Is Fission Classified as Capture or Noncapture?

[cooper7d7]

I apologize that me first post is such a silly one.

I am trying to solve problem 1.2 in Stacey. It asks for the probability of a 1 MeV neutron slowing to thermal. I can determine the number of collisions required in the water / uranium environment. But, when I go to select the proper crosssections, I think I should just use scattering but I was wondering if fission is considered capture or noncapture?

Thanks, I really enjoy this forum...

-Will

 

[Astronuc]

The fission cross-section would be considered capture, e.g. part of the total capture. It represents a loss during the slowing down process, but then it produces 2 or 3 fast neutrons.

 

[cooper7d7]

Ok

Makes sense, I was just thinking that maybe one of the 2 or 3 that are released was considered the same as the impacting neutron, in which case it was not captured. But fission is treated just like n,gamma.

Thanks,

-Will

 

[Morbius]

Makes sense, I was just thinking that maybe one of the 2 or 3 that are released was considered the same as the impacting neutron, in which case it was not captured..

Will,

You really can't do what you suggest of considering one of the fission neutrons as to
be the same as the impacting neutron.

Label all your neutrons with their energies. Now consider a case of "thermal fission";
that is a low energy or "thermal neutron" causes a fission; which in a thermal reactor
is the most common type of fission.

So the impacting neutron has low energy, while the 2-3 fission neutrons have high
energy. Because of the energy difference between the impacting neutron [ low energy ]
and the fission neutrons [ high energy ], you can't consider ANY of the fission
neutrons to be a "continuation" of the impacting neutron.

Dr. Gregory Greenman
Physicist