From what I can see for thermal neutrons the cross-sections for fission for the two nuclei are:
U235 - 346 barns
Pu239 - 695 barns
And for fast neutrons the the cross-sections are
U235 - 1.8 barns
Pu239 - 1.7 barns
So could I conclude from these that both nuclei are much more...
Why do different nuclei need different neutron speed to achieve fission??
My understanding is that U-235 requires a thermal neutron in order to be likely to cause a fission reaction, however Pu-239 requires the additional energy of a fast neutron in order to cause the nucleus to undergo fission...