Investigation of the effects of beam divergence on the response of neutron voluminous detectors

In the present study, the Monte Carlo calculations were carried out to evaluate the effect of beam divergence on the response of a spherical thermal-neutron counter at the center of a spherical moderating assembly. A model of ³He detector surrounded with 10 in. diameter polyethylene sphere was utilized to calculate the point isotropic and plane-parallel beams responses of twelve different energies, and obtain the geometry factor and its parameters (a ₄ and a ₅) based on general formulations. Furthermore, for 2, 3, 5, 8, 10 and 15 in. spheres exposed to three different radionuclide neutron sources with various energy spectra, the parameters of geometry factor were quantified, as a function of moderator radius and neutron energy. Obtained results based on this method were compared with the experimental data for one specific source. Finally, by these parameters the obtained values of the geometry factor based on two formulations were compared to each other.