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     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Peak structure of polarized bremsstrahlung upon scattering by coupled atomic electrons

Peculiarities of polarized bremsstrahlung (PB) of relativistic electrons produced upon scattering by coupled atomic electrons are discussed; in this case, PB is regarded as scattering of virtual photons of the electromagnetic field of a fast charge by atomic electrons with their coupling taken into account. In this case, the atomic electron during scattering can acquire recoil energy only through separate portions, as a result of which the spectrum of scattered photons degenerates into a series of narrow peaks.     

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z ² (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p ³ ? 1, where p is the electron momentum, and the condition ?ω ? p ² are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ? v _nei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.     

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。     

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。     

Electromagnetic radiation from positive-energy bound electrons in the Coulomb field of a nucleus at rest in a strong uniform magnetic field

A classical analysis is presented of the electromagnetic radiation emitted by positive-energy electrons performing bound motion in the Coulomb field of a nucleus at rest in a strong uniform magnetic field. Bounded trajectories exist and span a wide range of velocity directions near the nucleus (compared to free trajectories with similar energies) when the electron Larmor radius is smaller than the distance at which the electron-nucleus Coulomb interaction energy is equal to the mechanical energy of an electron. The required conditions occur in magnetic white dwarf photospheres and have been achieved in experiments on production of antihydrogen. Under these conditions, the radiant power per unit volume emitted by positive-energy bound electrons is much higher than the analogous characteristic of bremsstrahlung (in particular, in thermal equilibrium) at frequencies that are below the electron cyclotron frequency but higher than the inverse transit time through the interaction region in a close collision in the absence of a magnetic field. The quantum energy discreteness of positive-energy bound states restricts the radiation from an ensemble of bound electrons (e.g., in thermal equilibrium) to nonoverlapping spectral lines, while continuum radiative transfer is dominated by linearly polarized bremsstrahlung.     

PAMELA spectrum of electrons and positrons of cosmic rays in the energy range of 0.05–1.2 TeV

A proprietary method is used to process measurement data from a high-energy particle (protons, electrons, and positrons with Е ≥ 50 GeV) spectrometer in a near-Earth orbit. The data from three detector systems are used: a tracker in a constant magnetic field (TRK), a calorimeter (CAL), and a neutron detector (ND). A relatively simple and efficient way of isolating electrons and positrons from the total charged particle flux entering the PAMELA spectrometer is proposed. A technique for determining the energy of isolated primary particles and retrieving their energy spectra is described. The composite electron and positron spectrum (below, the total electron and positron flux is referred to simply as the electron flux) for energies up to 1.5 TeV is presented.     

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.     

Polarization bremsstrahlung in α decay

A mechanism of formation of electromagnetic radiation that accompanies α decay and is associated with the emission of photons by electrons of atomic shells due to the scattering of α particles by these atoms (polarization bremsstrahlung) is proposed. It is shown that, when the photon energy is no higher than the energy of K electrons of an atom, polarization bremsstrahlung makes a significant contribution to the bremsstrahlung in α decay.     

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.     

Geometry optimization of a barrel silicon pixelated tracker

We have studied optimization of the design of a barrel-shaped pixelated tracker for given spatial boundaries. The optimization includes choice of number of layers and layer spacing. Focusing on tracking performance only,momentum resolution is chosen as the figure of merit. The layer spacing is studied based on Gluckstern's method and a numerical geometry scan of all possible tracker layouts. A formula to give the optimal geometry for curvature measurement is derived in the case of negligible multiple scattering to deal with trajectories of very high momentum particles. The result is validated by a numerical scan method, which could also be implemented with any track fitting algorithm involving material effects, to search for the optimal layer spacing and to determine the total number of layers for the momentum range of interest under the same magnetic field. The geometry optimization of an inner silicon pixel tracker proposed for BESIII is also studied by using a numerical scan and these results are compared with Geant4-based simulations.     

Effect of sheath potential on electromagnetic radiation emitted from the rear surface of a metallic foil target

In ultra-intense laser--matter interactions, intense electric fields formed at the rear surface of a foil target may have strong influences on the motion of energetic electrons, and thereby affect the electromagnetic emissions from the rear surface, usually ascribed to transition radiation. Due to the electric fields, transition radiation occurs twice and bremsstrahlung radiation also happens because the electrons will cross the rear surface twice and have large accelerations. In the optic region, transition radiation is dominant. The radiation spectrum depends on the electric field only when the electrons are monochromatic, and becomes independent of the electric field when the electrons have a broadband momentum distribution. Therefore, in an actual experiment, the electric field at the rear surface of a foil could not be studied just with the measurement of optic emissions. In the terahertz region, both bremsstrahlung and transition radiations should be taken into account, and the radiation power could be enhanced in comparison with that without the inclusion of bremsstrahlung radiation. The frequency at which the maximum terahertz radiation appears depends on the electric field.     

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.     

Red shift of spectral lines in the Sun's chromosphere

An unexplained center-to-limb variation (CLV) of solar wavelengths has been known for 75 years. A theory is described which not only explains this variation but also predicts its amplitude without the use of any adjustable parameter. The model considered is based on the fact that the momentum transfer of the solar photons to the electrons of the atoms of the solar atmosphere produces secondary radiation due to bremsstrahlung. The energy of this bremsstrahlung radiation is taken away from the energy of the initial photons and leads to a red shift. No ad hoc parameters are used, and all constants are either fundamental physical constants or well-known solar parameters. The model can explain the variation of the CLV as a function of its position on the disk and near the limb, and even the change in the lineshapes especially near the limb     

Deexcitation of plasma by relativistic electrons

The characteristics of the electromagnetic radiation generated as a result of the interaction of relativistic electrons with a plasma are investigated. The main attention is paid to the analysis of the effect of broadening mechanisms on the bremsstrahlung energy spectrum of the relativistic electrons propagating in a carbon plasma upon deexcitation of its components.     

The Dynamics of Electron Orbits during the Acceleration of Electrons in a Betatron

This paper presents a system of equations that describe the motion of charged particles in the electromagnetic field of a betatron. This system of equation was successfully used to study the behavior of the electron orbits and to determine the principal parameters of the electron beam in the electromagnetic field of a betatron during the electron acceleration and deceleration. The results of this study may find application in developing systems designed to accelerate electron beams. It has been shown that in the course of acceleration there is no damping of the betatron oscillations by the law B _z ^–1/2 and, correspondingly, no decrease in beam cross section. In contrast to the existing belief, the initial departure of the kinetic energy (momentum) of the injected electrons from the energy (momentum) of the electrons following the equilibrium orbit is not preserved in the course of acceleration. In the betatron chamber, the electron beam, when accelerated, does not constrict to form a ring but occupies a broad zone, whose dimensions are determined by the initial double amplitudes of the vertical and horizontal oscillations. Despite the large double amplitude of the oscillations of the beam particles, the average energy of the electrons differs from the energy of the electrons following the equilibrium orbit only slightly, and the departure of the average energy from the energy of the equilibrium electrons varies proportionally to the (varying) field of the betatron.     

Explicit designs of spin chains for perfect quantum state transfer

For the process of electron-electron (e-e) bremsstrahlung the momentum and energy distributions of the recoiling electrons are calculated in the laboratory frame. In order to get the differential cross section and the photon spectrum for target electrons which are bound to an atom, these formulae are multiplied by the incoherent scattering function and numerically integrated over the recoil energy. The effect of atomic binding is most pronounced at low energies of the incident electrons and for target atoms of high atomic numbers. The results are compared to those of previous calculations.     

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.     

Electron acceleration by net inverse bremsstrahlung of a laser wave in a uniform magnetic field and longitudinal electric waves

The nonzero net dc (ponderomotive force) acting on high-energy beam electrons due to net inverse bremsstrahlung (the absorption by inverse bremsstrahlung minus the emission by stimulated bremsstrahlung) of the electromagnetic wave in a uniform magnetic field and longitudinal electric waves is calculated by using quantum kinetics in accordance with the correspondence principle. It is found that the ponderomotive force can be far stronger than the Lorentz force of the laser wave for an electron-energy range far beyond the free electron lasing regime.