I've been really interested in fission lately and have been trying to find out as much about it as I can. A lot of explanations I've read explain that the nucleus spits apart from the force of a neutron hitting it. I've also read that sometimes a nucleus will decay by spontaneous fission if it is unstable enough. This makes me wonder if it's the force of the neutron that makes it split. It makes sense to me for fissionable isotopes where the neutron has to have a certain energy to split it, but for fissile isotopes I get the impression that something like U-235 for instance would absorb the neutron and become U-236 for an instant then spontaneously fission. An article I found on Wikipedia on neutron cross-section had this written in the section "Actinide decay:" U-235+n=U-236*; U-236*(fission 81%) which looks like it shows the U-235 turning into U-236 for a moment before it fissions. I have no idea which is correct since I've seen it explained two different ways (or at least I think I have.) Can someone help clear this up for me? Another thing I'm curious about is how much energy a neutron has to have to be absorbed by a nucleus. Does it vary for different elements and isotopes of them? And one more: I read a bit on nuclear isomers where the particles in the nucleus have more energy than is normal. If fission happens when a nucleus absorbs a neutron that makes it unstable and then splits, does this explain that? (The * in the example above means that the nucleus is energized.) And if that's true, then does that mean that U-236 made from neutron absorption of U-235 would fission but U-236 made from beta decay of Pa-236 would not? Thanks for any help clearing this up for me.