What is a head-on collision? For that,state the impact parameter.

(N/A) head-on collision occurs when an alpha particle is directed straight towards the center of the nucleus of an atom. In this case,the alpha particle approaches the nucleus,comes to a momentary rest at the distance of closest approach,and then reverses its path back along the same line.
The impact parameter $(b)$ is defined as the perpendicular distance of the initial velocity vector of the alpha particle from the center of the nucleus.
For a head-on collision,the alpha particle is aimed directly at the center of the nucleus,meaning the perpendicular distance between the path of the alpha particle and the center of the nucleus is zero. Therefore,the impact parameter $(b)$ for a head-on collision is $0$.