Binary Collision Approximation
The binary-collision approximation (BCA) is a set of assumptions that tremendously reduce the computational complexity of simulating ion-material interactions. Ions interact with materials through nuclear and electronic stopping. These assumptions can be summarized as:
- Particles in the code represent many ions or atoms
- Energetic particles interact with material atoms through a series of discrete, binary collisions
- Nuclear collisions occur at mean-free-path distances
- Between collisions, particles travel in straight lines along which nonlocal electronic stopping is applied
- Binary collisions are calculated by solving the classical scattering integral for a chosen interatomic potential
- Material atoms are allowed to transfer momentum to tracked collision partners following any collision, but only collisions that transfer an energy greater than the displacement energy are removed from their original locations
- Particles are stopped when their kinetic energy drops below a cutoff energy in the material, or when they leave the simulation boundary outside of the material
- Particles leaving the material experience a locally planar surface binding potential
For an overview of the BCA, see Eckstein[1] and Robinson[2].
Coordinate system in the BCA as implemented in rustbca.
[1] W. Eckstein, Computer Simulation of Ion-Solid Interactions (1991)
[2] M. T. Robinson, The binary collision approximation: Background and introduction, Rad. Eff. and Def. in Solids (1994)