|MadSci Network: Physics|
Damoon, Thanks for the question. I think that you are right about the attraction of nucleons, but you are not considering something of fundamental importance to many quantum mechanical systems, the behavior of fermions. You are right about there being an attractive potential between nucleons. This potential operates over a short range, and becomes repulsive at very short ranges. If there is an attractive potential then why don't neutrons passing by a tritium get pulled into the nucleus? The reason has to do with the behavior of particles which obey the rules of interaction described by Fermi. Both neutrons and protons are particles that are fermions. Fermions are in a sense antisocial. They cannot ever share the same quantum numbers among themselves. What this means in a nucleus is that fermions each occupy an individual quantum state in the nucleus. The fermions try to occupy the states of lowest energy. A fermion that attempts to join the nucleus must join at an energy level that is higher than the energies of the fermions that are already there, since they have already taken the more "desirable" low energy states. The tritium nucleus has one proton and two neutrons. It so happens that the two neutrons fill what is called a "shell" so that another neutron that tries to join the nucleus must occupy a state in the next higher shell. This means that the neutron would have to join a much higher energy quantum state than the neutrons that are already there. There is a large gap in energy between the lower filled shell and the empty upper shell. The energy of the neutron in the higher shell for tritium is high enough that the neutron is not bound, so it can not join the nucleus in any long term sense. There is another way that nucleons with too many neutrons are unstable. If the energy level of the highest energy neutron is more than a few MeV above the energy of the lowest open energy level for a proton in the nucleus, then the neutron can "decay" into a proton, and electron, and an antineutrino. The electron and antineutrino escay the neucleus, and the proton "falls" into the open energy level. This process is called beta decay. This is just a sketch of what is going on with nucleons in a nucleus. There are other factors to be considered to explain nuclear structure, but I believe that the behavior of fermions has the most important effect.
Try the links in the MadSci Library for more information on Physics.