Radiation from relativistic positrons channeled in single crystals

The characteristic of the radiation emitted from relativistic positrons channelled between the major planes in single crystal have been calculated. The sharp peaks of the emission spectrum in the keV region is predicted correctly and is compared with previous calculations.     

Planar channeling potential in heavy ionic crystals

The planar channeling potential in finite-size heavy KCl-like crystals is studied.     

Modification of the theory of diffracted channeling radiation for axial electron and positron channeling

A new type of combinational channeling radiation induced by subbarrier (interband) transitions for the transverse motion of relativistic electrons (positrons) is studied. It is known as diffracted channeling radiation (DCR). The formula describing the DCR angular distribution in the case of axial channeling is obtained by taking into account the band structure of energy levels for the transverse motion of electrons (positrons). It is shown that, in the two-wave approximation of the wave function A(r) of virtual photons, the DCR matrix elements in the dipole approximation for axial and plane channeling coincide formally (with the dimension of the problem taken into account). However, the formulas for DCR angular distributions in the cases of axial and plane channeling differ considerably.     

Thermalization of positrons and positronium atoms in ionic crystals

It is shown that total positron scattering on acoustic and optical lattice vibrations in atomic semiconductors and ionic crystals causes total positron thermalization before annihilation at both high and low (T≤10 K) temperatures.     

Proton channeling in ionic and semiconducting single crystals

Measurements have been made of the energy distributions of protons passing through NaCl, KCl, Si, and Ge single crystals at various angles with respect to the {100} and {110} planes. The fraction of protons passing through lattice channels has been evaluated, and its dependences on the angle of incidence, the chemical composition of the sample, and the proton energy have been found.     

Spectral and total intensity of radiation of relativistic electrons and positrons with planar channeling

The spectral and total intensity of radiation of relativistic electrons and positrons is considered within the framework of a Langevin approach to the theory of transit of fast charged particles across a crystal. It is shown that one of the terms of the total intensity of radiation increases exponentially rapidly along those segments of the path that are in regions in which the potential of the planar channel of the crystal has negative Gaussian curvature. Surgut State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 42–44, September, 1997.     

Relativistic electron radiation spectra at axial channeling in crystals

Typical trajectories of relativistic electrons and the corresponding radiation spectra at axial channeling in silicon and diamond crystals are revealed and constructed using the ©Mathematica 6.01 computer code. The radiation spectrum of 300 MeV electrons averaged over the trajectories in the 〈100〉 direction in a diamond crystal well coincides with the experimental spectra measured in work [6].     

Large-angle channeling radiation from relativistic electrons in optically transparent crystals

In the work within the frame of quantum electrodynamics are obtained new formulae describing the large-angle photon emission from channeled electrons with taking into account of the dispersion of refractive index. Calculations based on these formulae show that the spectral and angular distributions of large-angle optical and ultraviolet radiation from planar channeled sub-GeV electrons in optically transparent crystal reflect the band structure of transverse energy levels of channeled electrons. Comparison with ordinary Cherenkov radiation spectrum reveals that channeling (depending on the beam energy) leads to sufficient change of the large-angle emission spectrum.     

Kinetics of positron annihilation in ionic crystals

A kinetic scheme of decay of positron and positronium states for the general case — a real ionic crystal with point lattice defects and a developed surface — is examined. The properties of positron and positronium states in the volume of the crystal, positron and positronium color centers, and surface positron and positronium states, are analyzed. It is shown that all the available experimental data qualitatively confirm the conclusion of an annihilation mechanism based on the postulated kinetic scheme of positron annihilation in real ionic crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–55, September, 1977.     

Influence of quantum fluctuations on the stochastic dynamics of the channeling effect of relativistic electrons and positrons

A system of equations describing multiple scattering by crystal nuclei and electrons, and also quantum fluctuations of the coordinate and momentum operators of fast charged particles has been obtained. Quantum corrections to the classical equations of motion have been determined in quasiclassical approximations. A computer simulation of 855-MeV electron and positron motion in the (110) planar channel of a Si crystal has been carried out. The inclusion of quantum fluctuations in the equation of motion affects the dynamics of the electron channeling effect in planar crystal channels considerably; in particular, intense dechanneling (heating) occurs. Intense rechanneling (cooling) occurs in the case of positrons.     

Generation of X-ray radiation during planar channeling of relativistic electrons in crystals

A classical model of the emission of radiation by relativistic electrons in a crystal has been developed using the form of the potential maximally close to its actual form. The dynamics of electrons with energies 20–25 MeV performing channeling in crystals is simulated numerically. The generation of electromagnetic radiation that accompanies this motion has been considered. It has been shown that, in the given electron energy range, this radiation corresponds to the X-ray spectral band with characteristic photon energies of up to 40 keV. The radiation yield is estimated. The requirements to the electron beam parameters are formulated based on the results of the simulation. It has been shown that numerical simulation gives results that correlate with the analytic results obtained earlier and with the experimental data.     

Anomalous passage of ultrarelativistic electrons in thick single crystals in axial channeling

The dynamics of ultrarelativistic axially channeled electrons in thick crystals is studied. It is revealed that a certain fraction of initial electrons have anomalously large dechanneling depths. It is shown also that the dechanneling depth in heavy and light crystals are comparable. In some cases, the number of channeled electrons can strongly increase at the expence of quasi-channeled electrons.

The problem of quasichanneling is also considered.     

Contribution of the second-order born approximation to the coherent bremsstrahlung cross section by relativistic electrons and positrons in crystals

Equations for the cross section and polarization of the coherent bremsstrahlung emitted by relativistic electrons and positrons in crystals are obtained taking into account the contribution from the second-order Born approximation. The radiation cross section and polarization in the field of the atomic plane is considered as a function of the charge sign of the particle.     

Lattice steering (channelling) of relativistic positrons from positive muons implanted in crystals

We report on the first observation of the lattice steering (“channelling”) of relativistic positrons (e⁺) resulting from the decay of thermalized positive muons (μ⁺), implanted in an Au single crystal at 100 K. The steering effect manifests itself in a blocking pattern in the lang100rang crystal directions (relative depth at least 20%, FWHM about 0.1°) and supports the view that at low temperatures μ⁺ in Au are well localized in octahedral interstices. The potential of the new technique is briefly outlined.     

Electromagnetic radiation of channeling electrons and positrons

General formulae for the spectral and angular distributions of radiation of relativistic channeled particles have been derived. These formulae are valid both for γ-quanta radiation and for radiation in the optical region. The problem of experimental detection of this radiation is discussed.     

Surface positron and positronium states in real ionic crystals

Positron annihilation has been used to examine the positron and positronium states at the surfaces of (100) and (110) faces of KCl and KCl-5% AgCl single crystals. The measured and theoretical parameters differ from the analogous bulk values; exposure of KCl to Ps atoms produces mainly A₊, F₊, and D₊ centers. It is shown that this method can be used to examine surface states and surface electron-density distributions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 76–81, January, 1978.