Low Energy Collision Cascades Research Articles

Molecular dynamics simulations of cascade events in AlN

The radiation tolerance and the ability to retain its piezoelectric response make aluminum nitride (AlN) a good candidate for emerging sensing technologies in nuclear reactor environments. Furthermore, it has been shown that doping the ceramic with additional metals can further improve the piezoelectric response. Fundamental understanding of the response of AlN to radiation is important for ultimately improving the material's properties. In this paper, we use molecular dynamics and a recently developed interatomic potential fitted to defect energies to investigate low energy collision cascades in AlN. We additionally investigate the electronic stopping effect in the damage production for 20 keV Al ions. Our results show that for all energies the number of Al surviving defects is larger than that of N defects. Additionally, we find that even though the ballistic energy loss is the dominating mechanism, it is important to take into account the electronic stopping in order not to overestimate the number of defects.

A ReaXFF carbon potential for radiation damage studies

Although molecular dynamics simulations of energetic impacts and collision cascades in graphite have been investigated for over 25years, recent investigations have shown a difference between the types of defects predicted by the commonly used empirical potentials compared to ab initio calculations. As a result a new ReaXFF potential has been fitted which reproduces the formation energies of many of the defects predicted by the ab initio calculations and the energy pathways between different defect states, important for investigating long term defect evolution. The data sets in the fitting have been added to the existing data sets used for modelling hydrocarbons and fullerenes. The elastic properties of the potential are less well modelled than the point defect structures with the elastic constants c33 being too high and c44 too low compared to experiment. Preliminary results of low energy collision cascades show many point defect structures develop that are in agreement with those predicted from the ab initio results.

Simulating radiation damage in Ga stabilised δ-Pu

Radiation events in Ga stablised δ-Pu are investigated by means of Molecular Dynamics simulations. Pu 5 at.% Ga is considered using the Modified Embedded Atom Method to govern the atomic interactions. Cascades were initiated with Primary Knock-on Atom (PKA) energies in the range of 0.4–10 keV, with trajectories deduced through comprehensive sampling of a representative set of directions, combined with different Ga atomic positions. The displacement threshold energy, E d , for Pu and Ga atoms was also determined through similar extensive studies to aid the understanding and interpretation of the cascade results. Values of E d between 5 and 40 eV were determined for Pu, with Ga PKAs requiring generally more energy to create a defect with E d between 8 and 70 eV. Low energy collision cascades, initiated with energies in the range of 0.4–1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone surrounded by isolated interstitial defects. A feature of these cascades is that the displaced Ga atoms return to lattice sites during the ballistic phase, leading to a lack of Ga-type residual defects. Higher energy cascades show similar features but with the development of an amorphous region at the cascade core of around 5 nm diameter at 5 keV. Quantitatively, the residual number of defects found shows no distinct variation to that for previous work on pure Pu, suggesting the inclusion of Ga does not significantly effect the susceptibility or resistance of Pu to initial cascade development.

Comparisons between statistics of low energy collision cascades generated by full molecular dynamics and by its binary collision approximation

Collision cascades in Cu, Au and Cu3Au are generated by full molecular dynamics (MD) and by its binary collision approximation (BCA) with the Marlowe program. Cu and Au primaries have 1 keV initial energy. The same Molière repulsive potential is used in both models for close encounters. In the MD model, this potential is carefully splined to the pair component of the N-body potential developed by Ackland and Vitek. In the BCA, this N-body interaction is roughly modeled by a constant isotropic 4 eV binding energy of the target atoms to their rest positions. Time distributions of the number of atoms moving with a total energy higher than a threshold value E d are compared and discussed. Recoil range distributions during the cascade development are discussed as well. The agreement between MD and BCA is fairly good in all cases for E d larger than about 3 eV. In the case of smaller E d-values, the BCA may result in an overestimate of the number of moving atoms in the late development of the cascades. This discrepancy is suggested to originate in the lack of attractive forces between the moving particles and the surrounding atoms in the BCA.

Production of energetic electrons in low-energy collision cascades in solids

One-electron models of kinetic electronic excitations in metallic solids bombarded by slow atomic particles (<2 keV) will be compared with kinetic electron emission experiments. As some of the experiments on energetic electrons cannot be explained in terms of the one-electron processes possible extension of emission models will be described. In particular, it will be assumed that electrons excited in collision cascades can strongly interact and this electron-electron interaction will be modelled either by a rapid (within few fs) or an instantaneous thermalization of excitations. The excitation processes will be studied at different temporal stages of the cascade.

Synthesis and ion beam modification of the Bi system superconducting ultrathin films

Superconducting Bi-Sr-Ca-Cu-O ultrathin films with thicknesses of 300, 150, 70 and 40 ̊a were synthesized by using single-target magnetron sputtering and by optimizing the heat treatment conditions. Ultrathin films thus obtained were highly oriented with the c axis perpendicular to the film plane. These film specimens were irradiated with 100 keV Ar ions at low temperatures, 10–20 K, and subsequently annealed at a relatively low temperature around 730 °C. For a film thickness of 300 ̊A, the zero-resistance transition temperature T c,o was raised to 108 K, which is equivalent to the maximum value so far observed for the bulk system. The critical current density J c was 10 4 A cm −2 at 77 K in zero magnetic field. Even for 40 A thick ultrathin film, the initial T c,o of 78 K was improved to 88 K, approaching T c,o of the high T c phase with a significant increase in J c of the order of 10 3 A cm −2 at 77 K. The role of 100 keV range Ar ions in the thin film modification was discussed in terms of a selective creation of “unit cell” scales of the low energy collision cascades and ion channelling in thin crystalline films.

Effect of temperature on the bulk atomic relocation in low-energy collision cascades in silicon: A molecular dynamics study

Abstract The production of damage in a Si lattice by internally starting 100 eV self-recoils has been studied using a MD simulation. Different initial lattice temperatures below the Debye temperature for Si have been considered. The number of stable atomic displacements and the amount of atomic mixing increase with the initial target temperature. The increase with temperature of atomic mixing is nonlinear -appreciable changes take place between 300 and 500 K, while the difference between the amount of mixing corresponding to 0 and 300 K is negligibly small. The size of the cascade zone in which stable atomic displacements occur doubles itself for temperature changes between 0 and 300 K, with a value for 500 K lying in between. This nonmonotonic variation with the initial target temperature of the size of the cascade zone may have its origin in the correlation between the initial direction of motion of the starting recoil and the directions of thermal velocities of the neighbouring atoms around this recoil.

Molecular dynamics simulation of low-energy collision cascades and atomic mixing in silicon

Abstract We investigate atomic relocation processes in silicon at OK, initiated by an internal 100eV silicon recoil. The molecular dynamics code MODYSEM is used, based on a Tersoff potential for silicon. A fitting procedure was used for the generation of 8 potential valid over the whole energy range of interest. The contribution of the collisional, spontaneous relaxation and thermalization stages to the atomic relocation process are discussed. A threshold distance for the definition of relocated atoms is determined, which separates atomic displacements into stable and unstable (or transient) groups. The atomic mixing process is quantified in terms of the first and second spatial moments over the relocation cross-section. These moments depend on the criterion used to define a relocated Si atom, with short-distance thermal-like atomic displacements, which appear during the thermalization stage, dominating the values of the spatial moments. However, the moments of the relocation cross-section calculated by considering only the stable displacements are generated mainly by collisional atomic relocations.

First-principles simulation of collision cascades in Si to test pair-potentials for Si-Si interaction at 10 eV–5 keV

Interatomic potentials for Si-Si interaction are tested at energies of 10 eV–5 keV for Si ions in ion-beam amorphized Si by simulating range distribution data with the molecular dynamics method. The range profile of 1 × 10 16 10 keV 30Si + cm −2 implanted into originally crystalline silicon was measured using a nuclear reaction technique. An interatomic repulsive potential from first-principles calculations is proposed for the Si-Si interaction. The dependence of the number of vacancies produced in low-energy collision cascades on the potential is demonstrated by simulations of the cascade dynamics.

Intermittency in low energy collision cascades

The method of scaled factorial moments is used to study fluctuations of the energy distributions obtained by Monte Carlo simulation of ion induced collision cascades in solids. It is shown that the intermittent behaviour is independent of the cross section. It is found that the energy fluctuation of power-law cascades is similar to the fluctuation of the self-similar cascade only in the case of a space-filling cascade.

Molecular dynamics of collision cascades with composite pair–many-body potentials

An empirical, composite interatomic potential is developed to describe interaction of energetic particles by pair potentials at high energies and many-body potentials at low energies. Molecular-dynamics studies of low-energy collision cascades are performed. The displacement threshold surface in copper is investigated and compared to experimental data. Our computer simulations show good agreement with the experimental results of King and Benedek at 10 K.

Energy sharing and sputtering in low-energy collision cascades

Abstract Using a non-linear transport equation to describe the energy-sharing process in an isotropic collision cascade, we have numerically calculated sputtered particle velocity spectra for several very low energy (<10 eV) primary recoil distributions. Our formulation of the sputtering process is essentially that used in the linear model and our equations yield the familiar linear model results in the appropriate limit. Discrepancies between our calculations and the linear model results in other cases may be understood by considering the effects of the linear model assumptions on the sputtering yield at very low energies. Our calculations are also compared with recent experimental results investigating ion-explosion sputtering. The results of this comparison support the conclusion that in insulators sputtering is initiated by very low energy recoil atoms when the energy of the incident beam is high enough that the stopping power is dominated by the electronic contribution. The calculations also suggest that energy spectra similar to those for evaporation may result from non-equilibrium processes but that the apparent temperatures of evaporation are not related in a simple way to any real temperature within the target.