Neutronic method of soil moisture measurement

The neutronic method is first outlined: analysis of neutron-nucleus interaction phenomena shows that the neutronic parameters of damp soils depend on the water itself, then on the dry density and the total chemical composition. A physical model representing the neutron moisture gage is worked out next, which leads to the establishment of a simple new mathematical expression applicable to two-dimensional geometry using the multigroup theory diffusion approximation. Following a brief description of the principle and technology of moisture gages two essential problems are dealt with: the calibration curve established by a quick and reliable method involving direct measurement of the thermal neutron constants of soils, and the measurement volume which must be approached by the use of a two-dimensional physical model to describe the geometrical conditions correctly. The problem of the vertical and horizontal resolution power of the neutron probe and the limiting case of surface measurements are discussed. Some possibilities offered by epithermal and fast neutron detection are suggested: epicadmic information represents a step forward as a complement to the thermal measurement since it supplies the principle of a moisture-gage independent of the dry density and allows the calibration curves of conventional instruments to be determined in situ. An experimental study of fast neutron space-energy distribution is described extended and specified by a physical model constructed on a Monte Carlo code; this gives the basis of a technique to measure water contents by fast neutron transmission.