1) Impact parameter (b) : Impact parameter is defined as the perpendicular distance of the initial velocity vector of the alpha particle from the central line of the nucleus, when the particle is far away from the nucleus of the atom.
2) Scattering angle `(theta)` : The scattering angle `(theta)` is the angle between the asymtotic direction of approach of the `alpha` - particle and the asymtotic direction in which it receeds.
3) The relation between b and `theta` is b `=(1)/(4 pi epsi_(0))(Ze^(2))/(E)cot""(theta)/(2)" where "E= K.E" of "alpha-" particle "=(1)/(2) mv^(2)`.
