Rutherford Scattering of an Alpha Particle

[ChrisWM]

Homework Statement

What kinetic energy does an a-particle have to have in order to just come into contact with the surface of an iron nucleus in a head on collision in a scattering experiment? (You may assume the iron is fixed and remains at rest during the interaction.) For the radius of the a-particle use ra = 2.60 fm and for the iron nucleus use rFe = 4.59 fm.

Relevant Equations

k=(z1e)(z2e)/(4pi*(eo)(rmin))

I have the equation but I am unsure of what my r min would be. Is it the sum of the radii or the difference? I am also confused on what z1 would be. I am fairly sure z2 is the atomic number of Fe(26) but I am unsure of this as well.
Edit: I just read that z1 could be 2, is this correct?

 

[Charles Link]

$r_{min}$ would be the sum of the radii, because in the potential energy formula, $r_{min}$ is the distance between the centers of the two charged nuclei. (The alpha particle is a helium nucleus, and yes $z_1=2$).

 

[ChrisWM]

$r_{min}$ would be the sum of the radii, because in the potential energy formula, $r_{min}$ is the distance between the centers of the two charged nuclei. (The alpha particle is a helium nucleus, and yes $z_1=2$).

Thank you so much!