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Soil Moisture Measurements: Neutron Sensing Technologies

Water (H2O), and subsequently hydrogen atoms, is present in soil and biomass moisture. Neutron sensing technologies count slow neutrons that have been thermalised by collision of fast neutrons (from the source) or cosmic rays (naturally occurring) with hydrogen in the soil and which have bounced back to the slow neutron detector. The behaviour of the neutron in response to changes in soil water (hydrogen) content enable small changes in soil moisture water content to be measured with high accuracy.

Neutron sensing technologies can measure neutrons from an emitting source, as in the Neutron Probe Smart503, or naturally occurring cosmic ray neutrons, as in the Finapp.

Cosmic Ray Neutron Sensor (CRNS) as used in the Finapp

Hydrogen atoms (H2O) are present in the surface soil and biomass moisture. By measuring incoming cosmic radiation compared to the backscattered radiation, changes in the soil and biomass moisture content can be monitored and calculated.

The Finapp CRNS relies on a Lithium-6 detector to measure the incoming fast mouns, and then the returning slow moving mouns (which have collided with the hydrogen within the soil and biomass moisture).

Emitting Source (as used in the Neutron Probe)

Using an emitting source (in this case, Americium-241 combined with Beryllium), neutrons are released into the surrounding soil. These fast neutrons subsequently collide with the hydrogen particles contained in the soil (as moisture), where they slow before returning to the detector, where they are then counted, and the volumetric soil water content calculated.

Image from Tuller, M., Babaeian, E., & Jones, S. B. (2023). Proximal sensing of soil moisture. In M. J. Goss & M. Oliver (Eds.), Encyclopedia of Soils in the Environment (Second Edition) (pp. 591–599). Academic Press. https://doi.org/10.1016/B978-0-12-822974-3.00157-9

Accuracy of the Neutron Sensing Technologies

Relying on first principles, the measurement of a fundamental element within water provides a highly accurate measurement of soil moisture content. ICT International, and Peter Cull in particular, have been working with neutron sensing technologies since the 1970s.