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while a neutron at the lowest uranium-235 resonance has an energy of 1.0 eV. From these values, the second term of (2.14) is seen to be much smaller than the first, since the largest ratio of the second to the first terra is approximately 0.001. Therefore, the second term is considered negligible in dealing with solid fuel resonance absorption. The third term of (2.14) is proportional to the component of momentum of the target nucleus parallel to the direction of motion of the incident neutron. Due to its large mass as compared to that of the neutron, the nucleus has a large enough momentum to cause the third term to have a significant effect on the relative energy. Denoting by Vz the component of the velocity of the nucleus which is parallel to that of the neutron, the third term can be written as


          Pn   PN             N)
               -  M +v nvV z                      (2.15)
           m+M         m+M           n


Using (2.15) and the expression for the relative kinetic
                                 1    2eqain214
energy for stationary nuclei, E  . - pvn , equation (2.14) is simplified as


        Er --       p v - vVn E - v-E Vz          (2.16)


        Since only the nuclear motion parallel to the neutron direction is considered to be significant, the