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Re: Adding neutrons

Submitted by Chatman on Thu, 12/24/2009 - 22:44

hey how hard is it too add a neutron and make the nucleus capture it to lets just say U-238 to make it U-239.


I suspect it is somewhat easier than proton capture where there would be repulsion from the positive nucleus, but not with neutral neutrons. However, energy will be needed to accelerate the particle.

Looking up on the internet I found a resource which says that a neutron can be packaged with a proton for acceleration, so my thought about no repulsion against neutrons would not be valid.

http://www.newton.dep.anl.gov/askasci/phy00/phy00734.htm


So as long as you have a particle accelerator and fire a neutron at the element it will capture it.


It is extremely difficult to add neutrons/protons to a nucleus. The strong nuclear force is the most powerful of the 4 fundamental forces (over an extremely small distance). The particle must first penetrate through the electron cloud, then squeeze itself somewhere in the nucleus. It can be done, however, with massive particle accelerators (atom smashers) that take up several city blocks. When you add additional protons/neutrons to some elements, they spontaneously decay to reestablish the ideal proton/neutron ratio. Although I suspect that protons would be somewhat more difficult to add a proton than a neutron, just as Kingchemist said.


Well if you fired hundreds of neutrons at it then would that probably work?


That would increase the probability of the neutrons hitting the nucleus, yes, but that doesn't make the atom any easier to penetrate


Okay so its basically just chance that the neutron is captured by the nucleus.


It is actually not that difficult to add neutrons to a nucleus, as long as you have the right nucleus, nor are large energies required.  One of the things associated with the nucleus are shells, in the same principle as electron shells, thus sometimes the acquisition of a neutron creates a more stable nucleus.

The energy of the incoming neutron also has a lot of importance on whether the resulting isotope will spontaneously undergo fission or not.  As an example lets take the case of U-238 and U-235 since this actually goes directly into why U-235 is better for nuclear energy and weapons vs U-238.

Neutrons absorption by both isotopes is quite common

238U + 1n ---> 239U

235U + 1n ---> 236U

One of the reasons that neutrons are actually not that difficult to capture is first because they are neutral and as such there is no Coulomb barrier as they approach the nucleus. Second they show almost no interaction with electrons and thus can penetrate teh atomic electron cloud easily.  Third because of the specifically nuclear force they are actually strongly attracted to the nucleus at short ranges.

When the neutron is in close range of an acceptable nucleus they are bound into this nucleus with a release of their binding energy which is on average about 7 MeV, (less than this in our examples).

Now back to the above example when U-235 captures a neutron it undergoes fission even if the neutron has negligible kinetic energy (slow neutron capture).  In the case of U-238, however, fission does not occur unless the neutron has kinetic energy greater than about 1 MeV.  In U-238 the captured neutron starts a new shell and is less tighly bound then the neutron caputed by U-235, which actually completes a nuclear shell.  That means that the energy released by the capture of U-235 is greater than that released by U-238, 6.6 MeV vs 4.9 MeV; fission in U occurs at about 5.5 MeVs.

Other examples of neutron capture occurs in the creation of the medicinal isotope P-32 from
P-31.

Probably the most obvioud example of neutron absorption is in control rods of nuclear reactors.  Both Cd-113 and B-10 are used as neutron absorbers, since they can easily capture neutrons in order to control the nuclear reactions taking place.

Now as a disclaimer, when using the terms easily and slow neutrons, lets not forget that we are talking in reltive terms to nuclear energies and velocities of subatomic particles.


okay So let me get this straight.
If you fire a neutron at an atom it will almost definatly capture it due to the fact that it is neutral and isnt effected by anything else in the atom. Depending on the type of atom it will undergo decay to get to a more stable form or as in the case of U-235 will undergo nuclear fission.
Is this correct?