MadSci Network: Physics

Re: Why is it that tritium does not absorb or attract neutrons?

Date: Sat Oct 2 21:54:56 1999
Posted By: Everett Rubel, MadSci Admin
Area of science: Physics
ID: 938723457.Ph


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 

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 

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.

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