The primary damage in Fe revisited by Molecular Dynamics and its binary collision approximation
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The primary damage in Fe revisited by Molecular Dynamics and its binary collision approximation C.S. Becquart1, M. Hou2 and A. Souidi3 1 Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Université de Lille I, 59655 Villeneuve d’Ascq Cédex, France 2 Physique des Solides Irradiés, Université Libre de Bruxelles, Bd du Triomphe, B-1050 Brussels, Belgium 3 Centre Universitaire de Saida, BP138, EN-NASR, Saida 2000, Algeria
ABSTRACT Molecular Dynamics (MD) is a very powerful tool for studying displacement cascades initiated by the neutrons when they interact with matter and thus evaluate the primary damage. The mean number of point defects created can be obtained with a fair standard error with a reasonable number of cascade simulations (10 to 20 [1]), however other cascades characteristics (spatial distribution, size and amount of defect clusters ...) display a huge variability. Therefore, they may need to be studied using faster methods such as the Binary Collision Approximation (BCA) which is several order of magnitude less time consuming. We have investigated the point defect distributions subsequent to atomic collision cascades by both MD (using EAM potentials for Fe) and its BCA. MD and its BCA lead to comparable point defect predictions. The significant similarities and differences are discussed. INTRODUCTION Displacement cascades have been studied for more than 30 years using numerical simulations. In this work, we have compared BCA results with some MD simulations of cascades in Fe using the EAM potential developed by Ludwig et al. [2]. Full MD does not allow accumulating statistics over large samples of collision cascades in the energy range of interest for radiation damage studies. This drawback is drastically reduced by the so-called BCA of MD, at the expense of an approximate treatment of multiple simultaneous interactions. The BCA is several orders of magnitude less time consuming than MD and therefore allows reasonably significant statistics in cases of widespread statistical distributions. However, since the consequences of this approximate treatment are not all fully identified, it is wise to ground the BCA on the basis of MD results as much as possible. MOLECULAR DYNAMICS AND ITS BCA The MD code used, DYMOKA, [3], is a slightly modified version of CDCMD, a user oriented code developed by J. Rifkin: http://www.ims.uconn.edu/centers/simul/index.htm#xmd. The Newtonian equations of motion are integrated using a fifth order Gear predictor-corrector algorithm. The neighbour search is done through a link cell method combined with a Verlet list so that the code is fully linear with the number of atoms. The interatomic potentials are tabulated and interpolation of the potentials is made through a 5th order Lagrange polynomial. To simulate displacement cascades, the following commonly used approximations were made: the effect of electron-phonon coupling has been ignored, the boundary atoms were not damped
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