Molecular dynamics study of structure transformation and H effects in irradiated silica F. Mota a,b, * , M.-J. Caturla c , J.M. Perlado b , J. Mollá a , A. Ibarra a a Materiales Para Fusion. Ciemat. Avda Complutense 22, Madrid 28040, Spain b Instituto de Fusion Nuclear, UPM, Madrid, Spain c Universidad de Alicante, Dep. Fisica Aplicada, Alicante, Spain article info abstract Fused silica is a key component in a number of diagnostics for the Safety and Control Systems of the ITER machine as well as in the final focusing optics of lasers for inertial fusion and it will be exposed to high radiation fields (neutron and gamma) in both environments. Silica properties of interest are closely related to the presence of defects and its changing structure. On the other hand, some experimental results show that radiation damage depends on its hydrogen content. In this work we present molecular dynamics (MD) simulations to study the effects of displacement cascades on the ring size distribution and on the presence of the hydrogen atoms. Changes in the ring size distribution and variation of the local relative densities have been observed to be a function of the primary knock-on atom energy. And on the other hand, the effects of the hydrogen atoms in the evolution of numbers of defects have been determined. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Vitreous silica is a material of great importance, both from the point of view of the fundamental physics, due to its use as model structure, and from the point of view of its great variety of techno- logical applications. The electrical, dielectric and optical properties, characteristic of vitreous silica, together with its chemical inert- ness and thermal stability are the main factors that determine the great versatility of applications of this material. Fused silica is a candidate material for optical and radio-frequency diagnostic systems in magnetic confinement fusion reactors and as final op- tics in inertial confinement fusion reactors [1–5]. In both cases this material will be exposed to high radiation fields, both neutrons (with energy up to 14 MeV) and gamma rays. Radiation induces optical absorption, creating point defects that can act as colour centers [6]. These defects which determine in last instance the properties of the material may be detected, among other methods, by studying the optical absorption bands or its photoluminescence. At the present time there is abundant information on the different defects and their optical properties [7]. In spite of this, we have to increase the knowledge of them and especially about of their evo- lution under irradiation. Molecular dynamics (MD) is a computer simulation technique that allows one to obtain information on the atomic structure of the system and on its physical properties. It has proved to be useful in the description of solid networks as well as the defects produced by irradiation at keV energies in metals and semiconductors [8,9]. However the number of MD studies in oxides and in particular, in amorphous systems, is more reduced. The objective of this work is the study through MD of the lattice structure modification and the defects variation that occur in fused silica produced by radiation. 2. Simulation modelling Fused silica is an amorphous system, formed by silicon atoms tetrahedrally bonded to oxygen atoms. The interatomic potential used for our calculations is the one developed by Feuston and Gar- ofalini [10,11]. This potential was fitted to reproduce the structure factor of this amorphous system as determinated experimentally through X-ray diffraction and neutron scattering data. The different vitreous silica materials studied in this paper were generated starting with the cubic b-cristobalite structure. The ini- tial amorphous structure was generated by melting a beta-cristo- balite lattice of SiO 2 (at 7000 K) and subsequently quenching down to 300 K (in step of 1000 K). The whole process takes a sim- ulation time total of 50 ps (in simulations steps of 0.5 fs). This pro- cedure was used previously in similar studies [12,13]. Simulations have been performed using a parallel molecular dynamics code MDCASK [14]. This code has been run in parallel computer SGI Al- tix 3700 (96 Itanium 2 processors at 1,3 GHz). In these calculations a simulation box with a total of 192 000 atoms was used, that occupies a volume of approximately 15 Â 15 Â 15 nm 3 . Hydrogen 0022-3115/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.12.062 * Corresponding author. Address: Materiales Para Fusion. Ciemat. Avda Complu- tense 22, Madrid 28040, Spain. E-mail address: fernando.mota@ciemat.es (F. Mota). Journal of Nuclear Materials 386–388 (2009) 75–78 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat