range, the uranium-235 microscopic absorption cross section, a (E), exhibits a large number of resonances

(25). The usual procedure in evaluating the integral in the numerator of (2.5) is to assume that each well defined resonance is accurately represented by the single level Breit-Wigner formula (26). This formula expresses the energy dependent absorption cross section as

                               rr
            a ()  wXgjny
            aE  =r - E0)2 + (r/2)2                (2.6)


The symbol E0 denotes the energy of the resonance and Er is the kinetic energy associated with the relative velocity between neutron and nucleus. In the case of a stationary nuclei, Er is given by
                         1    2
                    E  =   j Vn2                  (2.7)
                    r    2  n

where v  is the neutron speed in the laboratory coordinate system, and i is the reduced mass as defined by

                        _ mM                      (2.8)
                      m + M


with m and M representing the mass of the incident neutron and the target nucleus, respectively. The symbols ry and r are the radiative capture and neutron widths (27) which define a total width F as


S=   Y + Fn


- 25 -


(2.9)