Why can Uranium 235 absorb only slow moving electrons and Plutonium 239 absorb only fast moving electrons to undergo fission? What's the difference between them that makes this so?

Question

anonymous · Answer

It's neutrons, not electrons.

anonymous · Answer

Uranium 235 can absorb neutrons of any energy and fission, because it is fissile.
Plutonium 239 can also absorb neutrons of any energy, but only the fast ones will cause it to fission. This is because it is fissionable.

The difference is in comparing the binding energy involved in adding a neutron to the critical energy required to fission. If the binding energy is greater than the critical energy, the isotope is fissile. If the binding energy is lower than the critical energy, the isotope is fissionable and needs extra energy in the form of kinetic energy.