a N_Al a,Al N_Si a,Si 5.64  x  10^22atoms cm³ 0.23  x  10²⁴  cm² 2.85  x  10^21atoms cm³ 0.16  x  10²⁴  cm² 0.0134  cm¹ s N_Al s,Al N_Si s,Si 5.64  x  10^22atoms cm³ 1.49  x  10²⁴  cm² 2.85  x  10^21atoms cm³ 2.20  x  10²⁴  cm² 0.0903  cm¹ a 1 a 1 0.01345 cm¹ 74.3  cm s 1 s 1 0.0903  cm¹ 11.1  cm DOE-HDBK-1019/1-93 Reactor Theory (Neutron Characteristics) NUCLEAR CROSS SECTIONS AND NEUTRON FLUX Rev. 0 Page 13 NP-02 Step 2: The   macroscopic   cross   sections   for   absorption   and   scattering   are calculated using Equation (2-4). Step 3: The mean free paths are calculated by inserting the macroscopic cross sections calculated above into Equation (2-3). Thus, a neutron must travel an average of 74.3 cm to interact by absorption in this alloy, but it must travel only 11.1 cm to interact by scattering.