Axial Channeling Research Articles

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.

Deflection of high-energy negative particles in a bent crystal through axial channeling and multiple volume reflection stimulated by doughnut scattering

Different kinds of deflection in a silicon crystal bent along the 〈111〉 axis was observed for 150 GeV/c negative particles, mainly π− mesons, at one of the secondary beams of the CERN SPS. The whole beam was deflected to one side in quasi-bound states of doughnut scattering (DSB) by atomic strings with the efficiency (95.4±0.2)% and with the peak position close to the bend crystal angle, α=185 μrad. It was observed volume capture of π− mesons into the DSB states with a probability higher than 7%. A beam deflection opposite to the crystal bend was observed for some orientations of the crystal axis due to doughnut scattering and subsequent multiple volume reflections of π− mesons by different bent planes crossing the axis.

Analysis and interpretation of residence time distribution experimental curves in FM01-LC reactor using axial dispersion and plug dispersion exchange models with closed–closed boundary conditions

The liquid phase mixing flow pattern at low (20 < Re < 120) and intermediate liquid flow rate (120 < Re < 400) was studied by means of residence time distribution (RTD) experimental curve in an up-flow Filter Press electrochemical reactor (FM01-LC) bench scale. For this purpose, a plastic turbulence promoter was used with stainless-steel and platinised titanium structural meshes as electrodes in channel configuration. To visualize and determine the mixing flow pattern in the liquid phase, the stimulus-response technique was employed using dextran blue ( D M = 1.058 × 10 −11 m 2 s −1, 25 °C, in water) as model tracer. A theoretical analysis and approximation RTD experimental curves with axial dispersion model (ADM) and plug dispersion exchange model (PDE), with “closed–closed vessel” boundary conditions were used in order to establish a better approximation of the axial dispersion, stagnant zones, channelling and by-pass (preference flow) effects present at low and intermediate Re. RTD curves show that the liquid flow pattern in the FM01-LC deviates considerably from axial dispersion model at low Re, where the FM01-LC exhibits large channelling, stagnant zones, and dead zone. The PDE model represents fairly this deviation from ideal flow (less dead zone).

Structure of ultrathin silica films on Mo(112) studied via Classical and Quantum Mechanical Rainbow Scattering of Fast Atoms

AbstractThe structure of ultrathin silica films on Mo(112) has been investigated by grazing scattering of fast He atoms from the film surface. We discuss two recent experimental techniques which allow us to study the arrangements of surface atoms of these films in detail. The methods comprise: (i) classical rainbow scattering under axial surface channeling conditions; and (ii) fast atom diffraction. Our work demonstrates the attractive features of grazing fast atom scattering as a powerful analytical tool in studies on the structure of surfaces. From the analysis of our data we find a clearcut support for a structural model based on the arrangement of film atoms in terms of a 2D network. Copyright © 2010 John Wiley &amp; Sons, Ltd.

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.

Biaxially textured Ag films by grazing ion beam assisted deposition

The effect of grazing incidence 4 keV Ar + ion irradiation on the early stage of Ag thin film growth on amorphous Si was investigated. The double effect of axial and surface channeling resulted in grains oriented along the 〈110〉 axis in-plane, while the (111) out-of-plane texture was maintained. A slight average tilt of the (111) out-of-plane texture axis towards the ion beam direction is proposed to result from the difference between terrace and step edge sputtering yield. The observed tilt is consistent with a minimum erosion orientation of the surface profile.

NEGATIVE PARTICLE PLANAR AND AXIAL CHANNELING AND CHANNELING COLLIMATION

While information exists on high energy negative particle channeling there has been little study of the challenges of negative particle bending and channeling collimation. Partly this is because negative dechanneling lengths are relatively much shorter. Electrons are not particularly useful for investigating negative particle channeling effects because their material interactions are dominated by channeling radiation. Another important factor is that the current central challenge in channeling collimation is the proton-proton Large Hadron Collider (LHC) where both beams are positive. On the other hand in the future the collimation question might reemerge for electron-positron or muon colliders. Dechanneling lengths increase at higher energies so that part of the negative particle experimental challenge diminishes. In the article different approaches to determining negative dechanneling lengths are reviewed. The more complicated case for axial channeling is also discussed. Muon channeling as a tool to investigate dechanneling is also discussed. While it is now possible to study muon channeling it will probably not illuminate the study of negative dechanneling.

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].

Radiation processes accompanying the axially channeled positrons. Problem of stimulation of resonant transitions between different quantum states

An analytical expression is received for the effective interaction potential of a fast charged particle with the ionic crystal CsCl near the direction of axis ⟨100⟩ as a function of the temperature of the medium. By numerical analysis it is shown that the effective potential of axial channeling of positrons along the axis ⟨100⟩ of negatively charged Cl− ions practically doesn't depend on temperature of the media. The wavefunction and the energy spectrum of the localized state are investigated, and the possibility is proved for the formation of metastable, two-dimensional relativistic positron systems or the positron atoms (PA), i.e. the bound state of a positron in the present case with a negatively charged axis of ions. The problem of one-photon decay of PA is investigated. The possibilities of stimulation of resonant transitions between quantum states of PA by the external hypersound are investigated in detail.

High-efficiency deflection of high-energy negative particles through axial channeling in a bent crystal

Deflection due to axial channeling in a silicon crystal bent along the 〈111〉 axis was observed for 150 GeV/c negative particles, mainly π− mesons, at one of the secondary beams of the CERN SPS. The whole beam was deflected to one side with the efficiency of about 90% and with the peak position at the bend crystal angle α=43 μrad. The deflection occurs mainly due to doughnut scattering of above-barrier particles by the atomic strings of the crystal. However, due to a high probability of particle recapture into bound states with the atomic strings their contribution to the deflection should be about 15% for our case according to simulation results.

Channeling effect in electronic spectra produced by grazing impact of fast protons on insulator surfaces

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial channeling conditions. The differential electron emission probability is calculated within a distorted-wave formalism, taking into account realistic channeled trajectories. For electronic spectra along low-index crystallographic directions, we find that channeling effects on the electron emission yield depend strongly on the penetration rate of projectiles into the solid.

Azimuthal dependence and accurate determination of the half-angle in RBS channeling

Rutherford Backscattering Channeling (RBSC) is often used in the studies of single crystalline materials with regard to issues hinging upon the location of specific atomic species within the crystal lattice. Results are deduced from the characteristics of an angular scan curve representing the yield of close collision events from the constant scattering of the probing projectiles from the crystalline sample in a sequence of directions lying in a plane perpendicular to one particular crystal lattice direction. Such angular scans exhibit dips near major channels, and their angular widths are of concern. On a fundamental level, accurate determination of these angular widths in the case of axial channeling and their interpretation come up against uncertainties arising out of the absence of a preferred choice for the plane of scan. Here, we shall illustrate these uncertainties in a representative case, channeling of 2 MeV He + ions in the 〈1 0 0〉 direction of Si.

Structure of monolayer silica on Mo(1 1 2) investigated by rainbow scattering under axial surface channeling

The structure of a monolayer crystalline silica film grown on a Mo(1 1 2) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane (“axial surface channeling”) the corrugation of the interaction potential leads to an azimuthal out-of plane scattering with an intensity enhancement for the maximum deflection angle, the so called “rainbow”. From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.

Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.

Ion beam triangulation based on electron detection for studies on the structure of 1 ML Mnon Cu(001)

Recent developments in studies on the structure of surfaces based on ion beamtriangulation are discussed. We will outline recent experimental progress in the applicationof this method, which is closely related to the detection of the number of emitted electronsper incident ion during scattering under surface channeling conditions. Key features are thepronounced change of electron emission for the projectile beam aligned along a low indexcrystallographic direction in the surface plane (‘axial surface channeling’) and theinterpretation of data in terms of classical trajectory computer simulations. As arepresentative example we will discuss here studies on the structure of the low temperaturec(8 × 2) Mn/Cu(001) phase.

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast 3He and 4He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.

Diffraction of fast atoms under axial surface channeling conditions

For scattering of fast atoms under axial channeling conditions from surfaces diffraction effects have been observed in the distributions for scattered projectiles. Basic features concerning this recently observed new phenomenon in ion/atom surface scattering will be demonstrated for collisions of light atoms with well ordered clean and adsorbate covered surfaces of insulators and metals. We will focus our discussion on the scattering of 3He atoms with keV energies from a LiF(001) surface along low indexed axial surface channels and deduce the corrugation of the He–LiF(001) interaction potential from intensity modulations of the diffraction spots. Furthermore, we observe for the first time interference effects also for He+ ions which undergo charge exchange during scattering from the surface. The potential of the method for the investigation of structures of surfaces is discussed.

Critical parameters of channeling

Basic methods for calculating critical channeling parameters that play a decisive role in the problem of dechanneling of ions are reviewed. The analytic expressions derived for the threshold energy of channeling, the critical transverse energy, and the closest approach distance between particles and atomic chains or planes forming the channels, as well as critical angles of axial and planar channeling, are valid in a wide range of particle energies. The dependence of critical parameters of channeling on the temperature of crystals is analyzed.

High-Efficiency Deflection of High-Energy Protons through Axial Channeling in a Bent Crystal

Beam deflection due to axial channeling in a silicon crystal bent along the 111 axis was observed with 400 GeV/c protons at the CERN Super Proton Synchrotron. The condition for doughnut scattering of protons by the atomic strings of the crystal was attained. Such a condition allowed one to observe a beam deflection of 50 murad with about 30% efficiency. The contribution of hyperchanneled states of protons to the observed beam deflection was less than 2% according to simulation results.