How does one find interaction radius for Coulomb barrier?

[eigenmax]

For a proton striking an atomic nucleus (in a Cockcroft-Walton accelerator, for example), the Coulomb barrier must be overcome. The calculation of the Coulomb barrier is U = k Z₁Z₂ e² / r
r is interaction radius. How can I find that? For a proton to enter the atomic nucleus what would it be?

 

[mfb]

It is simply the electrostatic potential for two electric charges at distance r. What is unclear?
r will typically be about the sum of the nuclear radii, there are formulas to estimate that.

 

[eigenmax]

It is simply the electrostatic potential for two electric charges at distance r. What is unclear?
r will typically be about the sum of the nuclear radii, there are formulas to estimate that.

Ok, thank you! So it ought to be the radius of the proton + the target radius. Thank you!