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.     

Planar channeling potential in heavy ionic crystals

The planar channeling potential in finite-size heavy KCl-like crystals is studied.     

Mass exchange and angular distribution in a dynamical treatment of heavy ion collisions

We present a first numerical computation of the absolute value of the double differential cross section as a function of mass asymmetry and detection angle including a dynamical coupling between relative motion and mass asymmetry. We apply it to the⁶³Cu+¹⁹⁷Au experiment at two different energies. The equation of motion used is a Fokker Planck equation for the distribution function in classical phase space. The coefficients needed are those known from classical model calculations, besides a friction coefficient introduced for the mass asymmetry degree. We find encouraging agreement between the calculated and experimental curves.     

Computer studies of boron ion channeling in silicon single crystals

The motion of 200 keV B ions in the [111] direction in a silicon single crystal has been investigated using a computer simulation method. Profiles of the depth distribution of implanted ions for both an ideal crystal and a crystal with thermal vibrations have been obtained. A study of the defect distribution has been carried out.     

A study of small impact parameter ion channeling effects in thin crystals

We have recorded channeling patterns produced by 1–2 MeV protons aligned with ?1 1 1? axes in 55 nm thick silicon crystals which exhibit characteristic angular structure for deflection angles up to and beyond the axial critical angle, ψ _a . Such large angular deflections are produced by ions incident on atomic strings with small impact parameters, resulting in trajectories which pass through several radial rings of atomic strings before exiting the thin crystal. Each ring may focus, steer or scatter the channeled ions in the transverse direction and the resulting characteristic angular structure beyond 0.6ψ _a at different depths can be related to peaks and troughs in the nuclear encounter probability. Such “radial focusing” underlies other axial channeling phenomena in thin crystals including planar channeling of small impact parameter trajectories, peaks around the azimuthal distribution at small tilts and large shoulders in the nuclear encounter probability at tilts beyond ψ _a .     

Spatial charge distribution in fibrous dielectrics

In this work, an attempt was made to study the spatial charge distribution in fibrous dielectrics using pulsed electro-acoustic method. The polarized dielectrics displayed in-depth charge polarization and exhibited slow charge decay. The initial charge density was found to increase with an increase in polarization voltage as well as polarization time. The statistical analysis revealed that though the polarization voltage affected the initial charge density significantly, the polarization time was insignificant. A multiple linear regression model of initial charge density was developed and found to predict the experimental results quite satisfactorily.     

Energy and angular momentum dependence of heavy ion optical potentials

The influence of bombarding energy and angular momentum on the depth and shape of the real part of the optical ion-ion potential is studied in a model which uses oscillator wave functions for the ground states of the interacting nuclei and takes into account the relative motion of the nuclei by a multiplication with a plane wave factor. The calculations were done for α+α,¹⁶O+¹⁶ O,⁴⁰Ca+⁴⁰Ca,α +¹⁶O,α +⁴⁰Ca and¹⁶ O +⁴⁰Ca with the Skyrme force as interaction.     

Spatial distribution of electric field and charge in single-crystal ZnSe under pulsed electron irradiation

The linear electrooptical effect was used to study the spatial intensity distribution of the magnetic field induced in a ZnSe crystal by a nanosecond electron beam. Measured profiles of the electric field as a function of beam current density and the positions of the grounded electrodes on the specimen are presented. It is shown that the spatial distribution of the field is shaped by the distribution of the thermalized accelerator-beam electrons in the material, by the field of the space charge itself, and by radiation-induced conduction.Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 26–34, May, 1995.     

Fast-ion channeling in fullerene crystals

The motion of H⁺ and He⁺ ions with energies of 230 and 2000 keV in C₆₀ and K₃C₆₀ crystals was calculated by the Monte Carlo method. Ion channeling was shown to occur in the 〈100〉 and 〈111〉 directions. The main parameters characterizing channeling were determined. Medium-energy ions were found to be preferable in the detection of channeling in C₆₀ films.     

On channeling in LiH crystals

The thermal-oscillation-averaged potential of channeling along the charged $(1\bar 11)$ and (111) planes in a LiH crystal is derived. The theoretical value of the energy of an emitted photon that meets the transition between levels of adjacent potential wells is found to agree well with experimental data.     

Time scales for charge equilibration in heavy ion collisions

Charge equilibration was investigated in the reaction²⁰⁸Pb+¹¹⁰Pd at 1,180 and 1,280 MeV incident energy. It is found that the relaxation timeτ′ connected with this degree of freedom shows an increase with increasing bombarding energy. This behaviour can be explained within Langevin's classical theory of Brownian motion.     

A theoretical model for the charge equilibration in heavy ion collisions

We propose to treat the charge equilibration process as a collective high frequency mode and discuss the implications for the first stages of a heavy ion collision. We show how its dynamics can be treated by means of a quantal master equation. We solve numerically the dynamical equations for a two-dimensional model, using charge excess and mass asymmetry as dynamical degrees of freedom.     

Ranges and profiles of distribution of low-energy ions channeling in metal and semiconductor single crystals

In the present work peculiarities of trajectories and energy losses, ranges and profiles of distribution of low-energy different-mass ions channeling in thin single crystals of metals and semiconductors have been thoroughly studied by computer simulation in binary collision approximation. The character of oscillations of channeled-ion trajectories depending on their energies, aiming points from the axis of a channel, kind of interaction potential, crystal lattice type and temperature has been determined. It has been found that, in the case of light ions even at low energy, the main contribution to energy loss is made by inelastic energy losses, whereas for heavy ions, already at E < 10 keV, elastic energy losses exceed inelastic ones. Profiles of the distribution of channeled ions have been calculated depending on crystal lattice type, kind of ions and their energy.     

Effect of heavy ion charge fluctuations on Cherenkov radiation

Correlation effects in the Cherenkov radiation due to ions charge fluctuations in the matter are considered. Stochastic process of charge exchange leads to the washout of a radiation wave front and to the transformation of spectral-angular density. The effect is determined by a root-mean-square deviation of an ion charge. The additional radiation yield gives the nonzero contribution at the violation of the Cherenkov threshold condition. The interference of an electromagnetic field generated in the matter by the different ion's charge states along the trajectory is the cause of the additional radiation.