Coherent bremsstrahlung of relativistic electrons in antiferromagnets

The coherent bremssstrahlung (CB) of relativistic electrons in antiferromagnetic crystals is studied theoretically. It is shown that the internal magnetic field of an antiferromagnet has a negligible effect on CB. The complete CB spectrum is calculated for hematite, -Fe₂O₃.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 57–61, February, 1988.     

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     

Weak localization: The quasiclassical theory of electrons in a random potential

The recent subject of weak localization is an important area of research in condensed matter physics which has received a considerable amount of experimental and theoretical attention. In its existing form, the theory is founded entirely on the impurity technique of Green's function. In this paper, we present a theory of weak localization which is put rigorously on a quasiclassical basis thus providing a more intuitive understanding. For example, it becomes apparent that much of the underlying physics is a result of quantum mechanical interference effects in a very elementary sense. It is one of those unique cases where the superposition principle of quantum mechanics leads to observable consequences in the properties of macroscopic systems. In addition, all the important quantitative results obtained so far are recovered in the present quasiclassical approach.     

Polarization bremsstrahlung from relativistic electrons moving in a small-grained medium

Polarization bremsstrahlung from relativistic electrons moving in a medium consisting of very small crystals oriented at random is studied theoretically. The results of this analysis predict a sharp dependence of the spectral and angular features of the radiation on the crystal dimensions and on the angle of observation. The possibility of developing, on the basis of the phenomena considered in the present study, a newmet hod for studying lowly ordered media is highlighted.     

Influence of acoustic oscillations on coherent bremsstrahlung of electrons

The results of experiments on investigation of the influence of acoustic oscillations on the coherent bremsstrahlung of electrons with the energy 20 MeV are presented. The experiments were carried out on the extracted beam of the YerPhI linear accelerator LEU-50.     

The bremsstrahlung of electrons on atoms calculated in a broad range of photon energies

The total bremsstrahlung spectra for electrons colliding with atoms have been calculated for krypton atoms in a photon energy range from 10 eV to 25 keV and for lanthanum atoms in the vicinity of the 4d shell ionization energy. The generalized atomic polarizabilities were calculated using a simple semiclassical local energy density approximation and experimental data on the photon absorption. The results are compared to those obtained using the distorted partial wave approximation for Kr and to the experimental data available for La.     

Measurement of the polarization bremsstrahlung of relativistic electrons in polycrystalline targets

The spectra of the collimated polarization bremsstrahlung of 7-MeV electrons intersecting Al, Cu, and Ni polycrystalline films have been measured. Detailed quantitative comparison of the experimental data with theoretical predictions has been performed. It has been shown that the model of a polycrystal as an ensemble of randomly oriented single crystals adequately describes the experimental results. This makes it possible to expect the development of a new method of diagnostics of the atomic structure of partially ordered solids.     

On bremsstrahlung radiation of accelerated electrons in solar flares

We have calculated the spectrum and polarization of the bremsstrahlung hard X-ray radiation produced by nonrelativistic electrons that are accelerated during a solar flare. The distribution function of accelerated electrons is taken from the self-consistent solution of the kinetic equation allowing for Coulomb collisions and an electric field of reverse current in plasma. The computed polarization of the hard X-ray polarization is shown to be appreciably lower than the value given by simplified thick-target models without considering the reverse current.     

Diagnostics of nanodisperse polycrystals based on the polarization bremsstrahlung of relativistic electrons

The spectra of polarization bremsstrahlung are measured in the backscattering geometry during the interaction of 7-MeV electrons with a polycrystalline Ni foil. Measurement is conducted under conditions such that the size of the region of coherent X-ray radiation scattering in a target is on the order of 10 nm. The obtained results make it possible to suggest that using polarization bremsstrahlung as a new method for the diagnostics of the atomic structures of nanodisperse polycrystals is effective.     

Asymmetry of bremsstrahlung by moderately relativistic polarized electrons

The asymmetry of bremsstrahlung by transversely polarized electrons using Sommerfeld–Maue wave functions has been calculated. In order to simplify the final formula the relativistic approximation has been used. The results obtained show a linear dependence of the asymmetry on the atomic number Z for the range 4≤Z≤26. It is shown that the approach developed gives the zeroth asymmetry for ultrarelativistic energies as expected but for initial electron energies ～5 MeV the asymmetry may achieve a magnitude ～8% (for Z=26). Polarimetry of electrons with energy higher than 1 MeV (where conventional Mott polarimeters have large experimental difficulties) may be carried out using the bremsstrahlung process providing a reasonable asymmetry and high cross section.     

Features of bremsstrahlung from relativistic electrons in solid targets

It has been shown that the classical Bethe-Heitler result can be inapplicable for describing the bremsstrahlung spectrum of relativistic electrons in real solid targets.     

Effect of electron and γ polarization on two-photon bremsstrahlung of electrons in a nuclear field

The cross section for two-photon bremsstrahlung is integrated over the electron scattering angle and the (common) emission angle of the bremsstrahlung photons. Equations are found for the energetic, angular, and polarization distributions of the cross section. The cross section for two-photon bremsstrahlung for the case in which the momenta of the bremsstrahlung photons are parallel can be larger than the corresponding Bethe-Heitler cross section. The probabilities for certain polarization states of the electrons are also analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 13–18, July, 1972.     

Properties of coherent bremsstrahlung from electrons in thick single crystals

A technique for calculating the frequency and polarization of the coherent bremsstrahlung from relativistic electrons in single crystals of arbitrary thickness is proposed and implemented. It accurately includes multiple scattering of the emitting particles and the angular distribution and collimation of the radiation. The spectral properties of the collimated beam of radiation from electrons with energies of 900 MeV in thick (0.1 r.l.) single crystals of diamond, silicon, and tungsten are compared with the calculations. When the radiation is tightly collimated the dependence of the frequency and polarization properties of the radiation on the multiple scattering of the emitting particles in the crystals becomes weaker.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 21–43, June, 1991.     

Effectively two-dimensional mechanism of axion and neutrino bremsstrahlung in the interaction of electrons with nuclei in a magnetic field

Axion and neutrino bremsstrahlung from electrons in the processes $e(Ze) \to e(Ze)a(\nu \bar \nu )$ is considered within the proposed two-dimensional covariant method for calculating Feynman diagrams in an external magnetic field. General expressions for squared matrix elements, as well as for the probability and power of this radiation, are obtained for a nonrelativistic nondegenerate electron gas. The energy scale f _a characteristic of the violation of global Peccei-Quinn symmetry is constrained by comparing the contributions of the above processes to the emissivity of magnetic neutron stars.     

Influence of acoustic waves on coherent bremsstrahlung from 20 MeV electrons

We investigate coherent bremsstrahlung from electrons with the energy 20 MeV incident at small angles to crystallographic planes of quartz crystal periodically deformed by acoustic waves. Depending on the parameters of the acoustic wave and on the incidence angle, the presence of deformation can either enhance or reduce the bremsstrahlung cross-section.     

Role of bremsstrahlung radiation in limiting the energy of runaway electrons in tokamaks

Bremsstrahlung radiation of runaway electrons is found to be an energy limit for runaway electrons in tokamaks. The minimum and maximum energy of runaway electron beams is shown to be limited by collisions and bremsstrahlung radiation, respectively. It is also found that a massive injection of a high-Z gas such as xenon can terminate a disruption-generated runaway current before the runaway electrons hit the walls.     

Polarization bremsstrahlung at collisions of fast ions with multiatomic targets

The processes of polarization bremsstrahlung at collisions of fast ions with linear chains consisting of isolated atoms have been considered. The intensities and angular distributions of radiation spectra have been obtained for an arbitrary number of atoms in a chain. It has been shown that the interference of the photon emission amplitudes leads to a noticeable change in the spectral angular distributions of polarization bremsstrahlung as compared to the distributions at collisions with an isolated atom. The results allow standard generalization to the cases of polarization bremsstrahlung at channeling of fast ions over surfaces and in solid lattices.     

Quantum effects on electron-proton bremsstrahlung spectrum in a two-component plasma

The electron-proton low energy bremsstrahlung process is investigated in a two-component plasma. The corrected Kelbg potential taking into account the quantum effects is applied to describe the electron-proton interaction potential in a two-component plasma. The straight-line trajectory method is applied to the motion of the projectile electron in order to investigate the variation of the bremsstrahlung cross-section as a function of the scaled impact parameter, thermal de Broglie wavelength, projectile energy, and photon energy. The results show that the quantum-mechanical effects decrease the bremsstrahlung cross-sections when the de Broglie wavelength (λ) is greater than the Bohr radius (a₀). It is also found that the quantum effects are important only for the region of impact parameters b < 3a ₀. Received 13 March 2001     

Three-body recombination of ions with electrons in cooler-storage rings

Three-body recombination (TBR) of bare ions with free electrons of anisotropic velocity distribution is discussed in the context of recombination experiments in cooler-storage rings. The recombination rates are derived from the electron impact ionization rates using the modified Saha equation. Analytical expressions for the TBR rates and new scaling rules with main quantum number n, atomic number Z, and both transverse and longitudinal electron temperatures are derived for the low n-states regime probed in experiments in cooler-storage rings. In this context, the discrepancies found between measured rates and predictions for radiative recombination for the e + Ne¹⁰⁺ system [Gao et al., Phys. Rev. Lett. 75 (1995) 4381] are discussed. Present results show that TBR rates for the flattened electron beam velocity distribution cannot account for the recombination enhancement observed at very low relative energies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of the finite size of a nucleus having a magnetic dipole moment on the bremsstrahlung of longitudinally polarized electrons

Expressions are obtained in the Born approximation for the differential cross section and circular polarization of the bremsstrahlung radiated by longitudinally polarized electrons in the field of a nucleus having a magnetic dipole moment. Account is taken of the rms radius and the fourth moment of the charge distribution and the magnetic moment of the nucleus. The effects of r² and r⁴ on the spectrum of the charge and magnetic bremsstrahlung are studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 97–104, September, 1969.The authors thank professor A.A. Sokolov for discussion of these results.