- 30 -


probability of a neutron collision with a nucleus witi velocity V z is proportional to the number of nuclei with this velocity. The number of such nuclei is given by the one dimensional Maxwellian distribution MN                     1/2
          P(Vz) = (2)     e-MNVz2/2kT             (2.17)


Hence the effective cross section for a neutron with kinetic energy E is


          O(E)   f J°(Er)P(Vz)dVz                 (2.18)


Using equation (2.16), the effective resonance absorption cross section is

                           EE      dEr
          °a (E) = Ja(Er)P(        d r            (2.19)


This cross section is evaluated by substituting (2.12) and (2.17) into (2.19). Using the notation of Dresner (29), this substitution yields

                    r
                 (E) -Y  e(Ox)                    (2.20)
           a      o0 r

where

                         1 O      2 (X
                   e   r.a exp[- e(x - y),] T(e,x)          -]           1 +         - dy    (2.21)
                   2   -      1 wy