Distance of closest approach between an alpha particle to a gold nucleus

If an alpha particle with a kinetic energy E_a is fired directly towards a gold nucleus it will feel a repulsion which increases as it gets closer - climbing the potential hill surrounding the nucleus. When all the kinetic energy has been converted to potential energy the alpha particle (charge q) has reached its distance of closest approach (d₀) and comes to rest. At that point :



The diagram shows the paths of other alpha particles that were not travelling directly towards the centre of the nucleus. These are scattered.