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.     

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.     

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.     

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.     

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.     

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.     

Positron annihilation in neutron-irradiated p-type Ge

Abstract

Observations of sintering of fine particles of diamond, SiO₂ and Al₂O₃ and possibly Al in a neutron radiation field are reported. Possible mechanisms are discussed and it is concluded that the transport of interstitials to the surface is most probably the responsible mechanism. This may occur by diffusion, channeling or focusing, and it is suggested that measurement of radiation sintering at two temperatures will show if the effect is thermally activated diffusion and that examination of changes in stoichiometry of the sintered interface will show if focusing or channeling dominates as a transport mechanism. Computations are also presented which show that the solar wind on the lunar surface could sinter dust into the observed clumps in a reasonable length of time.     

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.     

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.     

Calculation of line width in channeling radiation by optical potential method

Abstract

The line width problem in channeling radiation is considered in terms of the optical potential method. It is shown that the plasmon excitations do not contribute to the line width.     

Study of fission fragments channeling in silicon monocrystals

Abstract

The research results of ²³⁵U fission fragments channeling in silicon semiconductor detectors of heavy ions are reported. The analysis of the pulse height defect enabled to obtain the experimental relationship between energy losses in silicon due to nuclear collisions and the energy of fission fragments in LSS-units. The experimental values obtained more than twice exceed the computed ones given by Heines and Whitehead based on Lindhard's theory.     

Channeling radiation: Past achievements and future prospects

Abstract

When a relativistic charged particle passes through a single crystal very nearly along a major crystalline plane or axis so that it is channeled in that direction, it undergoes periodic motion in the plane transverse to this direction and hence it can radiate. Quantum mechanically, this channeling radiation corresponds to a radiative transition between two eigenstates of the transverse crystalline potential; when the transition occurs between two bound states, a sharp spectral line is emitted. When there are only two bound states (for incident electrons), or when the interplaner potential is nearly harmonic (as for incident positrons), the emitted radiation is nearly monochromatic. Since the discovery of channeling radiation at the LLNL Electron-Positron Linear Accelerator, many of its properties have been delineated, both there and elsewhere. For example, channeling radiation is very intense, forward-directed, easily tunable, and for the planar case, linearly polarized. Channeling radiation has been used as a probe both of the interplanar potentials and other properties of perfect crystals and of the effects of impurities and defects in imperfect crystals. Finally, channeling radiation has great potential use as a photon source for numerous other applications in several fields of science and technology.

This paper is intended to keynote the first International Conference on Coherent Radiation Processes in Strong Fields by recalling some history of the discovery and exploitation to date of channeling radiation. Studies of channeling radiation, in addition to elucidating the physics of the process itself, its application to the determination of properties of perfect and imperfect crystals, and its potential application to a large variety of fields by its use as an intense, monochromatic, forward-directed, tunable, and polarized photon source, have spawned an entire industry of studies of other coherent radiation processes, all consisting of photon production from beams of relativistic charged particles traversing periodic structures, which constitute the principal subject matter of this Conference. This paper will be limited to the discussion of channeling radiation and some of its applications. It will be in the nature of an illustrative exposition, showing many of the features of channeling radiation and its applications in a qualitative way. Several detailed studies of channeling radiation will be presented later in the Conference.     

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.     

Binary encounter electron spectroscopy under channeling incidence conditions

Abstract

This paper briefly reviews fundamental and applied aspects of our recent study of binary encounter electrons induced by MeV/amu ions. The binary encounter electron spectroscopy (BEES) under channeling incidence conditions enables precise measurements of the ion-beam shadowing effect, providing unique information on ion-solid interactions.     

Theory of coherent bremsstrahlung

Abstract

We review the phenomenon of coherent bremsstrahlung (CB) of electrons in a crystal. This well-studied process in a new light after the realization by Andersen [J. U. Andersen, Nucl. Inst. and Methods 170, 1 (1980)] that CB and the phenomenon of channeling radiation (CR) are two aspects of the same physical process. These two types of radiation may be described in a unified framework by the use of Bloch functions for the electron in the crystal. The kinematics of CB is discussed from this viewpoint and a model calculation for CB in a strained-layer superlattice is presented.     

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.     

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.     

Peculiarities of heavy ion channeling

Abstract

Depth distributions of implanted Mg⁺- and Ca⁺-ions and the corresponding radiation damage were studied for different channeling orientations of silicon crystals. The shape of the implantation profiles is discussed by using simple models for dechanneling and energy loss processes. A correlation between dechanneling, damage production and depth distributions of the channeled ions could be observed. This correlation is seen by the maxima shifts in damage and implanted ion distributions between channel and random incidence.     

Channeling in single-wall nanotubes: Possible applications

The relevant potentials are calculated and used to investigate the trajectories and various characteristics of axial channeling of high-energy positively charged particles in the recently discovered single-wall nanotubes (SWNTs). The application of SWNTs in high-energy physics, specifically, in future colliders, is discussed, in view of the fact that the dechanneling length in SWNTs is much longer than in single crystals. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 304–307 (10 September 1997)