auger electrons, and Compton electrons arising in an irradiated specimen is proposed. Calculations have shown that the contribution of the Compton electron bremsstrahlung shows up for monochromatic primary radiation of high-energy photons. For the primary radiation of x-ray tubes, only the bremsstrahlung spectrum of photoelectrons and Auger electrons is significant. The factors affecting the proportions between these components are considered. The bremsstrahlung spectral distribution of the mentioned electrons shows considerable deviations from that predicted by the Kramers theory that are due to the large depth of their occurrence and to the ambiguity of their energy. The region of the spectrum has been determined where the intensity of the electron bremsstrahlung is greater than the intensity of the x-ray tube polychromatic primary radiation scattered by the irradiated object.     

Bremsstrahlung target for studying photonuclear reactions in the giant dipole resonance energy region

Studying the interaction mechanism between gamma-quanta and atomic nuclei in the giant dipole resonance energy region gives us significant information about atomic nuclei. A large number of experiments for studying giant dipole resonance structure were accomplished using bremsstrahlung gamma-quanta. In such experiments cross sections of interactions are retrieved from reaction yields. To retrieve cross sections one needs to know the shape of the bremsstrahlung spectrum and to choose the optimal characteristics of the bremsstrahlung target. The purpose of this article is to choose the optimal characteristics of the bremsstrahlung target for experiments in the energy region E < 30 MeV. The analysis that was carried out based on numerical simulation showed that the optimal bremsstrahlung target consists of two parts. The first part is a tungsten plate 2.5 mm thick. The second part is aluminum plate 3 cm thick.     

强流电子束入射叠靶能量沉积计算

计算了多脉冲相对论强流电子束入射钽-石墨叠靶的能量沉积和轫致辐射谱。能量沉积采用Geant4程序计算,轫致辐射谱根据基本的辐射理论和蒙特卡罗方法计算。结果显示,各层的热区能量沉积呈由大到小的递减分布,截面轫致辐射分布和电子束径向分布主要受钽层的影响。石墨层的低能量沉积率和高热容能改善叠靶的性能。对于单脉冲,钽-石墨层厚比为1∶1时,石墨能全部吸收相邻钽层的热沉积,轫致辐射效率为35.4%;4脉冲情况下,钽-石墨层厚比应为1∶13,总轫致辐射效率降到19.9%。考虑轫致辐射剂量和质量,钽-石墨两者的厚度比为1∶5时,钽层的总厚度应为1.2 mm;当钽-石墨层厚比为1∶10时,钽层的总厚应降到0.7 mm。