I understand how they calculated a maximum radius for the gold atom nucleus. What I don't realize is how did they know that the alpha particle wasn't hitting the nucleus. I mean: if they had used particles with 2 times that kinetic energy, they would have obtained half the value for the radius of the nucleus. If they had used particles with less kinetic energy they would have obtained a bigger radius.
I think the solution to this problem would have to increase the kinetic energy until the particles actually hit the nucleus. But how would they know that?