Simulation Of Radiation Damage Research Articles

Gaussian approximation potentials for body-centered-cubic transition metals

We develop a set of machine-learning interatomic potentials for elemental V, Nb, Mo, Ta, and W using the Gaussian approximation potential framework. The potentials show good accuracy and transferability for elastic, thermal, liquid, defect, and surface properties. All potentials are augmented with accurate repulsive potentials, making them applicable to radiation damage simulations involving high-energy collisions. We study melting and liquid properties in detail and use the potentials to provide melting curves up to 400 GPa for all five elements.

Impulsive generation of 〈100〉 dislocation loops in BCC iron

The conditions for the formation of 〈100〉 dislocation loops in body-centered cubic (BCC) iron were investigated via molecular dynamics simulations using a simplified model intended to mimic conditions in high energy collision cascades, focusing on the possible coherent displacement of atoms at the boundary of a subcascade. We report on the formation of 〈100〉 dislocation loops due to the fast displacement of a few hundred atoms with a coherent acceleration, in agreement with previous results for much larger cascade simulations. We analyze in detail the resulting atomic velocities and pressures, and find that they cannot be described within the usual formalism for a shock regime, since the pressure pulse only lasts less than 1 ps and does not match expected values from a Hugoniot shock. Our simulations include two interatomic potentials: Mendelev, which is extensively used for radiation damage simulations, and Ackland, which has been used for shock simulations because it can reproduce the experimentally observed transition from BCC to hexagonal close-packed structure at around 25 GPa, at high deformation rates. They both show similar evolution of defects, also indicating departure from a shock regime which is extremely different for these potentials.

Assessing the influence of electronic effects on molecular dynamics simulations of primary radiation damage in tungsten

Molecular dynamics (MD) simulation is extensivelyusedintheresearch on the evolution of cascade damage. Considering their capability of transferring energy from atoms to electrons, the electronic effects, including electronic stopping (se) and electron–phonon coupling (EPC), cannot benegligible in the MD simulations. However, how to incorporate them is somewhat debatable, and their influence on simulation results is still not very clear. In the present study, thesensitivity researchof collision cascades in tungsten caused by individual primary knock-on atoms to the parameters of se and EPC is approached methodologically. The results show that se influences the defects produced in the thermal spike and the size distribution of residual defect clusters more significantly, whereas EPC affects the number and space distribution of residual defects more remarkably. In addition, the simulation results depend appreciably on the choice of cutoff energies of se and EPC.

Evaluation of tungsten interatomic potentials for radiation damage simulations

The structure and properties of materials under neutron irradiation are an important basis in future fusion reactors. In the absence of fusion neutron sources for irradiation experiments, it is increasingly important and urgent to carry out neutron irradiation simulations on fusion reactor materials and then establish complete databases of defect properties and collisional cascades, where the first and foremost step is to select suitable interatomic potentials for atomistic-level simulations. In this work, six typic interatomic potentials for tungsten (W) are evaluated and reviewed systematically for radiation damage simulations. The relative lattice stability and elastic constants of bulk W are considered first with those potentials; then, the properties of point defects and defect clusters at interstitial sites and vacancies are obtained by molecular statics/dynamics simulations. The formation energies of interstitial/vacancy clusters, 1/2 〈111〉 and 〈100〉 dislocation loops in W and the threshold displacement energies along different directions are also determined. In addition, the extended defects are further investigated, such as free surfaces and the energy profiles of 1/2 〈111〉 {110} and 1/2 〈111〉 {112} stacking faults. The current results provide a reference for selecting W potentials to simulate the radiation damage.

Simulation of Primary Radiation Damage in Nickel

The process of radiation damage formation in collision cascades initiated by primary knock-on atoms (PKAs) with energy EPKA = 5, 10, 15, and 20 keV in nickel at temperatures T = 100, 300, 600, 900, and 1200 K was studied using the molecular dynamics method. To ensure the statistical validity of the results, a series of 24 cascades was modeled for each pair of (EPKA, T) parameters. The simulation results were analyzed to determine the number NFP of Frenkel pairs, fractions of vacancies σvac and interstitial atoms σSIA in clusters of point defects, average sizes of vacancy 〈Nvac〉 and interstitial 〈NSIA〉 clusters, and average numbers of vacancy 〈Yvac〉 and interstitial 〈YSIA〉 clusters produced in collision cascades as functions of the PKA energy and simulation temperature. It was found that the relation 〈NFP〉 = 2 ± 0.9 × $$E_{\text{PKA}}^{{1.1 \pm 0.1}}$$ holds true at all the examined values of (EPKA, T). The functional dependences of 〈σvac〉 and 〈σSIA〉 on EPKA were identical. The dependence of 〈σvac〉 follows that of 〈Yvac〉, while 〈σSIA〉 is governed by 〈NSIA〉 and the mobility of interstitials. The value of 〈Nvac〉 depends on the irradiation temperature and the thermal stability of vacancy clusters. These clusters are stable at T ≤ 300 K, and 〈Nvac〉 ∝ EPKA; at 600 ≤ T ≤ 900 K, 〈Nvac〉 ≈ 6 and 10, which corresponds to the sizes of regular stacking fault tetrahedra. The value of 〈YSIA〉 is proportional to 〈NFP〉 and, consequently, to EPKA in the entire range of PKA energies.

Geant4 electromagnetic physics progress

The Geant4 electromagnetic (EM) physics sub-packages are a component of LHC experiment simulations. During long shutdown 2 for LHC, these packages are under intensive development and we report progress of EM physics in Geant4 versions 10.5 and 10.6, which includes faster computation, more accurate EM models, and extensions to the validation suite. New approaches are developed to simulate radiation damage for silicon vertex detectors and for configuration of multiple scattering per detector region. Improvements in user interfaces developed for low-energy and the Geant4-DNA project are used also for LHC simulation optimisation.

Dynamical stability of radiation-induced C15 clusters in iron

Density functional theory predicts clusters in the form of the C15 Laves phase to be the most stable cluster of self-interstitials in iron at small sizes. The C15 clusters can form as a result of irradiation, but their prevalence and survival in harsh irradiation conditions have not been thoroughly studied. Using a new bond-order potential optimised for molecular dynamics simulations of radiation damage, we explore the dynamical stability of the C15 clusters in iron under irradiation conditions. We find that small C15 clusters make up 5–20% of the interstitial clusters formed directly in cascades. In continuous irradiation, C15 clusters are frequently formed, after which they remain highly stable and grow by absorbing nearby single interstitial atoms. Growth of C15 clusters ultimately leads to collapse into dislocation loops, most frequently into 1/2⟨111⟩ loops and only rarely collapsing into ⟨100⟩ loops at low temperatures. The population, size, and collapse of C15 clusters during continuous irradiation correlates well with their formation energies relative to dislocation loops calculated at zero Kelvin.

Reduced-order atomistic cascade method for simulating radiation damage in metals

Atomistic modeling of radiation damage through displacement cascades is deceptively non-trivial. Due to the high energy and stochastic nature of atomic collisions, individual primary knock-on atom (PKA) cascade simulations are computationally expensive and ill-suited for length and dose upscaling. Here, we propose a reduced-order atomistic cascade model capable of predicting and replicating radiation events in metals across a wide range of recoil energies. Our methodology approximates cascade and displacement damage production by modeling the cascade as a core-shell atomic structure composed of two damage production estimators, namely an athermal recombination corrected displacements per atom (arc-dpa) in the shell and a replacements per atom (rpa) representing atomic mixing in the core. These estimators are calibrated from explicit PKA simulations and a standard displacement damage model that incorporates cascade defect production efficiency and mixing effects. We illustrate the predictability and accuracy of our reduced-order atomistic cascade method for the cases of copper and niobium by comparing its results with those from full PKA simulations in terms of defect production as well as the resulting cascade evolution and structure. We provide examples for simulating high energy cascade fragmentation and large dose ion-bombardment to demonstrate its possible applicability. Finally, we discuss the various practical considerations and challenges associated with this methodology especially when simulating subcascade formation and dose effects.

Machine-learning interatomic potential for radiation damage and defects in tungsten

We introduce a machine-learning interatomic potential for tungsten using the Gaussian Approximation Potential framework. We specifically focus on properties relevant for simulations of radiation-induced collision cascades and the damage they produce, including a realistic repulsive potential for the short-range many-body cascade dynamics and a good description of the liquid phase. Furthermore, the potential accurately reproduces surface properties and the energetics of vacancy and self-interstitial clusters, which have been long-standing deficiencies of existing potentials. The potential enables molecular dynamics simulations of radiation damage in tungsten with unprecedented accuracy.

Molecular dynamics investigation of threshold displacement energies in CaF2

Understanding the propagation of radiation damage in a material is paramount to predicting the material damage effects. To date, no current literature has investigated the Threshold Displacement Energy (TDE) of Ca and F atoms in CaF2 through molecular dynamics and simulated statistical analysis. A set of interatomic potentials between Ca-Ca, F-F, and F-Ca were splined, fully characterizing a pure CaF2 simulation cell, by using published Born-Mayer-Huggins, standard ZBL, and Coulomb potentials, with a resulting structure within 1% of standard density and published lattice constants. Using this simulation cell, molecular dynamics simulations were performed with LAMMPS using a simulation that randomly generated 500 Ca and F PKA directions for each incremental set of energies, and a simulation in each of the [1 0 0], [1 1 0], and [1 1 1] directions with 500 trials for each incremental energy. MD simulations of radiation damage in CaF2 are carried out using F and Ca PKAs, with energies ranging from 2 to 200 eV. Probabilistic determinations of the TDE and Threshold Vacancy Energy (TVE) of Ca and F atoms in CaF2 were performed, as well as examining vacancy, interstitial, and antisite production rates over the range of PKA energies. Many more F atoms were displaced from both PKA species, and though F recombination appears more probable than Ca recombination, F vacancy numbers are higher. The higher number of F vacancies than Ca vacancies suggests F Frenkel pairs dominate CaF2 damage.

Molecular dynamics simulations of high-energy radiation damage in W and W–Re alloys

High-energy collision cascades with an energy of up to 300 keV for the primary knock-on atom (PKA) have been initially simulated in W and W–Re alloys containing 5 or 10 at.% Re atoms using the molecular dynamics method with recently fitted W–Re interatomic potentials. The effects of PKA energy and Re concentration on defect production, defect clustering and states of dislocation loops have been quantitatively analysed. The results show that the presence of Re atoms does not significantly affect either the number of surviving defects or their clustered fractions. In addition, the interstitial dislocation loops are dominated by the 1/2<111> loops. Mixed interstitial loops with 1/2<111> and <100> Burgers vectors and interstitial loops that have the same Burgers vectors but are located on different habit planes have also been observed. Further analysis indicates that the pinning effect induced by the Re atom segregation leads to the lower mobility of the interstitial clusters and interstitial 1/2<111> loops in W–Re alloys than in pure W, which is expected to influence the subsequent evolution of radiation-induced defects in W–Re alloy.

Atomistic-object kinetic Monte Carlo simulations of irradiation damage in tungsten

We describe the development of a new object kinetic Monte Carlo (kMC) code where the elementary defect objects are off-lattice atomistic configurations. Atomic-level transitions are used to transform and translate objects, to split objects and to merge them together. This gradually constructs a database of atomic configurations- a set of relevant defect objects and their possible events generated on-the-fly. Elastic interactions are handled within objects with empirical potentials at short distances, and between spatially distinct objects using the dipole tensor formalism. The model is shown to evolve mobile interstitial clusters in tungsten faster than an equivalent molecular dynamics (MD) simulation, even at elevated temperatures. We apply the model to the evolution of complex defects generated using MD simulations of primary radiation damage in tungsten. We show that we can evolve defect structures formed in cascade simulations to experimentally observable timescales of seconds while retaining atomistic detail. We conclude that the first few nanoseconds of simulation following cascade initiation would be better performed using MD, as this will capture some of the near-temperature-independent evolution of small highly-mobile interstitial clusters. For the 20keV cascade annealing simulations considered here, we observe internal relaxations of sessile objects. These relaxations would be difficult to capture using conventional object kMC, yet are important as they establish the conditions for long timescale evolution.

Comparison of Hartree–Fock and Hartree–Fock–Slater approximations for calculation of radiation damage dynamics of light and heavy atoms in the field of an x-ray free-electron laser

Simulations of radiation damage in single-molecule imaging using an x-ray free-electron laser use atomic rates calculated in the lowest order. We investigate the difference in ion yield predictions using Hartree–Fock and Hartree–Fock–Slater approximations for light and heavy elements of biological significance. The results show that for the biologically abundant elements of the second and third rows of the periodic table both approximations agree to within about 6%. For the heavier elements beyond the fourth row the discrepancy rises to 11% for the range of pulse parameters covered in this work. The presented analysis can be used for error estimation in a wide range of ab initio simulations of the x-ray pulse interaction with biological molecules. We also discuss other atomic structure effects and show that their account has considerably smaller effect on the ion yields of respective elements compared to the choice of the approximation.

Mechanisms of grain boundary migration and growth in nanocrystalline metals under irradiation

Atomistic simulations of radiation damage uncover how grain boundaries (GBs) migrate and coalesce under irradiation in bicrystalline Cu. Planar GB migration biased by defect cluster-mediated attraction first leads to slow and steady motion. Subsequently, adjoining GBs coalesce into curved surfaces, where curvature-driven migration with a velocity three orders of magnitude higher than that of a planar boundary dominates motion, triggering rapid grain growth. This study reveals the atomistic mechanisms of radiation-induced grain growth, and has practical implications towards engineering radiation-tolerant nanostructures.

Molecular dynamics simulation of primary radiation damage in vanadium and alloy V-4Ti

In the framework of the molecular dynamics method, simulation of the primary radiation damage in crystallites of bcc both V and alloy V-4Ti was carried out. The initial temperature of the crystallites was 700 K, which corresponds to the operating temperature of the alloy when used as the material of the first wall of a thermonuclear reactor. The interatomic interaction in V and alloy V-4Ti was described on the base of modern potentials of interatomic interactions which took into account the interaction of screened ions at small interatomic distances and allowed to simulate correctly radiation damage. The impact of the decay particle on the atoms of the simulated crystallites, as a result of which a cascade of atomic displacements was generated, was simulated by assigning a pulse to one of the lattice atoms, the primary knocked out atom. The main characteristics of atomic displacement cascades in simulated crystallites were calculated: the number of defects at different stages of cascade development, the size of the radiation-damaged regions, and an analysis of the estimation of the number, types and sizes of the surviving radiation defects in crystallites. The results obtained are compared for vanadium and alloy V-4Ti.

Derivation of Transferable Pair Potentials and the Calculation of Intrinsic Defect Properties for Xenotime

A new force field has been empirically derived that is transferable across the YPO4, Y2O3, and P2O5 phases, utilizing a reverse Monte Carlo method. This method employs a simulated annealing technique with a logarithmic quench to fit potential parameters to observed crystallographic and mechanical properties, producing a force field suitable for simulating radiation damage events in an atomistic molecular dynamics regime. These potentials are used to investigate the defect properties of xenotime, where a wide range of intrinsic defects including Schottky, Schottky-like, Frenkel pairs, and anti-site defects have been investigated, both at infinite dilution and as defect clusters. A common feature in the lowest energy defect configurations was the presence of polymerized phosphate tetrahedra, forming P2O7 units. The trend in the formation energies for the Frenkel pair defects at infinite dilution was in good agreement with previously published simulations. However, the binding energy associated with the aggr...

E.D.R.A., the Argentine facility to simulate radiation damage in space

E.D.R.A. (Ensayos de Daño por Radiación y Ambiente/Test of Radiation and Environmental Damage) is a system composed of a beam line and a vacuum chamber installed at Tandar (a 20 MV Van de Graaff tandem accelerator). It has been designed to analyze devices' behaviour under simulated space conditions. The facility features: large irradiation area with uniform beam current, controlled target temperature in a range from LN2 to 300 °C, good target connectivity by multiple BNC and DB25 feedthrough connectors for in situ measurements and control during sample irradiations, and capability of irradiation under very low fluxes. Using this last feature, it is possible to measure space solar cells under proton irradiation to study Total Non Ionizing Dose (TNID), or semiconductor devices under operation to assess their response to Total Ionizing Dose (TID) or Single Event Phenomena (SEP) effects. Multiple sample studies are easily made by using remotely controlled rotary target holders, and a solar simulator illumination source is also available at the facility. The beam current and its uniformity can be assessed and monitored by a system of multiple, electron suppressed, Faraday cups. For SEP measurements, PIN diodes particle detectors are used.

The Influence of Photoelectron Escape in Radiation Damage Simulations of Protein Micro-Crystallography

Radiation damage represents a fundamental limit in the determination of protein structures via macromolecular crystallography (MX) at third-generation synchrotron sources. Over the past decade, improvements in both source and detector technology have led to MX experiments being performed with smaller and smaller crystals (on the order of a few microns), often using microfocus beams. Under these conditions, photoelectrons (PEs), the primary agents of radiation-damage in MX, may escape the diffraction volume prior to depositing all of their energy. The impact of PE escape is more significant at higher beam energies (&gt;20 keV) as the electron inelastic mean free path (IMFP) is longer, allowing the electrons to deposit their energy over a larger area, extending further from their point of origin. Software such as RADDOSE-3D has been used extensively to predict the dose (energy absorbed per unit mass) that a crystal will absorb under a given set of experimental parameters and is an important component in planning a successful MX experiment. At the time this study was undertaken, dose predictions made using RADDOSE-3D were spatially-resolved, but did not yet account for the propagation of PEs through the diffraction volume. Hence, in the case of microfocus crystallography, it is anticipated that deviations may occur between the predicted and actual dose absorbed due to the influence of PEs. To explore this effect, we conducted a series of simulations of the dose absorbed by micron-sized crystals during microfocus MX experiments. Our simulations spanned beam and crystal sizes ranging from 1μm to 5μm for beam energies between 9 keV and 30 keV. Our simulations were spatially and temporarily resolved and accounted for the escape of PEs from the diffraction volume. The spatially-resolved dose maps produced by these simulations were used to predict the rate of intensity loss in a Bragg spot, a key metric for tracking global radiation damage. Our results were compared to predictions obtained using a recent version of RADDOSE-3D that did not account for PE escape; the predicted crystal lifetimes are shown to differ significantly for the smallest crystals and for high-energy beams, when PE escape is included in the simulations.

Nanohardness measurements of heavy ion irradiated coarse- and nanocrystalline-grained tungsten at room and high temperature

Heavy ion irradiation was performed on coarse and nanocrystalline-grained tungsten (CGW and NGW, respectively) at room temperature (RT) and 1050 K from 0.25 to 4 dpa to simulate radiation damage for fusion energy applications. TEM and nanohardness measurements of irradiated samples were made to quantify the radiation tolerance of these two candidate materials. In this case, TEM is used to quantify the defect morphology at low dpa values and determine the barrier strength coefficients of the different defects using the dispersed barrier hardening (DBH) model. Nanohardness measurements and the determined barrier strength coefficients are then used to estimate the defect morphologies at higher dpa values where quantification with TEM is not reliable. Quantification of the damage at low dpa (0.25 dpa) showed different loop (at RT and 1050 K) and void (at 1050 K) densities and sizes for the two grain sizes. Nanohardness measurements performed on the samples showed a very small change in hardness for RT ion irradiation and a higher but more scattered increase in hardness for 1050 K ion irradiation. Using the Dispersed barrier hardening (DBH) model and the average loop/void density and size, the dislocation barrier (α values) values were shown to be very small (0.003 and 0.03 for the CGW and NCW respectively) for loop damage and of weak to moderate strength (0.13 for NCW and CGW) for void damage. The small barrier strength values for fine grained materials and the limitations of using nanohardness measurements with the DBH model to characterize irradiation damage are discussed.

Molecular Dynamics Simulations of Radiation Damage in Ti-Al based Structural Intermetallics

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