Planar Channeling Conditions Research Articles

Metastable Ar17+ (2 s) production by Stark-assisted resonant coherent excitation

We report on the selective production of the 2s metastable state of 390-MeV u−1 H-like Ar17+ ions by the resonant coherent excitation (RCE). The RCE to the 2s state was achieved by using Stark mixing of n = 2 states under planar channeling conditions in a 1-m thick Si crystal. The population of the 2s state was quantitatively estimated from the ionization probability in a stripper foil installed 50 mm downstream of the crystal. The 2s population was found to be 0.05 of the total number of Ar17+ ions at the stripper. The result is in good agreement with a theoretical calculation performed with the density matrix approach.

Backscattering/transmission of 2 MeV He++ ions quantitative correlation study

In this work we report on detailed findings of planar channeling oscillations of 2MeV He++ particles in (110) silicon crystal. The exact correlation and coherence mechanism between confined particles oscillating trajectories are analyzed theoretically and experimentally in backscattering/transmission geometry. Regular patterns of channeled He++ ion planar oscillations are shown to be dominated by the crystal harmonic-oscillator potential and multiple scattering effect. For the first time it was shown that under the planar channeling conditions trajectories of positively charged particles exhibit observable correlation dynamics, including the interference effect. Quantitative estimation of channeling efficiency is performed using path integral method.

First studies of 500-nm Cherenkov radiation from 255-MeV electrons in a diamond crystal

The first experiment on Cherenkov light from 255-MeV electrons passing through a 50-μm-thick diamond crystal in a special geometry allowing extraction of 500-nm Cherenkov light at a right angle with respect to the electron beam direction has been performed at the injector linac of SAGA Light Source accelerator facility. The dependence of 500-nm Cherenkov light intensity (separated by a band-pass filter) on the crystal rotation angle was measured by a CCD detector. The experimentally obtained rocking curve with an intense maximum is theoretically explained as the projector effect of Cherenkov light deflected by the exit surface of the crystal. The width of the rocking curve is explained by the convolution of the standard Tamm–Frank angular distribution of Cherenkov radiation with chromatic aberration, the multiple scattering of electrons in a crystal, and initial electron beam angular divergence. In addition, it is found that the Cherenkov light intensity did not change under the (220) planar channeling condition, which is consistent with a recent theory.

On the deflection of a positron beam by the miscut surface of an oriented crystal

Trajectories of relativistic positrons moving in the electric field of oriented crystal near its surface have been simulated. A positron beam enters the field of the crystal at a small angle to crystallographic planes. Thus, planar channeling conditions are satisfied. The miscut surface of a crystal has a specific shape of a step sequence of terraces. It has been shown that such a surface can deflect a noticeable fraction of the beam from the crystal surface by means of quasichanneling. The possibility of the experimental observation of this phenomenon has been analyzed.

Studies of relativistic electron scattering at planar alignment in a thin Si crystal

Abstract Experiments on 255-MeV electron scattering under (220) planar channeling conditions in a Si crystal were carried out at the linac of the SAGA Light Source. The spatial and angular distributions of electrons penetrating through a 20-μm thick Si crystal at different incident angles with respect to the (220) plane were measured, and features characteristic of the planar alignment were identified. The experimental results were compared with computer simulations, and showed a reasonable agreement. A comparison with doughnut scattering at axial channeling in the same crystal was also performed. It was confirmed that the planar alignment effect is weaker than the axial alignment effect. These studies are important for understanding the basic mechanism of electron scattering and radiation processes in a crystal.

Parallel code NSBC: Simulations of relativistic nuclei scattering by a bent crystal

The presented program was designed to simulate the passage of relativistic nuclei through a bent crystal. Namely, the input data is related to a nuclei beam. The nuclei move into the crystal under planar channeling and quasichanneling conditions. The program realizes the numerical algorithm to evaluate the trajectory of nucleus in the bent crystal. The program output is formed by the projectile motion data including the angular distribution of nuclei behind the crystal. The program could be useful to simulate the particle tracking at the accelerator facilities used the crystal collimation systems. The code has been written on C++ and designed for the multiprocessor systems (clusters). Program summaryProgram title: NSBC (Nuclei Scattering by Bent Crystal)Licensing provisions: Standard CPC license, http://cps.cs.qub.ac.uk/license/license.htmlProgramming language: C++ (g++, icc compilers)Computer: multiprocessor systems (clusters)Operating system: any OS based on LINUX; the program was tested under Novell SLES 10Has the code been vectorized or parallelized?: Yes. The code contains MPI directivesRAM: about 1 MB per processorNumber of processors: >1Supplementary material: the user manual readme.pdf, utility to generate the beam of particles Beam_Generator.exe, the pdf presentation that is commented in the Sample A.Classification: 7.10, 11.10External routines: MPI library for GNU C++, Intel C++ compilersRunning time:In general, the running time T depends on the number of both processors N and particles P hitting the crystal, as well as on the crystal thickness. It can be estimated by the ratio T[min]=3⋅10−5⋅αR[μrad]⋅P/(N−1) for the 2.66 GHz processors, where αR is the crystal bending angle. In our tests the simulations were performed for a few thousands of particles into the crystal up to several mm thickness. The number of the 2.66 GHz processors used counted up to 30. The running time of about 5 min was registered at above mentioned conditions.Nature of problem:Here we deal with planar channeling of fast particles in a bent crystal. The channeled projectile moves along bent planes being in such a way deflected at large angles from the initial direction of motion. This effect is recognized as accelerator techniques to shape the beam. Another attractive phenomenon is known as the volume reflection of quasichanneled projectiles. Volume reflected projectiles can also be deflected at essential angles. In general, channeled and reflected particles are deflected in opposite directions and the initial beam is split into two beams. Hence, there is the practical interest to model the beam tracking in a bent crystal, to obtain the characteristic angles of deflection, to estimate the number of particles, which can be effectively deflected at large angles.Solution method:Initially the beam of relativistic nuclei hitting the bent crystal is considered. The velocity of a particle is defined by two components. The component along the beam direction is relativistic, while the transverse component is nonrelativistic. The particle trajectory in a crystal is defined by the continuous potential of bent planes. Hence, to obtain the trajectory the classical equation of motion is solved numerically. The initial position of a nucleus in the channel is suggested to be random that can be obtained from the uniform distribution. To take into account the multiple scattering of projectiles on crystal both electrons and nuclei the corrections to the trajectory is introduced from time to time. Finally, at the projectile fly-out from the crystal one can obtain the transverse velocity as well as the deflection angle.Restrictions:As known the theory of the channeling effect implies the critical Lindhard angle θL. Channeling takes place when the angle θ0 between the bent planes and the velocity of a particle at the crystal entrance face undergoes the condition |θ0|<θL. The quasichanneling appears when |θ0| exceeds the value θL but remains close to this value. Thus, it is not recommended to input large values of the crystal orientation angle |θC| which defines the range of angles θ0 (see in Section 2). Nevertheless, in our simulations we found the program gives correct results in the broad range of crystal orientation angles, for example, −18θL≤θC≤4θL for 400 GeV protons. Characteristic values of critical angle θL are about 10 μrad for the energy of projectile about 100 GeV/uamu.

Resonant coherent excitation of ions in planar channel of crystal with inclined beam incidence

Resonant coherent excitation of relativistic heavy ions in crystals under planar channeling conditions is considered at beam incidence inclined with respect to the channeling plane. The beam inclination leads to effective exclusion of contribution of ions with small energy of transverse motion from observables of resonant coherent excitation. This effect can be used for resolution of the observables by individual ion trajectories.

Deflection of MeV Protons by an Unbent Half-Wavelength Silicon Crystal

The interaction of a 2MeV proton beam with an ultrathin unbent Si crystal was studied through simulation and experiment. Crystal thickness along the beam was set at 92nm, i.e., at half the oscillation wavelength of the protons in the crystal under planar channeling condition. As the nominal beam direction is inclined by less than the critical angle for planar channeling with respect to the crystal planes, under-barrier particles undergo half an oscillation and exit the crystal with the reversal of the transverse momenta; i.e., the protons are "mirrored" by the crystal planes. Over-barrier particles suffer deflection, too, to a direction opposite that of mirroring with a dynamics similar to that of volume reflection in a bent crystal. On the strength of such coherent interactions, charged particle beams can be efficiently steered through an ultrathin unbent crystal by the same physical processes as for thicker bent crystals.

Polarization of photons emitted in the process of resonant coherent excitation of relativistic ions under planar channeling conditions

Owing to the recent observation of the strong anisotropy of characteristic X-ray radiation accompanying the resonant coherent excitation of relativistic Fe24+ ions under planar channeling conditions in a silicon crystal, the resonant coherent excitation method can be considered as a candidate for a source of polarized X-ray radiation. The Stokes parameters of the radiation have been calculated using the density matrix approach. The behavior of the polarization characteristics of the radiation in various directions has been explained by considering the properties of the resonance part of the crystal field, which excites an ion and has the form of an elliptically polarized electric field.

Resonant coherent excitation of Li-like Ar15+ Ions in a Thin Si crystal

We investigated resonant coherent excitation (RCE) of 445 MeV/u Li-like Ar15+ under the planar channeling and non-channeling conditions. The K-shell and L-shell excitations are observed as decreases in the survival fraction of the Ar15+ ions after passing through a thin Si crystal. RCE under the planar channeling condition provides the opportunity to study a strong static Stark effect on the Li-like system with K-shell or L-shell vacancy. Under the non-channeling conditions, the observed excitation energies and the reported theoretical calculations were in reasonable agreement.

Angular anisotropy of characteristic x-radiation and auger electrons during the resonance coherent excitation of relativistic ions under planar channeling conditions

The results of new experiments on the resonance coherent excitation of relativistic highly charged ions under planar channeling conditions are analyzed by a method of the generalized master equation for the density matrix of ions. The results of calculations pertaining to the angular anisotropy of characteristic electromagnetic radiation, to the survival fraction of ions as a function of the trajectory of their motion in the channel, and to the excitation of autoionizing states of helium-like ions under the double resonance coherent excitation are in close agreement with measurement data. New experiments are proposed.

Channeling radiation from relativistic electrons in a thin LiF crystal: When is a classical description valid?

Calculation of channeling radiation from relativistic electrons under (1 1 1) planar channeling conditions in a LiF crystal is performed using both classical and quantum methods. The complicated shape of the periodic (1 1 1) planar potential allows the existence of a different number of discrete energy levels (quantum states) in the potential well formed by planes consisting of only Li atoms and the potential well consisting of only F atoms is large enough to apply the classical method. With increasing electrons energy, the classical method becomes valid to describe CR from under-barrier electrons in a deeper potential well, while one should still apply the quantum method to calculate CR from electrons in neighboring planes. Thus, the quantum feature of a CR spectrum from relativistic electrons maintains its quantum characteristics up to relativistic factors 1000.

Trajectory dependent resonant coherent excitation of planar-channeled ions in a thin Si crystal

We observed resonant coherent excitation (RCE) of 1s electron to the n=2 states in 390MeV/u Ar17+ ions passing through thin Si crystals of about 1μm in the {220} planar-channeling condition by measuring both the exit charge state and the exit angle of the emerged ions simultaneously in a list mode. The yield of the de-excitation X-rays from the Ar17+ ions was also measured. The thin crystal is suitable to study trajectory dependent RCE dynamics, because the exit angle of the high energy planar-channeling ion is uniquely related to the ion trajectory. From the de-excitation X-ray yield on resonance, we learned that RCE occurs with a fairly large probability within the traveling length of about 1μm, and a majority of the excited ions survive from ionization by the collisions with target electrons or nuclei, and de-excite into the ground state by emitting the X-rays. On the other hand, the observed resonance profile of the exit charge state is shallow and broadened by the Stark effect. We found that the ionization of the excited ions takes place when the ions travel close to the atomic plane from the observed relation between the charge state and the exit angle of the emerged ions.

Probing surface magnetism with ion beams

Ion beams can be used to probe magnetic properties of surfaces by a variety of different methods. Important features of these methods are related to trajectories of atomic projectiles scattered from the surface of a solid target and to the electronic interaction mechanisms in the surface region. Both items provide under specific conditions a high sensitivity for the detection of magnetic properties in the region at the topmost layer of surface atoms. This holds in particular for scattering under planar surface channeling conditions, where under grazing impact atoms or ions are reflected specularly from the surface without penetration into the subsurface region. Two different types of methods are employed based on the detection of the spin polarization of emitted or captured electrons and on spin blocking effects for capture into atomic terms. These techniques allow one to probe the long range and short range magnetic order in the surface region.

X-ray yields from high-energy heavy ions channeled through a crystal: their crystal thickness and projectile dependences

X-rays emitted from Ar17+, Fe24+ and Kr35+ ions of about 400MeV/u transmitting through a thin Si crystal of about 20μm thickness have been measured in a planar channeling condition and compared with those in a random incident condition. We have found that the X-ray yield from Ar17+ ions is larger for the channeling condition than for the random incidence, while those from Fe24+ and Kr35+ ions are rather smaller. Such tendencies are explained by considering the projectile dependences of excitation and ionization probabilities together with X-ray emission rates. A crude simulation has qualitatively reproduced these experimental results. When the crystal thickness is small, the X-ray yield is smaller in the channeling condition than in the random incident condition, because excitation is depressed. However, for thicker crystals, the X-ray yield is larger, since the survived population of projectile-bound electrons is larger due to small ionization probabilities under the channeling condition. This inversion occurs at a specific crystal thickness depending on projectile species. Whether the thickness of the used crystal is smaller or larger than the inversion thickness determines enhancement or depression of the X-ray yield in the channeling condition.

Effect of exit surface on neutral fraction in mega-electron-volt hydrogen beams transmitted through aluminum epitaxial foils under a planar channeling condition

Neutral fractions are measured for (0.4-0.8) MeV H{sup +} ion beams transmitted through aluminum epitaxial foils under the (100) planar channeling condition. Two foils are prepared: the first ({theta}=90 deg.) is almost perpendicular to the direction of the beam and the second ({theta}=30 deg.) is inclined at 60 deg. compared with the first. The measured neutral fractions, which are in the 10{sup -3} range, exhibit a minimum at the (100) channeling direction, however, the channeling effect is enhanced for the foil with {theta}=30 deg. The fraction is calculated for the two foils from the equilibrium fraction and the charge exchange between the ions and the surface atoms in a vacuum. Both the equilibrium fraction and the charge exchange depend on the exit positions of ions from the midplane of the channel. The measured energy dependence of the ratio (channeling/random) of the fractions is reproduced by calculation. Thus the results indicate that the transmitted charge-state distributions are modified by the charge exchange after the emergence of ions in a vacuum.

Convoy electron emission from resonant coherently excited 390 MeV/u hydrogen-like Ar ions

Energetic ions traveling through a single crystal are excited by an oscillating crystal field produced by a periodic arrangement of the atomic strings/planes, which is called Resonant Coherent Excitation (RCE). We have observed enhancement of convoy electron yields associated with RCE of 1s electron to the n =2 excited states of 390 MeV/u hydrogen-like Ar 17+ ions passing through a Si crystal in the (2 2 ̄ 0) planar channeling condition. Lost electrons from projectile ions due to ionization contribute to convoy electrons emitted in the forward direction with the same velocity as the projectile ions. With combination of a magnet and a thick Si solid-state detector, we measured the energy spectra of convoy electrons of about 200 keV emitted at 0°. The convoy electron yield as a function of the transition energy, i.e. the resonance profile, has a similar structure to the resonance profile observed through the ionized fraction of the emerging ions. It is explained by the fact that both enhancements are due to increase in the fraction of the excited states from which electrons are more easily ionized by target electron impact in the crystal than from the ground state.

Highly charged ions impinging on a stepped metal surface under grazing incidence

We report on energy loss measurements and charge state distributions for 60 keV ${\mathrm{N}}^{6+}$ and 75 keV ${\mathrm{N}}^{5+}$ ions scattered off a $\mathrm{Pt}(110)(1\ifmmode\times\else\texttimes\fi{}2)$ single crystal surface. In particular, the influence of surface steps on the energy loss and the outgoing charge states is discussed. The scattering angle and the angle of incidence are varied. We use grazing incidence conditions, i.e., the momentum perpendicular to the surface is low enough to prevent penetration through the first atomic layer. Image charge effects are observed leading to an additional projectile acceleration towards the surface. Outgoing charge states are detected from $1+$ up to $3+.$ Axial channeling conditions are defined by scattering along the azimuthal [001] direction, i.e., the projectiles are guided along the [001]-atomic rows. The energy loss for axial channeling is found to be somewhat larger and the energy distribution to be broadened in comparison with the spectra for planar channeling conditions. The broadening is attributed to the interaction of the projectiles with the side wall potentials of the surface channels. We find a strong increase in the energy loss with increasing scattering angle, which exceeds the calculated contribution of the elastic energy transfer by orders of magnitudes. These increased energy losses are attributed to the interaction of the ions with the surface steps.

Atom location by channeling enhanced microanalysis study of site distributions of alloying elements in titanium aluminides

The original equation of Spence and Taftø for quantitative determination of site occupancy using atom location by channeling enhanced microanalysis method has been extended to take into account both delocalization effect and the influence of point defects (antisite atomic distribution and vacancies). The outcome of this treatment suggests that, for crystals free of antisite defects, the accuracy of site occupancy is influenced by delocalization effect but is independent of both thermal and structural vacancies. For crystals free of structural vacancies, the accuracy of site occupancy is influenced by both delocalization effect and antisite defects, but is independent of thermal vacancies. The delocalization effect was shown to vary with channeling strength at a given channeling condition. For a binary ordered phase in which at least one of the host elements exhibits weak delocalization effect (as is the case for many transition-metal aluminides), a tangent-line method for obtaining the value of k (a parameter necessary for the calculation of site occupancy) was proposed, allowing the determination of the delocalization correction factor for the other host element. Application of this method to estimating the delocalization effect of Al in Ti3Al and TiAl under axial and planar channeling conditions, respectively, was demonstrated.

Alchemi Analysis of Cation Distributions in Spinel-Structured Compounds Using Oxygen as a Marker Element

Abstract The establishment of ALCHEMI (atom location by channeling-enhanced microanalysis) as a viable microanalytical technique owes much to its early success in determining cation distributions in crystals having the structure of the mineral spinel, MgAl2O4. The first application of ALCHEMI was to a synthetic chromite spinel, and geological applications to natural mineral spinels containing multiple 3d transition elements provided many of the early applications of the technique. Spinel has three distinct regular lattice sites, two cation sites and one anion site, occupying the 8a, 16d and 32e Wyckoff positions of the Fd3m space group. In projection along (800), tetrahedrally coordinated (8a) cations occupy one set of alternate planes, whereas octahedrally coordinated (16d) cations and, to a good approximation, the anions occupy the other set. In principle, therefore, ALCHEMI performed at an 800 planar channeling condition could be used to analyze site distributions in spinel, using oxygen as a marker for the planes containing the octahedral cations, as was early noted.