Binary collision approximation

In condensed-matter physics, the binary collision approximation (BCA) is a heuristic used to more efficiently simulate the penetration depth and defect production by energetic ions (with kinetic energies in the kilo-electronvolt (keV) range or higher) in solids.

In the method, the ion is approximated to travel through a material by experiencing a sequence of independent binary collisions with sample atoms (nuclei).

[3][7] However, at very low energies (below ~1 keV, for a more accurate estimate see [8]) the BCA approximation always breaks down, and one should use molecular dynamics ion irradiation simulation approaches because these can, per design, handle many-body collisions of arbitrarily many atoms.

If the code does not account for secondary collisions (recoils), the number of defects is then calculated using the Robinson extension of the Kinchin-Pease model.

The BCA simulations give naturally the ion penetration depth, lateral spread and nuclear and electronic deposition energy distributions in space.

[12][13] BCA codes can, however, be extended with damage clustering and recombination models that improve on their reliability in this respect.

Schematic illustration of independent binary collisions between atoms
Schematic illustration of a linear collision cascade . The thick line illustrates the position of the surface, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material. The purple circle is the incoming ion. Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively. In between the ballistic collisions the ions move in a straight path. BCA can in "full cascade mode" describe well linear collision cascades.