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collisions with uranium. The relative importance of scattering collisions in graphite and uranium can be determined by comparing the scattering mean free paths of neutron in the two materials. Graphite has an atom density of Nc = 0.08 X i024/cc and an epithermal microscopic scattering cross section of ac    4.8 barns.  The corresponding scattering mean free path is

          Xs         =1I/Nca   = 2.6cm
            graphite        c  


Nuclear criticality calculations (which are discussed in the following chapter) indicate that a typical critical uranium-235 mass can be achieved with an atom density of approximately Nu = i019/cc. With an average epithermal scattering cross section of 10 barns, the scattering mean free path in uranium is

           Xs =1NUa S =14 C
                =1/Nua  = i0~ cm
           U        Uu


These mean free path values show that a neutron slowing down from fission energy to thermal energy will have about 4000 scattering collisions in graphite for each scattering collision in uranium. Thus the neutron spectrum will be determined primarily by collisions in graphite. This conclusion greatly simplifies the computation of effective absorption cross sections in the fuel region since the