Neutron Dose Rate Attenuation by Iron and Lead

Effective removal cross sections for iron and lead that can be applied to fast-neutron dose rate calculations have been determined from calculated spatial-spectral neutron distributions as a function of succeeding polyethylene thickness. These cross sections increase with polyethylene thickness, and for large polyethylene thicknesses, they are in agreement with those derived from experiment. From the spatial-spectral neutron distributions, relative contributions of various neutron energy ranges to the neutron dose rate have been calculated as a function of succeeding polyethylene thickness. For polyethylene thicknesses > 30 cm, fast (E > 302 keV), epithermal (302 keV > E > 0.625 eV), and thermal (E < 0.625 eV) neutrons contribute 83, 6, and 11%, respectively, to the neutron dose rate.