The Influence of PKA Direction on Displacement Cascade Evolution
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The Influence of PKA Direction on Displacement Cascade Evolution Roger E. Stoller Oak Ridge National Laboratory, Oak Ridge, TN, USA, [email protected] ABSTRACT An extensive database of atomic displacement cascades in iron has been developed using molecular dynamic simulations. More than 300 cascades have been completed at 100K at energies between 100 eV and 100 keV, with fewer simulations at 600 and 900K. A systematic evaluation of the database has revealed an unexpected influence of PKA direction in low energy simulations. For primary knockon (PKA) directions that lie in the close-packed {110} planes, the cascade tends to be constrained to develop in only two dimensions. This planar “channeling” leads to much higher point defect survival than when a random high-index PKA direction is used to initiate the cascade. For example, the average number of stable Frenkel pair produced in 300 eV cascades is about 2.1 for cascades initiated with a [135] PKA, and 3.2 for [114] cascades. Some influence of this PKA direction effect was observed for energies up to 2 keV. The interstitial clustering behavior also appears to be affected in cascades with high defect survival. INTRODUCTION The exposure of materials to irradiation by high-energy particles leads to the formation of atomic displacement cascades. Depending on the energy of the initiating particle, these events involve from a few atoms to a few tens of thousands of atoms, and occur on the time scale of a few pico-seconds. The method of molecular dynamics (MD) was first applied to simulate cascade evolution around 1960 [1], but only in the last ten years have the capabilities provided by modern computers made it possible to simulate the high energy cascades relevant to nuclear energy systems [2-12]. For example, these same computational capabilities have permitted the development of an extensive library of atomic displacement cascade simulations in iron with cascade energies as high as 100 keV [12]. The analysis discussed in Ref. [12] provided a basis to determine the number of simulations required to obtain statistically-relevant mean values for several primary damage parameters as a function of cascade energy and temperature. This database has also permitted meaningful comparisons of primary radiation damage parameters in various irradiation environments to be made [13]. In order to avoid lattice effects such as channeling and directions with particularly low or high displacement thresholds, most MD cascade simulations are initiated using a high index PKA direction. For example, most of the simulations in the database discussed in Ref. [12] were generated using a [135] PKA direction. An initial investigation of PKA direction effects using 1 keV cascades indicated that mean values obtained with [135] PKA should be representative of the average behavior at this energy [10]. However, further analysis of the cascade database revealed unexpected phenomenon in the lower energy cascades that lead to relatively large differences in point defect production. These differences are discu
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