Linear polarization of channeling radiation from axially channeled electrons

Abstract

The degree of polarization of channeling radiation emitted by axially channeled electrons has been calculated using the many-beam method. The polarization was found to be substantial, and to increase monotonically with the channeling angle. The many-beam results are compared with those obtained from the single-string approximation and are found to be significantly different.     

Steering of Sub-GeV electrons by ultrashort Si and Ge bent crystals

We report the observation of the steering of 855 MeV electrons by bent silicon and germanium crystals at the MAinzer MIkrotron. Crystals with 15 \(\upmu \)m of length, bent along (111) planes, were exploited to investigate orientational coherent effects. By using a piezo-actuated mechanical holder, which allowed to remotely change the crystal curvature, it was possible to study the steering capability of planar channeling and volume reflection vs. the curvature radius and the atomic number, Z. For silicon, the channeling efficiency exceeds 35%, a record for negatively charged particles. This was possible due to the realization of a crystal with a thickness of the order of the dechanneling length. On the other hand, for germanium the efficiency is slightly below 10% due to the stronger contribution of multiple scattering for a higher-Z material. Nevertheless this is the first evidence of negative beam steering by planar channeling in a Ge crystal. Having determined for the first time the dechanneling length, one may design a Ge crystal based on such knowledge providing nearly the same channeling efficiency of silicon. The presented results are relevant for crystal-based beam manipulation as well as for the generation of e.m. radiation in bent and periodically bent crystals.     

Stopping power of fast channeled protons in Hartree-Fock approximation

The electronic energy loss of MeV-protons in crystals is investigated in the Hartree-Fock approximation for the core electrons of the target. It turns out that each core electron can be regarded as stopping the proton independent of all other electrons without restriction by the Pauli principle. For the channeling stopping power the impact parameter dependent energy loss of a proton moving rectilinearly past a crystal ion is calculated in first Born approximation. Low excitations of the core electrons lead to a long range impact parameter dependence, whereas high excitations contribute to the energy loss proportional to the electron density sampled along the proton trajectory. The results are applied to 4 MeV proton channeling along the main channels of Si using Clementi wave functions for the core electrons and a free electron approximation for the valence electrons. The comparison with the experimental results of Clarket al. yields good agreement. In the high velocity limit a reduction of the channeling stopping power to 0.64(0.83) of the random value is predicted for Si(C).     

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.     

Spectral and polarization characteristics of (111) planar channeling radiation from relativistic electrons in diamond crystal

Channeling radiation of 900 MeV electrons in (111) diamond planar channels has been investigated using the computer simulation method. Two characteristic maxima have been obtained in the radiation spectra, one of which is due to molecular-type channeling. The polarization characteristics of the channeling radiation from relativistic electrons in the molecular states and quasi-atomic ones are obtained.     

Feasibility of an electron-based crystalline undulator

The feasibility to generate powerful monochromatic radiation of the undulator type in the gamma region of the spectrum by means of planar channeling of ultrarelativistic electrons in a periodically bent crystal is proven. It is shown that to overcome the restriction due to the smallness of the dechanneling length, an electron-based crystalline undulator must operate in the regime of higher beam energies than a positron-based one does. A numerical analysis is performed for a 50 GeV electron channeling in Si along the (111) crystallographic planes.     

Features of the formation of radiation spectra at (111) planar channeling of relativistic electrons in a Si crystal

The spectral intensity of (111) channeling radiation from electrons is numerically calculated at the energy varying from 100 to 900 MeV and different electron incidence angles relative to the (111) planes in thin Si crystals. The calculation results show that the channeling radiation’s spectra have a more complicated structure, and the total channeling radiation’s yield is several times larger than that at (100) or (110) channeling.     

Diffracted channeling radiation for axial electron channeling

It is shown that, in contrast to diffracted channeling radiation (DCR) in the case of planar channeling, transitions of type 2 → 1, 3 → 2, 4 → 3, and so on make the main contribution to DCR in the case of axial channeling along an isolated axis. The angular DCR distributions for channeling of electrons with the energy corresponding to the Lorentz factor γ = 100 are calculated for an isolated axis using the Coulomb-type potential.     

Radiation of channeled 800 MeV electrons in diamond crystals

Measured data on γ-ray emission for the low-energy region of the bremsstrahlung spectra produced by 800 MeV electrons in diamond crystals are presented. The obtained spectral and orientational characteristics of the γ-ray yield indicate that the low-energy radiation is associated with the channeling of electrons.     

Energy losses of electrons in oriented crystals

Abstract

Within the framework of the multistring model of axial channeling of electrons the method for calculating the losses to radiation in channeling is proposed. The physical model includes the multiple scattering, the beam and target characteristics. The simulated results are compared with the experimental data.     

Complete reorganization of the mode structure of the wave function of a channeled electron in a crystal with a superlattice

The dynamics of relativistic electrons subjected to planar channeling in distorted crystals is investigated using step-by-step numerical integration of the Schrödinger equation. It is shown that periodic distortion of a planar channel (the formation of a “superlattice”) can cause complete reorganization of the mode structure of the wave function. In the limit of weak perturbations of the crystal lattice, the simulation results are consistent with the analytical results from perturbation theory. The position of each line in the spontaneous emission spectrum in the case of resonant distortions depends on the length of the superlattice period.     

Channeling of neutral particles in layered structures

The wave equation for neutral particles (including photons) in periodical structure is considered. Its solution gives a general expression describing in two opposite cases Borman effect and the phenomenon of channeling. The channeling of ?-quanta and neutrons is found to be possible as a result of their interaction with either electrons or nuclei of the medium in the layers of different physical or chemical properties. This channeling may occur when the width of the layers is equal to or exceeds several lattice parameters. The critical channeling angles θc in the cases considered may range from one minute to one degree. The peculiarities of channeling at resonant energies are investigated. The conditions of the ring channeling are obtained.     

“Defect” states of channeling electrons

We develop a theory of quantum states caused by defects of the crystal lattice during planar channeling of electrons. Points defects are considered. We obtain expressions for the wave function and the transverse energy of the defect states and make estimates for {110} planar channeling in silicon by electrons with energies of 1–10 MeV. Substitutional, interstitial, and vacancy impurity atoms are examined. We propose using properties of the electromagnetic radiation of electrons in defect states for diagnostics of single crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp.68–72, April, 1990.     

Electromagnetic radiation of MeV electrons in thick aligned single crystals

Abstract

Earlier published theoretical models for MeV electrons are generalized in this paper. Different theoretical predictions for planar channeling and accompanying electromagnetic radiation in thick crystals are presented. A comparative analysis of theoretical and experimental spectra of photon radiation is given. Thickness dependences of channeling quantum state populations, radiation line broadening and photon flux intensities are obtained. The existence of planar electron channeling at 54 MeV in a silicon single crystal with a thickness of several millimetres is shown.     

Simulation of radiation energy losses by channeled relativistic electrons in a crystal

A computer code is developed to calculate the radiation energy losses (RELs) of electrons during both 〈100〉 axis and (100) plane channeling in a thin Si crystal. A computer simulation of these losses is carried out by taking the initial angular divergence of the beam into account, and the REL dependences on the angle of electron entry into the crystal are obtained for both axial and planar channeling (orientational dependences). The calculations are carried out in connection with experiments on the interaction of 20–255 MeV electrons with crystals conducted at the SAGA Light Source linear accelerator (Tosu, Saga, Japan). The simulation results show the possibility of using the orientational dependence of the RELs of channeled electrons in thin crystals to diagnose the initial angular divergence of the electron beam and to orient crystals.     

Mechanism for origination of a new kind of radiation during the channeling of relativistic electrons in a crystal

Singularities on the spontaneous radiation of relativistic electrons moving in the axial channeling mode in a crystal due to longitudinal electron vibrations are analyzed by quantum electrodynamics methods. A comparison is made with published experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 99–103, January, 1991.     

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].     

Volume capture of ultrarelativistic electrons in the axial channeling regime

Using computer simulation the volume capture effect of ultrarelativistic electrons in the axial channeling regime in crystals is investigated. The thickness, temperature and orientational dependences of the beam fraction captured in the channeling regime in the volume of the crystal are obtained.     

Impurity lattice location and recovery of structural defects in semiconductors studied by emission channeling

Doping of semiconductors by ion implantation usually requires implantation doses below 10¹³ cm^–2 to obtain typical impurity concentrations of <10¹⁸ cm^–3. The lattice location of impurities as well as the defect recovery after such low dose implantations can be studied using the emission channeling technique. In this technique, single crystals are doped with radioactive probe atoms and the channeling effects of electrons, positrons or -particles emitted from these atoms are measured. We present a quantitative analysis of electron emission channeling measurements after heavy-ion implantation into Si and III–V compound semiconductors by comparison with calculated channeling profiles based on the dynamical theory of electron diffraction. For In atoms implanted into Si, complete substitutionality was found after rapid thermal annealing to 1200 K. For lower annealing temperatures, the observed channeling effects indicate small mean displacements (of about 0.2 Å) of the In atoms from substitutional sites, caused by residual implantation defects. For GaAs, GaP and InP implanted at low temperatures with In or Cd isotopes, pronounced recovery stages around 300, 400 and 350 K, respectively, were observed and substitutional fractions close to 100% were derived after annealing above the stage.     

Kanalisierungseffekte von Elektronen und Positronen in Silizium- und Anthrazenkristallen

Channeling of 148 keV electrons and 247 keV positrons in monocrystals of silicon and anthracene is found in transmission experiments. In anthracene the measurements are carried out atT=130 K. The number of transmitted electrons incident along the [110] axis of silicon and the [001] axis of anthracene is remarkably reduced. The number of transmitted positrons incident along these axes shows pronounced peaks. The theoretical critical angles for channeling are compared with the observed values.