ISSN:0033-7579    

DOI:10.1080/00337577208231114

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

The critical angle for axial proton channeling has been measured as a function of temperature for a number of different crystals. The results are consistently below the values predicted from simple analytical estimates. The magnitude of this discrepancy does not seem to depend on the specific lattice structure. In point of fact, all the results are consistent with a relationship ψ1/2= α(p/a). ψ1, where ψ1is Lindhard's characteristic angle, and α(p/a)is a function of the thermal vibrational amplitudep, measured in units of the Thomas-Fermi screening distancea. A formula for α(p/a), derived by Barrett from computer simulations, on the average predicts the magnitude of α quite well, but the observed functional dependence onp/ais considerably stronger than predicted.

ISSN:0033-7579    

DOI:10.1080/00337577208231115

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

Measurements are presented of ψ1/2, the width of the dip in the yield of CuL X-rays when Ne+, Ar+, Kr+and Xe+of 150–350 keV are channeled in the <110> axis of copper. Absorption of the X-rays and the slowing down of the beam are shown to limit the depth from which the detected X-rays can come. The magnitude of the effect of multiple scattering on the measured values of ψ1/2is shown to be small for all the projectiles except nech. The data has a dependence on (Z1/E) which is intermediate between that of the critical angles ψ1and ψ2. A numerical calculation of critical angle gives good agreement with the data. At low energy the calculation uses a potential which is a sum of screened Coulomb and Born-Mayer terms; at higher energy screened Coulomb alone is adequate. A simple analytical expression for the critical angle, based on a 1/r2atomic potential, may be used as an approximation to the results.

ISSN:0033-7579    

DOI:10.1080/00337577208231116

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

One of the major advantages of using wide-angle scattering-yield measurements to study channeling is the possibility for confining the observed yield to a thin zone at a chosen, variable depth beneath the target surface. The thickness of this thin zone is, at least at a not too large depth, proportional to the energy resolution of the experimental apparatus used, i.e. that it is usually proportional to the energy resolution of the applied particle detector. In the present paper is described the application of a magnetic particle spectrometer which, in a random substance, yields a depth resolution of ∼15 −30Å. Although this depth resolution loses some of its relevance when channeling occurs, it is possible to study details of channeling which are inaccessible in measurements where a solid-state detector is used. An example of this is demonstrated in an investigation of one single collision between particles and atomic strings and planes.

ISSN:0033-7579    

DOI:10.1080/00337577208231117

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

The channeling properties of thin BaTiO3crystals at temperatures both above and below the ferroelectric Curie point (Tc≈ 120°C)have been measured with 3.8 MeV protons. In the cubic phase (T>Tc), 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<Tc), measurements were made with single ferroelectric domains. The channeling characteristics of the (100) planes in BaTiO3are 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.

ISSN:0033-7579    

DOI:10.1080/00337577208231118

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

Axial channeling along the [00.1]c-axis and the [2λ.0]A2-axis of α-quartz as well as planar channeling along planes containing one of these two axes were studied. The effects of silicon and oxygen sublattices were separated by using backscattering for the silicon sublattice and the nuclear reactioq O16(d,p)O17* for the oxygen sublattice. Experiments were carried out with H+, D+and He4+in the 0.5 MeV to 2 MeV range. Results represented using well chosen reduced coordinates follow single laws satisfactorily. For axial channeling the full width at half minimum l1/2of the surface extinction curves depends on √Z1Z2/Ed. For the channeled fraction the reduced depth isxp2dZ1/E,p2corresponding, even for the silicon sublattice. to the thermal vibrations of the oxygen atoms. The results obtained for the minimum yields are in good agreement with those calculated. The depth dependence ofl1/2shown. A rather weak effect is observed for silicon planes (not larger than 50 per cent). For the oxygen planes, the effects are very small. Detailed calculations are carried out along the (01.1) plane for the silicon sublattice. An anglel1/2of 18′ is measured at the surface for 0.9 MeV deuterons. A diffusion model taking into account the multiple scattering by the oxygen atoms, considered as randomly distributed scattering centers, compares favourably with the experiments.

ISSN:0033-7579    

DOI:10.1080/00337577208231119

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

A beam of 1.2–MeV positrons was obtained by accelerating in a 1–MeV Van de Graaff positrons emitted by58Co. Channeling in thin silicon crystals was studied by measuring as a function of incidence angle the yield of positrons scattered through a few degrees into an annular detector. The measured axial and planar dips are compared partly to channeling dips in scattering yield for protons of corresponding energy, partly to classical calculations as well as calculations based on the dynamical theory of electron diffraction. In general, the results support the conclusion of a previous study of positron channeling in gold, that channeling of positrons may essentially be described by classical mechanics. In the present measurement, however, fine structure due to interference is clearly resolved. Also, for very weak planes, a complete departure from classical estimates is observed. Finally the dechanneling rate has been studied for axial channeling by measuring the dip in crystals of different thicknesses. It is found to be very close to the dechanneling rate for protons of similar energy, in contrast to the anomaleously slow rate indicated by the work of Behnisch,et al.

ISSN:0033-7579    

DOI:10.1080/00337577208231120

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

Analytical estimates and computer simulations were undertaken to perceive the motion of negative particles through a lattice structure, the interaction being classical binary scattering. Three distinct modes of particle motion along atomic strings were found depending on the magnitude of the transverse energy and the angular momentum L of the particle with regard to the string axis. At small and largeLincreased scattering on the strings as compared with random penetration dominates. At mediumLand negative transverse energy (bound state particles in the attractive potential) a rosette motion along the string occurs. In this case small impact parameters to the string atoms are avoided and thus an increased penetrability of the negative particles results. The influence of thermal lattice vibrations on these motions was studied.

ISSN:0033-7579    

DOI:10.1080/00337577208231121

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

The trajectories of Kr ions in the energy range 50 eV to 5 keV incident on the (100) and (110) surfaces of W have been followed using digital computational techniques, in which the interaction was treated as a multiparticle process rather than a sequence of binary collisions. At the higher energies (>2 keV), the latter process was found to be a good description also of the multiparticle event but at lower energies, many atoms were involved simultaneously in the ion scattering process. Probabilities of ion penetration into the surfaces were derived and compared with experimental data of Kornelsen and Sinha and penetration probability-energy functions shown to be similar. For the (100) surface penetration was prohibited below an ion energy of∼400 eV and angular and energy distributions of backscattered particles in this energy range were determined. Angular analysis showed a pattern illustrative of surface atomic symmetry but the energy distribution shows a sharply defined probability of backscattering in narrow energy range which lies between the maximum recoil energy in a binary collision and recoil from an infinitely hard surface. This indicates that, as energy decreases, the multiple collision process can be approximately characterised as a ‘soft’ surface of effective mass greater than the target atom mass. Because of the sharply defined energy of recoil and a quite well defined narrow cone of backscattering, these results suggest a method of producing quite mono-energetic neutral atomic beams by recoil of ions from surfaces, since neutralization is usually highly efficient at low ion energies. The energy of ions recoiling in a collision on an axis of symmetry between surface atoms is compared with the theoretical non-binary scattering model of Andersen and Sigmund.

ISSN:0033-7579    

DOI:10.1080/00337577208231122

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor

摘要:

The angular width of blocking has been studied for 120 keV protons back-scattered from Si single crystal in various crystallographic directions. It was shown that for the <111> string an effective interatomic distance which determines the critical angle of channeling is larger than the mean interatomic distance. The blocking half-width dependence on the implantation dose with boron 40 keV ions and on the following annealing temperature is obtained.

ISSN:0033-7579    

DOI:10.1080/00337577208231123

出版商:Taylor & Francis Group    

年代:1972

数据来源: Taylor