Fission: how far from the center of a U-235 nucleus can a neutron strike

[1907Quarter]

and successfully fission. let's say a neutron approaches the spherical nucleus from a particular distance. its possible route to the nucleus has a conical shape. how far from the center can it strike and still fission? If it just nicks it just inside a tangent line will it fission?

 

[Janus]

It's not quite that simple. The property of interest here is the cross section of fission for the nucleus, and that depends on more than 1 factor.

For example, the the cross-sectional area of a U235 nucleus is about 1 barn. However, for slow moving neutrons, the cross section for fission can be as large as 1000 barns.