The determination of a neutron source position in an unknown homogeneous medium; the 3D case

The possibility of localisation of an unknown neutron source in various bulky homogeneous media (box) was studied. Previous study showed that for the planar case, neutron source could be localised with accuracy better than 7%, by using the count rates ratio of two ³He detectors, positioned on the opposite faces of the box. The influence of the deviation of the source from the virtual plane connecting the two ³He detectors was studied. It was found that for detectors placed on the centres of the sample faces, larger distance of the source from the central line leads to larger error, up to 15%. It was found that while using the measurement of the ratio R(0) of external source placed on the box face coplanar with unknown source, accuracy does not depend on the distance from central line. Different ways to determine the plane of the unknown source were investigated. It was found that it is impossible to determine it by looking for maximum count rate of one detector or geometric mean of a pair of detectors. The experimental results were confirmed with Monte-Carlo N-particle (MCNP) simulations.