Todays discussion is on Uranium-236. So as we know during fission uranium-236 atom rapidly decays into an atom of Ba-141 (barium), an atom of Kr-92 (krypton) and three neutrons. Barium and Krypton are the fission products. As you know When a neutron passes near to a heavy nucleus, for example uranium-235 (U-235), the neutron may be captured by the nucleus. So when this happens Uranium-235 changes to Uranium 236. U236 is not stable and lasts a few nanoseconds. If we compare U 235 with U 238, U238 has the 3 extra neutrons in comparison to U 235 that provide additional Strong Nuclear Force to hold the nucleus together. So now the question is that U 236 has another extra neutron in comparison to U 235 so the strong force should make it stable then why it decays so easily? Adding extra neutrons to a nucleus doesnt just add more attractive force because of the strong interaction; it also adds repulsive force because of the Pauli exclusion principle--no two neutrons can be in the same state. When adding neutrons to a nucleus, there are certain points where the binding energy of the next neutron is significantly less than the last one. In laymans inaccurate terms, this means that, although two particles can be in the same place as each other, if they are, they will be moving at different velocities and so will shortly no longer be in the same place as each other (Pauli’s Exclusion Principle). Therefore nuclei which have a full outer neutron shell will have a higher binding energy than other nuclei with a similar total number of neutrons. I will explain this: If there are 11 neurons in an atom then 1st shell has 2 and 2nd shell has 8 so the third shell has only 1 neutron so the 3rd shell becomes unstable and repulsive force grows. So neutrons (and protons) in a nucleus have to occupy energy shells, in much the same way as electrons in an atom do. So adding a neutron to a nucleus could make it more stable, if, for example, the added neutron fills an energy shell; but it could also make it less stable, if, for example, the added neutron has to go into a new shell that wasnt occupied in the old nucleus. And there are lots of possible intermediate cases as the numbers of nucleons in each shell get larger. There are also effects due to the interaction between the spins of the protons and neutrons, which have to be taken into account.
Posted on: Sun, 29 Jun 2014 05:14:34 +0000
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