U-238 Neutron Capture: Clarifying Misconceptions about Fast and Slow Neutrons

[morage_key_ri]

Wikipedia said U-238 is better with capturing fast neutrons than slow neutrons. shouldn't it be the other way round? I thought U-238 is not fissile but is fissionble by neutrons with high enough K.E, and doens't fast neutrons have higher K.E.??

 

[malawi_glenn]

U-238 is even-even and can't be fissioned by thermal (slow) neutrons, so it requires fast neutrons to be fissioned. The activation energy is 6.6MeV.

Where is the contradiction? You say that wiki claims that U-238 is better to capture fast neutrons than slow. You says that U-238 require neutrons with high enough K.E.

Where is your confusion?

 

[morage_key_ri]

Thank you for the reply,

wiki said fast neutrons required for U-238 to capture and become U-239, which then decays into Pu-239.

my confusion is: Won't fast neutrons more likely to cause fission than transform 238 to 239?

 

[malawi_glenn]

It depends on K.E

There is no contradiction since you are talking about two different things.

What you really need is a diagram over cross section for the two processes: "Pu-239 formation" and "fission" as a function of neutron energy.