Parametric channeling and collapse of beams of charged particles with internal structure in crystals: 1. Features of parametric channeling of molecular ions in crystal channels

A new type of self-organized orientational motion of charged particles with internal energy structure (molecules, atoms, and in some cases nuclei and electrons) in crystals, i.e., parametric channeling, is proposed and studied in detail. A feature of this mode of motion is associated with the strong parametric coupling of orientational oscillations controlled by the interaction of the moving particle charge with averaged fields of crystal plane axes and vibrations caused by intraparticle processes. The features of parametric processes during channeling of molecular diatomic ions are studied. It is shown that this effect can cause “parametric collapse,” i.e., beam self-cooling, compression, and a decrease in its phase-space volume. For molecules whose axis is perpendicular to the channel axes, periodic parametric collapse occurs. For molecules oriented along the channel, beam self-cooling is an asymptotic, aperiodic, and irreversible process.     

Generation of X-ray radiation during planar channeling of relativistic electrons in crystals

A classical model of the emission of radiation by relativistic electrons in a crystal has been developed using the form of the potential maximally close to its actual form. The dynamics of electrons with energies 20–25 MeV performing channeling in crystals is simulated numerically. The generation of electromagnetic radiation that accompanies this motion has been considered. It has been shown that, in the given electron energy range, this radiation corresponds to the X-ray spectral band with characteristic photon energies of up to 40 keV. The radiation yield is estimated. The requirements to the electron beam parameters are formulated based on the results of the simulation. It has been shown that numerical simulation gives results that correlate with the analytic results obtained earlier and with the experimental data.     

Role of vacancies and adsorbed atoms in the channeling of molecular particles in CNTs

The channeling of atomic and molecular particles in carbon nanotubes is considered in the presence of vacancies on the walls and of adsorbed atoms inside them. It is shown that the significant influence of the indicated disturbance of nanotube structures greatly affects channeling, which makes it possible to use beams of atomic and molecular particles to probe nanotubes.     

Ion channeling in natural and synthetic beryl crystals

The transmission of ions by channeling through natural beryl and synthetic emerald has been studied extensively. The transmission ratios depend upon the angle of incidence with a full half width of less than 0.32°. While the maximum ratio obtained up to now is only 4×10⁻⁴ for 350 keV protons through a crystal of 21 μm thickness, the energy of the transmitted ions is high, the loss being in the order of a few keV/μm. About 60–80% of the particles emerging from the rear surface are ionized. By varying the ion species transmission could be observed up to atomic number 9. It is assumed that the transmission is facilitated by the existence of an electron free channel core. Higher transmission ratios can be expected for sufficiently perfect crystals.     

Electromagnetic radiation of relativistic channeling particles (the Kumakhov effect)

A new type of electromagnetic radiation characteristic for channeling in single crystals relativistic particles which has been recently predicted by Kumakhov and proved experimentally is reviewed. It is observed as one or several strong peaks in the soft part of the Bremsstrahlung, surpassing it by a factor of nearly one hundred. At moderate electron energies sharp peaks corresponding to spontaneous transitions between quantum eigenstates of the channeling particles were detected. But for high energy channeling electrons these peaks merge into one broad classical peak with the frequency determined by the collective potential of an atomic row or plane. This radiation also strongly depends on the charge sign, velocity and mass of the particle.     

Quantum effects for parametric X-ray radiation during channeling: Theory and first experimental observation

The theory of X-ray radiation from relativistic channeled electrons at the Bragg angles—parametric X-ray radiation (PXR) during channeling (PXRC)—is developed while accounting for two quantum effects: the initial population of bound states of transverse motion and the transverse “form-factor” of channeled electrons. An experiment was conducted using a 255 MeV electron beam from a linac at the SAGA Light Source. We have identified a difference in the angular distributions of PXR and PXRC and obtained a fairly good agreement between the theoretical and experimental results.     

Radiation source for medical X-ray examination,based on the use of electron radiation during channeling in crystals

One of the possible applications of relativistic electron radiation during channeling in crystals, i.e., the use of radiation for medical visualization, is considered.A setup scheme is proposed, the results of numerical simulation of electron dynamics in crystals and radiation generation and the results of calculations of dose loads are presented.     

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.     

Laser channeling in millimeter-scale underdense plasmas of fast-ignition targets

Two dimensional particle-in-cell simulations show that laser channeling in millimeter-scale underdense plasmas is a highly nonlinear and dynamic process involving longitudinal plasma buildup, laser hosing, channel bifurcation and self-correction, and electron heating to relativistic temperatures. The channeling speed is much less than the linear group velocity of the laser. The simulations find that low-intensity channeling pulses are preferred to minimize the required laser energy but with an estimated lower bound on the intensity of I approximately 5x10(18) W/cm(2) if the channel is to be established within 100 ps. The channel is also shown to significantly increase the transmission of an ignition pulse.     

Spatial and angular distribution of the heavy ion charge under channeling in crystals

A kinetic theory of passage of multiply charged heavy ions through crystals is developed that allows for diffusion in the transverse momentum space and ion-crystal charge exchange. The theory provides an adequate explanation for the observed angular distributions of heavy ions passing through oriented crystals, makes it possible to calculate the partial angular distributions of different charge states, and treats the discovered effects of “cooling” and “heating” of channeled ion beams in physical terms. The angular and spatial distribution of channeled ions with different energies is calculated. Whether a channeled beam of multiply charged heavy ions will be cooled or heated is related to the dependence of the electron capture and loss probabilities on the impact parameter when the ions interact with atomic chains. This interaction governs the run of the angular and spatial distribution of the channeled ion charge.     

共振相干激发的最新实验研究(英文)

在满足沟道效应条件下 ,高能离子在穿过晶体时 ,离子穿过的是周期性的原子序列或者有序平面阵列 .在入射离子的坐标系中 ,离子感受到一种振动场的作用 .如果与此振动频率相关的能量与离子的电子态跃迁能量相匹配 ,则离子有可能被激发 ,这种现象就称为共振相干激发 (RCE) .1998年 ,成功地观测到了类氢Ar17+ 离子以相对论速度穿过Si晶体时 ,其 1s电子到n =2态的共振激发 .此后 ,有关出射离子电荷态分布以及退激X射线谱的实验研究取得了很大进展 ,近期的实验研究还观测到了类氢离子的 1s电子到更高电子态 (n =3,4 ,5 )的RCE和更重离子的RCE ,以及类氦离子的RCE .这些实验结果有助于详细研究这种共振现象 .实验结果表明 ,RCE也能够作为研究高精度原子谱学的一种潜在的工具 . When energetic ions penetrate into a crystal in a channeling condition, they travel across a periodic array of the atomic strings or ordered planes. These ions feel an oscillating field in the projectile frame. If the energy corresponding to this frequency matches with the transition energy of the electronic states of the ions, the ions have a chance to be excited. It is called Resonant Coherent Excitation (RCE). We have succeeded in observing resonant excitation of 1s electron to the n=2 states in H-like...     

Effect of Moving Mass Center on Atomic Transition Probability in a Single-Mode Cavity

By considering a two-level atom coupling with a single-mode cavity field which is prepared in three different initial states respectively, the influence of spatial motion of the atomic centre of mass on the atomic transition probability is studied. It is shown that the oscillation with collapse and revival in atomic transition probability may be suppressed due to Doppler effect.     

Complex structure of resonant coherent excitation peaks for heavy relativistic hydrogen-like ions under planar channeling

The computer model for the resonant coherent excitation of heavy relativistic ions under planar channeling in crystals taking into account the fine structure of the energy levels of the orbital electron and the ion ionization from both the ground and first excited state is presented. The model has been used to explain the experiments carried out under planar channeling of 390 MeV/n ¹⁷⁺Ar ions. Reasonably good agreement for the calculated and experimental data has been obtained.     

Electron channeling,structure factor phases,polarity and atom site determination in crystals

We relate element characteristic electron energy losses and the accompanying X-ray emission under electron channeling conditions to the phases of structure factors. We discuss examples illustrating that structure factor phase information can be extracted from two-beam channeling experiments. This information has been used by several investigators to determine the absolute orientation of non-centrosymmetric crystals. We further discuss how the success of electron channeling in locating atoms in the crystal unit cell, ALCHEMI, is tied to phase information beyond the conventional one. From the perspective of diffraction-based crystallography, ALCHEMI provides information about the phase with which a particular element scatters into a reflection. Therein lies the strength of the ALCHEMI technique as compared to relying on diffraction intensities for the location of small concentrations of elements and neighbors in the periodic system.     

High-resolution atomic channeling using. velocity-selected atomic beam

High-resolution atomic channeling using velocity-controlled atoms may be able to overcome precision limitations of the conventional atom lithography. We have experimentally clarified the dependence of line width and contrast of atomic patterns in the channeling region on the velocity spread of the atomic source for the first time. Thermal or velocity-selected atomic beams prepared with a one-dimensional magneto-optical trap were employed as the atomic sources. We investigated the channeling characteristics by measuring the frequency shifts of the atomic absorption spectra in an intense standing wave light field. As a result, we can show that narrower line width and higher contrast atomic patterns are obtained as the velocity spread becomes narrower. An atomic pattern with an estimated line width of 57 nm was generated when the velocity spread of the atomic source was almost 50 m/s, that is, 1/6 that of the thermal beam. Received: 16 June 2001 / Published online: 7 November 2001     

Temperature dependence of the efficiency at which electrons are captured into axial channeling

A procedure for calculating the efficiency at which electrons are captured into axial channeling with a realistic axis potential is discussed. That region in the space of the electron's total transverse energy E and angular momentum L (integrals of motion) which corresponds to stable bound states of the electron with an atomic row is found. The temperature dependence of the capture coefficient is analyzed. As the crystal temperature is increased, the capture coefficient decreases, to a particularly noticeable extent for beams making a small angle with the crystallographic axis and for crystals having a high atomic number z.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 69–74, May, 1988.     

Chaotic behavior of channeling particles

Channeling describes the collimated motion of energetic charged particles along the lattice plane or axis in a crystal. The energetic particles are steered through the channels formed by strings of atomic constituents in the lattice. In the case of planar channeling, the motion of a charged particle between the atomic planes can be periodic or quasiperiodic, such as a simple oscillatory motion in the transverse direction. In practice, however, the periodic motion of the channeling particles can be accompanied by an irregular, chaotic behavior. In this paper, the Moliere potential, which is considered as a good analytical approximation for the interaction of channeling particles with the rows of atoms in the lattice, is used to simulate the channeling behavior of positively charged particles in a tungsten (100) crystal plane. By appropriate selection of channeling parameters, such as the projectile energy E(0) and incident angle psi(0), the transition of channeling particles from regular to chaotic motion is demonstrated. It is argued that the fine structures that appear in the angular scan channeling experiments are due to the particles' chaotic motion.     

Enhanced planar channeling of MeV protons through thin crystals

At certain tilt alignments between a MeV proton beam and a planar channeling direction, a single interface lattice rotation within a crystal can result in a lower rate of dechanneling than at planar alignment in a perfect crystal. Such planar channeling enhancement arises when the beam passes through a layer thickness which is a half-multiple of the oscillation wavelength and then encounters a small interface rotation which is matched to the beam tilt angle. The beam is projected into the center of the phase space ellipse below the interface, resulting in certain trajectories undergoing a reduction in their transverse energy, in a manner analogous to stochastic cooling or atom laser cooling.     

New mechanism of atomic particle ionization in crystals

It is shown that at channeling in a crystal the electron loss by a fast ion due to “inelastic” collisions with the target electrons may be of importance as the “elastic” electron loss which is dominating in an amorphous target can be strongly reduced in a crystalline one.