Uranium

[Ed Aboud]

Hi all.
Why does Uranium-238 absorb fast moving neutrons very well but not slow moving ones?
My intuition tells me it should be the other way around but its not.
Thanks.

[strongmotive]

http://www.answers.com/topic/fast-fission?cat=technology

Maybe this can help ;)

[Ed Aboud]

Hmm, I still don't really get it.

[vanesch]

U-238 captures slow neutrons as well as fast, and even a bit more slow than fast. However, U-235 fissions MUCH MORE easily with slow neutrons than with fast, and hence the ratio for fission in U-235 over capture in U-238 is more favorable for slow neutrons than for fast.

[Ed Aboud]

Oh ok, I get it now. Thanks very much.

[malawi_glenn]

it has to with the pairing force, the answer vanesh gave you.