Soil chemical elements macroscopic cross section influence on the calibration curve of neutron probes

The thermal neutron flux in a homogeneous soil, at the point where is located a fast neutron source, depends on the microscopic cross sections of the soil elements and also on their density and water content. When the soil water content increases, both hydrogen atomic density and thermal neutron flux increase because more neutrons are slowed down by H nuclei. Microscopic cross sections of the soil elements were obtained from the literature on nuclear data for all soil elements. Thermal neutron fluxes were calculated using values of water content from air-dry soil to saturation. The results showed that the relation between thermal neutron flux and soil water content could be considered linear in a range of water content. The theoretical calibration curve for a known soil has an excellent agreement with that obtained in field conditions for the same soil. (author)