Soil water sensing by neutron scattering

Soil water content can be sensed by counting thermal (energy at room temperature) neutrons that are scattered from collisions of more energetic (fast) neutrons with the nuclei of atoms. Repeated collisions reduce the energy of fast neutrons until they are at thermal energy, a process called moderation. Hydrogen atoms, mainly found in soil water, are more effective moderators than are most elements found in soils, so the count of thermal neutrons is a good indicator of the amount of water in soil. Fast neutron sources can be man-made or natural (cosmic ray collisions with atoms). Neutron scattering approaches can be used for access tube measurements or larger-scale near-surface measurements using the cosmic ray neutron scattering method.