Ordean S. Oen

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

Cross sections for atomic displacements in solids by fast positrons

Abstract The Mott series has been used to calculate the cross section for atomic displacements produced in elastic collisions between relativistic positrons and atomic nuclei. The Kinchin and Pease displacement model was used. Several elements spanning the atomic table were treated using positron energies ranging from threshold to several tens of MeV. The results are compared with previous calculations for relativistic electrons. It is found that for the same energy and atomic number the positron cross sections are always smaller (up to a factor of 5 or more). It is also found that the McKinley-Feshbach formula, which is frequently used in radiation damage analysis, is even less reliable for positrons than for electrons.

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.

Cross Sections for Atomic Displacements in Solids by Gamma Rays

Cross sections for the displacement of lattice atoms by gamma rays for energies up to 5 Mev have been calculated. The principal contribution is found to come from the Compton effect, in which the atoms are displaced by electrons produced by the incident gamma rays. The calculations have been based on the assumption of a sharp threshold energy for displacement, but cross sections have been computed as a function of the assumed threshold energy. Thus, from the present work it is relatively easy to obtain cross sections based upon other displacement functions.

Computer simulation of the reflection of hydrogen and the sputtering of hydrogen from metal hydrides

Abstract Reflection of 0.1 to 2 keV H atoms from Ti, Fe and their “metal hydrides”, together with the H sputtered from the latter, have been calculated using the binary collision cascade program MARLOWE. The fraction of particles and energy reflected is found to decrease with increasing hydrogen content of the metal hydride and this decrease is independent of the incident ion energy. It is found that the heavy metal atoms of the metal hydride are responsible for the reflection and that most of the sputtering is produced by the reflected ion as it exits through the surface layer. It is also found that tritum ions sputter H from “FeH” much more effectively than H ions sputter T from “FeT”.

THE EFFECT OF CHANNELING ON DISPLACEMENT CASCADE THEORY

It is shown that channeling of energetic atoms in crystal lattices may be very importent in determining the total number of displaced atoms in a cascade. (C.E.S.)

Computer studies of the scattering of low energy hydrogen ions from polycrystalline solids

Reflection of 50 eV to 10 keV H atoms from polycrystalline Cu, Nb and Au targets has been calculated using the binary collision cascade program MARLOWE. The fractions of particles and energy reflected (backscattered) increase with increasing atomic number of the target and decrease with increasing incident energy. The results indicate that the effects of polycrystallinity are modest, reducing the amorphous reflection coefficients by about 25 percent. The calculations agree quite well with the experimental data for Cu and Au, but are about a factor of two larger than is observed for Nb.

MONTE CARLO RANGE CALCULATIONS FOR A THOMAS-FERMI POTENTIAL

Our earlier Monte Carlo calculations of the ranges of atoms having energies from 1 to 100 keV, slowing down in a random solid through binary elastic collisions, have been extended by using a Thomas‐Fermi potential to represent the interaction between the moving atom and a lattice atom. The screening radius of the potential is that derived by Firsov. The calculations have been made for a wide variety of target‐to‐projectile mass ratios. Except for the highest energies, the calculated ranges are considerably shorter than those found previously using the exponentially screened Coulomb potential. Most of the experimental range data lie between the range curves calculated for these two potentials, although the Thomas‐Fermi potential gives somewhat better over‐all agreement. The shapes of the calculated range distributions give close agreement with those found by experiment in amorphous solids. Average ranges calculated by integrating the reciprocal of the stopping power agree fairly well with these Monte Carlo...

Atomic structure of collision cascades in ion-implanted silicon and channeling effects

Abstract We have investigated the atomic structures of collision cascades in Bi + -implanted silicon. The formation of subcascades or bunching of the primary cascades is clearly observed. The central regions of the cascades were found to be amorphous with a high density of disorder in the surrounding regions. The experimental results are discussed in terms of the computer simulation of the deposited damage energy profiles. The effect of channeling on the deposited damage energy profile is examined. The peak and integrated damage energies are considerably lower in channeling directions compared to random directions.

A Simple Approximation for Classical Scattering at Large Angles

The classical scattering for repulsive potentials is treated approximately by a matching method. The matching potential is a cut‐off potential which matches the value and the slope of the actual potential at the distance of closest approach. The method is thought to be successfully applicable to interatomic potentials of the screened Coulomb type. Comparison is made with machine results for an exponentially screened Coulomb potential. The differential cross section and the stopping power are compared for various energies. The agreement in the differential cross section is fair except for very low energy transfers. The stopping powers agree over a wide energy range with an error of only a few percent. Further comparison is made with another approximation given by Lehman and Shapiro.