U235 vs U238
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.
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
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.
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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Update: June 2012