Radiation from 39 and 45 MeV electrons channeled in Lithium Niobate

Abstract

Channeling radiation from 39 and 45 MeV electrons channeled along the <0001> axis, the (0110) plane and the (1210) plane of a 30 μm thick LiNbO ₃ crystal has been measured. Calculations of the planar crystal potentials were performed by means of the many-beam formalism. Good agreement between theory and experiment is obtained for the planar channeling radiation. Associated with channeling additional radiation lines have been observed, which may be explained by a periodic perturbation of the continuum potential.     

The channeling of light ions in the cubic and tetragonal (ferroelectrically polarized) phases of BaTiO3

Abstract

The channeling properties of thin BaTiO₃ crystals at temperatures both above and below the ferroelectric Curie point (T _c ? 120°C)have been measured with 3.8 MeV protons. In the cubic phase (T > T _c ), values for critical angles and minimum yields, have been measured for the major crystal axes and planes by detecting backscattered and transmitted protons, and characteristic X-rays. These values are compared with those predicted by current theories of ion channeling. In the tetragonal phase (T < T _c ), measurements were made with single ferroelectric domains. The channeling characteristics of the (100) planes in BaTiO₃ are found to be strongly dependent on the relative orientations of the electric polarization vector P, the (100) planes, and the beam direction. For example, when P lies in the channeling plane, strong channeling occurs; but when P is normal to the plane, the incident beam is rapidly dechanneled. These effects are attributed to the relatively large ionic displacements, and the strong internal electric fields in the ferroelectrically polarized state.     

Measurement of quadrupole moments of unstable nuclei during channeling and the possibility of revealing the effect of vector polarization of tensor-polarized beams of nuclei

General relations are deduced which describe the spin dynamics of beams of nuclei with the initial tensor and vector polarizations upon planar channeling in bent crystals. Our analysis demonstrates the possibility of revealing the effect of vector polarization, as predicted by Baryshevsky and Sokolsky, which occurs upon the planar channeling of a beam of nuclei with initial tensor polarization. The planar channeling of a beam of nuclei with the initial tensor and vector polarizations may be used to determine the quadrupole moments of unstable nuclei with a small lifetime of up to the order of 10^?7 s. The quadrupole moments of nuclei with a lifetime on the order of 10^?7 s cannot be measured by known techniques, including optical methods.     

Vector polarization effect appearing in tensor polarized beams of nuclei under channeling conditions and during measurements of the quadrupole moments of unstable nuclei

General relations describing the spin dynamics of beams of nuclei with initial tensor and vector polarizations have been derived upon planar channeling in bent crystals. The performed analysis indicates that the vector polarization effect predicted by Baryshevsky and Sokolsky can be detected, occurring upon the planar channeling of a beam of nuclei with initial tensor polarization. The planar channeling of a beam of nuclei with initial tensor and vector polarizations can be used to determine the quadrupole moments of unstable nuclei with small lifetimes, up to 10⁻⁷ s. The quadrupole moments of nuclei with lifetimes of about 10⁻⁷ s cannot be measured via known methods, including optical methods.     

Channeling radiation: Theory,semi-classical simulations

Abstract

An introduction to channeling and channeling (Kumakhov) radiation is given. Relativistic and quantum-mechanical effects are discussed in function of the electron or positron energy. Phenomena of Quantum Electrodynamics in strong macroscopic field, which can be tested in channeling conditions, are described. Recipees for semi-classical Monte-Carlo simulations are presented and one of them compared with experiments at 10 and 150 GeV.     

Alternate dissociation of the screw dislocations in a (0 0 1) buried small-angle twist boundary in silicon

The square dislocation network of a (0 0 1) buried small-angle boundary in silicon was observed by dark-field transmission electron microscopy to examine the structures of more than 100 dissociated dislocation segments. Images were taken with g = (2 2 0), using a many-beam case along the reciprocal lattice row. Dissociation occurs on alternate close-packed planes without systematic rule, although a degree of ordering is taking place. Most of the dislocation segments have lengths equal to half of the square network period. Image simulation studies revealed that their experimental contrasts cannot be explained from the usual assumption of straight dislocations running in an infinite crystal. However, if these dislocations are supposed close and parallel to a nearby free surface, a reasonable agreement is found between the micrographs and the simulated images. A three-dimensional elastic model is proposed to explain the contrasts of the dislocation network.     

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.     

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.     

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.     

Carrier removal by implanted IONS in GaAs

Abstract

A model for calculation of the range distribution of energetic ions with taking into account the channeling effect is proposed. The measurement of the depth distributions of boron ions in silicon crystals implanted at 13.6 and 91 MeV revealed significant difference between the measured and the calculated range profiles when the channeling effects have not been included in the calculation. In spite of deminishing the critical angles of channeling with growing ion energy the probability of the capture of ions into the channeling regime is significant in case of high energy implantation even when the incident angles are 7–10° off the main crystallographic directions.     

Damage and lattice location studies in Hg implanted Hg1–xCdxTe

Abstract

Rutherford backscattering (RBS) and ion induced X-ray (PIXE) channeling experiments have been used to study the damage accompanying Hg and Al implantations into Hg_0.8Cd_0.2 Te and its annealing as well as to determine the location of Hg in the crystal.

The damage induced by the implantation of 300 keV Hg and 250 keV Al ions at room temperature was found from RBS channeling studies to reach a saturation level at doses of 1 × 10¹⁴ cm^?2 and 3 × 10¹⁴ cm^?2 respectively. The damage resembles that characteristic for extended defects and it anneals at ≈ 300°C.

The location of the constituents of Hg implanted Hg_0.8 Cd_0.2 Te was studied by PIXE channeling observing the characteristic X-rays for each element. Angular scans indicate that the channels are mostly blocked by Hg atoms for both unannealed and, to a lesser extent, annealed crystals. This observation supports the suggestion that interstitial Hg atoms may be responsible for the conductivity of Hg implanted Hg_1–x Cd_g Te.     

Proton channeling studies in thin crystals with a supercollimated beam

Abstract

A supercollimated beam of 4 MeV H^? ions with an angular spread of 1.5 × 10^?3 degrees, a diameter of 25 μ and a current of 10 picoamps was used to study the axial and planar channeling characteristics of single crystal silicon samples ranging in thickness from 0.5 to 1.0 μ. Since the angular spread of the beam is much smaller than most of the gross angular phenomena associated with channeling, it is possible to study the detailed characteristics of both planar and axial channeling with greater precision than before. Preliminary results indicate that this technique will allow a direct study of interatomic or continuum potential distributions and will also be useful for studying nuclear multiple scattering as a function of the tranverse energy of channeled particles relative to atomic rows and planar directions.     

Resonant influence of hypersound on the radiation of an axially channeled electron

Abstract

The spectral intensity of the radiation emitted by an axially channeled electron in a single crystal excited by a longitudinal hypersonic wave propagating along the channeling direction has been calculated for the energy range 10MeV ≤ E ≤ 100 MeV. It has been shown that under the influence of acoustic vibrations excited in the single crystal a resonant intensification of the electron channeling radiation, a variation of its spectral distribution as well as inverse radiative transitions are possible.     

Toward a three-dimensional treatment of channeling radiation

Abstract

Many authors have studied channeling radiation theoretically by means of the so called many beam approach. For axial channeling this approach tends to involve rather large eigenvalue problems if high accuracy is desired. With the extension to three-dimensional problems in mind we have therefore investigated the possibilities for a more efficient solution of these eigenvalue problems. The Lanczos algorithm [2] used in the preparation of Fig. 1 provides a substantial reduction of the numerical effort. We have also obtained some preliminary results for three-dimensional corrections to the tranverse problem. These corrections were found to be three-dimensional rather than longitudinal in character and tend to reduce the photon energy. The corrections are smaller than 1 percent for 4 MeV electrons in silicon. A more thorough investigation of these effects for the low energy part of the spectrum and a study of their impact on the high energy radiation [3] is now in progress. The conventional transverse many beam approach based on the Lindhard continuum approximation does not describe the latter effect.     

Fast-ion channeling in fullerene crystals

The motion of H⁺ and He⁺ ions with energies of 230 and 2000 keV in C₆₀ and K₃C₆₀ crystals was calculated by the Monte Carlo method. Ion channeling was shown to occur in the 〈100〉 and 〈111〉 directions. The main parameters characterizing channeling were determined. Medium-energy ions were found to be preferable in the detection of channeling in C₆₀ films.     

Effect of light polarization on holographic recording in glassy azocompounds and chalcogenides

Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As₄₀S₁₅Se₄₅ film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As₄₀S₁₅Se₄₅ with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As₂S₃ film, LiTaO₃:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO₃:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.     

Lattice site of Er in LiNbO3:Mg,Er crystals

Abstract

The lattice site of Er has been determined in LiNbO₃:Mg, Er crystals by the Rutherford backscattering channeling technique. It has been shown unambiguously that Er occupies Li site in the crystal lattice. OH absorption band has not been found in the 3500–3525 cm^?1 region. The RBS result confirms the assumption that OH^? ions may form defect complexes only with those trivalent impurities which replace niobium.     

Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L _A , andL _G scatterers, according to their polarization characteristics. TheS, L _A , andL _G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.     

Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

We present and apply a new method to measure directly weak magnetization in thin films. The polarization of a neutron beam channeling through a thin film structure is measured after exiting the structure edge as a microbeam. We have applied the method to a three-layer thin film structure acting as a planar waveguide for polarized neutrons. The middle guiding layer is a rare earth based ferrimagnetic material TbCo₅ with a low magnetization of about 20 mT. We demonstrate that the channeling method is more sensitive than the specular neutron reflection method.     

Hg ION implantation in silicon: Lattice disorder created and hg atom lattice location

Abstract

The lattice disorder produced by 42-keV and 75-keV Hg ions implanted in Silicon at room temperature and the lattice location of the Hg atoms were studied by means of the channeling technique with a 2.0 MeV ⁴He⁺ beam. The damage produced was found to increase linearly with ion dose until a saturation value, connected to the ion range, is reached. The number of Si atoms displaced for Hg ion implanted was evaluated and compared with the theoretical expectation. The substitutional Hg fraction is connected to the disorder produced: the replacement mechanism is discussed.