- 26 -


The reduced neutron wavelength is given by


                   Xh
                   27r  27 (2pE r) 1/2            (2.10)


The statistical spin factor gj (28) is equal to (2J + 1)/ 2(21 + 1), where J is the spin quantum number of the compound nucleus formed by the target nucleus and the neutron, and I is the spin quantum number of the target nucleus. The peak value of the total (absorption plus scattering) cross section, o0, occurs at E = E0 and has the value

                          F
               4 4.g7r 2   n                      (2.11)


Using (2.11) and (2.6), the absorption cross section becomes

                          r1
               a (Er) = 0          1      2       (2.12)
                               1 + , r/2



        If the absorbing nuclei were stationary, Ea. would be evaluated from (2.5), with a (E r) given by (2.12) and Er given by (2.7). In reality, all absorbing nuclei have a kinetic energy at least as great as that corresponding to the temperature of the surrounding medium. The thermal motion associated with the kinetic energy effectively