Mark T. Robinson

Computer studies of the reflection of light ions from solids

Abstract Reflection of 10 eV to 20 keV H, T and He atoms from amorphous Al, Cu, Nb and Au has been studied using the binary collision cascade simulation program MARLOWE. The fractions of particles and energy reflected increase with increasing atomic number of the target and decrease with increasing incident energy. The peak in the energy distribution of reflected particles shifts to lower fractional energy with increasing incident energy. The number fraction reflected increases for increasing incident angle such that total reflection occurs for angles greater than a critical angle. Fair to poor agreement is found between the calculations and the experimental data available.i

Sputtering Experiments with 1‐ to 5‐keV Ar+ Ions

Sputtering yields have been determined with 1‐ to 5‐keV Ar+ ions normally incident upon targets of type 304 stainless steel, three different types of polycrystalline Cu, a wide variety of Cu monocrystals, and monocrystals of Si and Ge. Ejection patterns have been recorded from these targets and from a monocrystal of InSb. The sputtering yield of polycrystalline Cu depends on the source of the metal, apparently due to variations in preferred orientation. The yield from Cu monocrystals is strongly dependent on orientation, the effect becoming more pronounced as the energy is increased. A simple model is presented which accounts for this behavior in terms of the variation with direction of the initial mean free path of the incident ion. The ejection patterns leave little doubt that focusing collision chains are primarily responsible for the transport of momentum to the surfaces of close‐packed metals.

Basic physics of radiation damage production

Abstract The basic physical processes underlying the production of displacement damage in irradiated solids are briefly discussed, including topics from nuclear, atomic, and solid-state physics. Following a general introduction, the concepts of elementary cascade theory are presented as a basis for intuitive descriptions of the damage process. Then the production of primary recoils, mainly by nuclear processes, is discussed in enough detail to prepare a basis for calculating the primary-recoil energy spectra in typical irradiation facilities. The slowing down of fast atomic particles in solids is next discussed as a basis for developing atomistic models of damage production. Finally, several aspects of damage production, as revealed by atomistic simulation models, are outlined.

Computer studies of low energy scattering in crystalline and amorphous targets

Abstract The scattering of 1 keV Ar atoms in thin amorphous and monocrystalline Cu targets has been studied, using the computer simulation program MARLOWE. An improved procedure for evaluating groups of nearly simultaneous collisions has been developed. Particle reflection is found to be a near surface effect and shows crystallographic dependence associated mostly with the surface. Transmission of the incident particles shows strong axial channeling effects and smaller effects of planar channels. The importance of surface forces in governing the entry of low energy particles into channels is shown.

The influence of the scattering law on the radiation damage displacement cascade

Abstract The asymptotic solution to the integral equation describing the radiation damage displacement cascade is derived under weaker assumptions than made previously. The results of the earlier calculation have been completely confirmed, even for inverse power scattering laws having infinite total scattering cross sections, as long as the stopping cross sections are finite.

Computer simulation of low-energy sputtering in the binary collision approximation

The sputtering of amorphous Cu targets by low-energy atoms has been investigated in the binary collision approximation using the computer program MARLOWE. Particular attention was given to the influence of the surface binding model on the results. Calculations were made of the dependence of the sputtering yield on the incident particle direction, energy, and mass. Angular-, energy-, and yield-distributions of the ejected atoms were evaluated. Comparisons with experimental results on polycrystalline targets show that the planar surface binding model is to be preferred over the isotropic surface binding model, especially with regard to the angular- and energy-distributions. Calculated yields are in reasonable agreement with experiment at energies below about 1 keV, but deviate at higher energies, apparently because of crystal correlation effects that are neglected in the amorphous model.

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Computer simulation of the self‐sputtering of uranium

The sputtering of polycrystalline α‐uranium by uranium ions of energies below 10 keV has been studied in the binary collision approximation using the computer simulation program marlowe. Satisfactory agreement of the computed sputtering yields with the small amount of available experimental data was achieved using the Moliere interatomic potential, a semilocal inelastic loss function, and a planar surface binding barrier, all with conventional parameters. The model is used to discuss low energy sputtering processes and the energy and angular distributions of the reflected primaries and the sputtered target particles.

Ranges of Energetic Atoms in Solids

The ranges in solids of atoms having energies from 1 to 100 keV have been calculated using Monte Carlo techniques. The model assumes that the moving atom loses all of its energy through binary elastic collisions with the atoms of the solid. The potential of interaction, principally studied, is an exponentially screened Coulomb (Bohr) potential, and the scattering angles are calculated explicitly. It is found that neither the hard sphere approximation nor the inverse r‐squared approximation to the Bohr potential is particularly good. To obtain correspondence with experimental results it is found that the Bohr screening length must be increased as the atomic number of the interacting atoms increases.

The energy spectra of atoms slowing down in structureless media

Abstract Collision densities have been computed for a uniform, isotropic source of mono-energetic atoms slowing down by elastic collisions with the atoms of an infinite, homogeneous, structureless medium. It is found that, because of the indistinguishability of the particles proceeding from a collision, the results are relatively insensitive to the shape of the differential elastic scattering cross section, whether this is described by a Legendre polynomial expansion, or by the interaction of the atoms through the Born-Mayer or the Thomas-Fermi potentials. For energies not too close to the source energy, the collision densities vary as 1/E 2, in agreement with the result for hard-core scattering and with certain available experiments.