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The binary collision approximation (BCA) signifies a method used in ion irradiation physics to enable efficient computer simulation of the penetration depth and defect production by energetic (with kinetic energies in the kilo-electronvolt (keV) range or higher) ions 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). Between the collisions, the ion is assumed to travel in a straight path, experiencing electronic stopping power, but losing no energy in collisions with nuclei.〔R. Smith (ed.), (Atomic & ion collisions in solids and at surfaces: theory, simulation and applications ), Cambridge University Press, Cambridge, UK, 1997 ISBN 0-521-44022-X〕 ==Simulation approaches== In the BCA approach, a single collision between the incoming ion and a target atom (nucleus) is treated by solving the classical scattering integral between two colliding particles for the impact parameter of the incoming ion. Solution of the integral gives the scattering angle of the ion as well as its energy loss to the sample atoms, and hence what the energy is after the collision compared to before it.〔 The scattering integral is defined in the centre-of-mass coordinate system (two particles reduced to one single particle with one interatomic potential) and relates the angle of scatter with the interatomic potential. It is also possible to solve the time integral of the collision to know what time has elapsed during the collision. This is necessary at least when BCA is used in the "full cascade" mode, see below. The energy loss to electrons, i.e. electronic stopping power, can be treated either with impact-parameter dependent electronic stopping models ,〔L. M. Kishinevskii, Cross sections for inelastic atomic collisions, Bull. Acad. Sci. USSR, Phys. Ser. 26, 1433 (1962)〕 by subtracting a stopping power dependent on the ion velocity only between the collisions,〔J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping and Range of Ions in Matter, 1985 ISBN 0-08-022053-3 and references therein.〕 or a combination of the two approaches. The selection method for the impact parameter divided BCA codes into two main varieties: "Monte Carlo" BCA and crystal-BCA codes. In the so-called Monte Carlo BCA approach the distance to and impact parameter of the next colliding atom is chosen randomly for a probability distribution which depends only on the atomic density of the material. This approach essentially simulates ion passage in a fully amorphous material. (Note that some sources call this variety of BCA just Monte Carlo, which is misleading since the name can then be confused with other completely different Monte Carlo simulation varieties). SRIM and SDTrimSP are Monte-Carlo BCA codes. It is also possible (although more difficult to implement) BCA methods for crystalline materials, such that the moving ion has a defined position in a crystal, and the distance and impact parameter to the next colliding atom is determined to correspond to an atom in the crystal. In this approach BCA can be used to simulate also atom motion during channelling. Codes such as MARLOWE operate with this approach. The binary collision approximation can also be extended to simulate dynamic composition changes of a material due to prolonged ion irradiation, i.e. due to ion implantation and sputtering. At low ion energies, the approximation of independent collisions between atoms starts to break down. This issue can be to some extent augmented by solving the collision integral for multiple simultaneous collisions.〔 However, at very low energies (below ~1 keV, for a more accurate estimate see ) 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. The MD simulations can either follow only the incoming ion (''recoil interaction approximation'' or RIA ) or simulate all atoms involved in a collision cascade . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Binary collision approximation」の詳細全文を読む スポンサード リンク
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