الفهرس 
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Abstract
The neutron transport is a complex phenomenon that involves absorption, scattering, moderation and alteration of the neutrons properties in the medium. Both of the mathematical models and the computational tools are used to simplify and evaluate the extent of the disturbances resulting from the presence of a sample or any substance in the neutron field when used in various applications. The present work focuses on two very important phenomena about the use of neutrons in the various applications of neutron activation. The first phenomenon is the neutron self-shielding factor of different materials having different shapes and compositions, the phenomenon associated with reduction of the neutron flux affecting the material as it penetrates the material to deeper portions. The second focal point is the perturbations occurred in the energy distribution of neutron in consequence of the existence of a sample in the neutron field, the phenomenon associated with absorbing material. The two phenomena, together, represents the biggest problems in the neutron applications.
A mathematical model, based on neutron transport theory, was developed to obtain the self-shielding factor and the attenuation of the flux as neutrons travels further inside the sample, while the Monte Carlo N-Particle Transport Code (MCNP) was used to investigate the fate of neutrons upon existence of a material (absorber or scatterer). Some experimental results together with reported experimental values of the neutron self-shielding factors was used to verify the mathematical model.
The results of the first scope of this thesis was a sigmoid form of the neutron self-shielding factor in the form
G_(energy domain)= (Σ_t/Σ_s )x(1+ l ̅/((1-exp(-Σ_t l ̅P_escape )) ) (Σ_t Σ_a)/Σ_s P_escape )^(-1)
The derived mathematical model was adopted to use the well-known integral absorption, scattering, and total cross-sections of the materials (Σa, Σs, and Σt respectively) input parameter instead of the large amount of spectroscopic input parameters used in the previous models. In addition, exact theoretical formulae for the average neutron chord-length (l ̅) and probability of neutron interactions are given for specific shapes and energy domains. In thermal neutrons energy range, the energy bin escape probability pescape=1; while in epi-thermal neutron field, the adopted value of pescape= 2 √(2&|Σ_a-Σ_s | )/(√(2&Σ_a ) ∛(Σ_s l ̅ )). The most important parameter was the average chord length, which can be considered a measure of the migration length of neutrons inside the material, enabled correlating the measurements of the infinite dilution self-shielding factor to samples having undiluted reference samples as directed from the agreement between values of Gth for zinc and mercury and the expected values obtained mathematically. The thermal neutron self-shielding factor for indium, gold, zinc, and mercury was determined experimentally. We compared our findings with those reported in the literature using our mathematical. Results showed that neutron self-shielding has to be determined in samples with high macroscopic absorption cross-sections, especially in nuclear activation techniques due to the absorption of neutrons in the sample itself. The perfect match between these types of data supports the validity of linking the average chord length in the mathematical model with the neutron chord length in the material having a convex shape. The validity extends to epithermal energy region with the modified probability of neutron interactions in the epithermal energy region.
The second scope of the thesis is associated with neutron energy perturbation was done using the Monte Carlo N-Particle Transport Code (MCNP). The experiment’s geometry was created to simulate an isotopic neutron field with a neutron source 241AmBe, whose production physics for neutrons depend exclusively on interactions with alpha-beryllium and are unaffected by what happens to them after they are produced. The geometries were created to provide consistent neutron concentrations in every neutron energy group up to 10 MeV within a sphere with a radius of 15 cm. To study the field perturbation, absorbers of different dimensions were positioned inside the volume. The phenomena was correlated to the integral cross section of the neutron absorber using several neutron absorbers. The spatial neutron flux distribution produced by the source and the setup without the absorber was used as a guide when determining the flux density inside and outside the absorber volume. In particular, in the neutron resonance area, our work showed that absorbers of different dimensions cause the neutron field to be perturbed in a manner that depends on the absorption and scattering cross-sections. Contrary to the straightforward scenario in which the neutron number density is decreased, it was discovered that the perturbation greatly affects the moderation of neutrons in the medium above 1 MeV.
The discussion showed that not only the reduction of the neutron flux (or neutron density) that affects the neutron application, but also the disturbance in the energy distribution itself which is reflected in the none unit value of energy bin escape probability Pescape in epi-thermal neutron field. Both phenomenon showed that the complexity of the neutron transport phenomenon throws its shadows on every physical system wherever neutron is produced or absorbed.