Geometrical effects on thermal neutron reflection of hydrogenous moderators using 241Am-Be source

The variation of thermal reflection parameter with thicknesses of water and paraffin moderators was measured using an instrument consisting of 1 Ci 241Am–Be isotopic neutron source and 3He neutron detector. Based on experimental data obtained from two different source–detector–sample geometries, analytical expressions were derived for calculation of reflection parameters for various moderator thicknesses. The measured data for the two geometries exhibit saturation curves with saturation achieved at about 8 and 16 cm when sample container was placed at top and side of the source holder, respectively. By locating the source holder along the vertical axis of the sample container, measurements showed that the reflection parameter was very sensitive with the axial location and was strongly dependent on the moderator and its thickness.