Calculate the macroscopic absorption cross section for aluminum given that a thermal flux of 2.91x10 8 neutrons/cm sec (E = 0.0253 eV) is incident on

Calculate the macroscopic absorption cross section for aluminum given that a thermal flux of 2.91x10⁸ neutrons/cm² sec (E = 0.0253 eV) is incident on a 4.8 mm thick aluminum foil, and that after passing through the foil the beam contained 2.89x10⁸ neutrons/cm² sec.