Fission cross sections are a measurable property related to the probability that fission will occur in a nuclear reaction. Cross sections are a function of incident neutron energy, and those for U and Pu are a million times higher than U at lower neutron energy levels. Absorption of any neutron makes available to the nucleus binding energy of about 5.3 MeV. U needs a fast neutron to supply the additional 1 MeV needed to cross the critical energy barrier for fission. In the case of U however, that extra energy is provided when U adjusts from an odd to an even mass. In the words of Younes and Lovelace, "...the neutron absorption on a U target forms a U nucleus with excitation energy greater than the critical fission energy, whereas in the case of n + U, the resulting U nucleus has an excitation energy below the critical fission energy."