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Name: allen
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 1999 


Question:
Why is U(235) less stable than U(238) ? At least that's the impression I get from reading about efforts to separate them for making the A-bomb. I would think that the whole stability thing was directly related to atomic weight. How could adding some neutrons (and such a small percentage) to an unstable mass make it more stable.


Replies:
Stability of a nucleus is a fight between the strong attractive nuclear force, which is very short range and which binds nucleons without paying any attention to whether they are protons and neutrons, and the relatively weak repulsive electromagnetic force between protons, which is long range. Since neutrons attract protons and since they don't repel each other or protons, the more neutrons the better. However, neutrons are inherently unstable particles, and they decay into protons, electrons, and antineutrinos if it's energetically possible for them to do so.

"Also, what determines the mass of the fission products of U(235). I read that they are usually Barium and Krypton. Why these, and not a range of elements?"

It's partly a statistical thing. One way of looking at a nucleus is as a swirling collection of caucuses involving collections of nucleons that make up all possible arrangements of the available protons and neutrons. If a collection of two protons and two neutrons (a particularly stable arrangement called an alpha particle, also called a helium nucleus) happens to form near the surface of the nucleus, for example, it may just split off and go its own way. You might imagine all the nucleons deciding what would be the most energetically favorable arrangements for themselves, with this decision tempered by how closely the existing arrangements of nucleons actually resemble those arrangements.

" I hope this can also give me some insight into what happens to an atom (nucleus) of crystaline Silicon when struck by a Neutron."

First of all, you can forget about that word "crystalline". The silicon nucleus could not care less how it is arranged in space with other silicon nuclei. The binding that produces a crystal is of valence electrons, and they are so far away from the nucleus and their binding energies are so weak that their influence on what happens inside is negligible.

I don't have a table of the nuclear isotopes handy, and I don't know right off what happens, but I can tell you that silicon, with 14 protons and 14 neutrons, does not want another neutron. The atomic weight of natural silicon is 28.086, which tells you that only a very small percentage (roughly 1 out of 280) of silicon nuclei have 15 neutrons. The next stable arrangement of nucleons (P) has 15 protons and 16 neutrons.

Tim Mooney


The rate of decay of a radioactive nucleus depends in a complicated way on a number of factors. It's not just a matter of the amount of energy released by the decay. It's true that U-235 has a shorter half-life than U-238. Perhaps someone with a better nuclear physics backgroung than mine can tell you why.

As for the yields of fission products, there IS a distribution of products. Most frequently, U-236 fission (U-235 after neutron capture) gives a lighter and a heavier daughter fragment. Barium and krypton are not the only products; everything from germanium to dysprosium arises. The greatest yields are around masses 90 and 138.

Richard Barrans Jr., Ph.D.



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