Fast Neutron Attenuation and Energy Loss in Extended Shields

Abstract

This work is concerned with the investigation of the attenuation and energy degradation of fast neutrons passing through materials widely used in reactor shielding. Measurements and calculations have been performed for shields of iron and for homogeneous and heterogeneous media of iron-graphite and iron-polypropylene in the form of a cylinder having a monoenergetic point neutron source of energy 14.1 MeV produced by D(T,n)He reaction at its centre. All measurements were made with a proton recoil scintillation spectrometer using NE-213 liquid organic scintillator, and gamma ray background was discriminated by pulse shape discrimination using the zero crossing technique. Calibration of the spectrometer energy scale was made with monoenergetic neutrons of energies 2.48 MeV and 14.1 MeV from the D(d,n)He and D(T,n)He reactions respectively. These fixed points were need to scale the energy pulse height distribution determined by other workers for use in the conversion of integral pulse height distribution into neutron energy spectrum. This conversion was achieved by a method based on numerical differentiation. Calculations have also been carried out with the multigroup diffusion and removal diffusion equations in one dimension by approximating the experimental assembly to a spherical geometry. These calculations provided a test for cross section data sets compiled by Abagyan and Yiftah and Sieger. Measured and calculated spectra show reasonable agreement and shields containing hydrogen show stronger neutron attenuation than those containing graphite. Removal cross sections at 14 MeV have been experimentally determined for iron, carbon and hydrogen from the variation in transmitted neutron intensities with thickness for iron (steel) and double layers of iron and graphite or polypropylene. These have been compared with theoretical predictions by other workers and good agreement has been found.