Soil Moisture Measurement‐Neutron

Abstract The neutron probe is based on the principle of neutron thermalization. Hydrogen nuclei are nearly the same size and mass as neutrons and therefore, can scatter and slow neutrons. High‐energy (0.1–10 Mev.), fast (1600 km/s) neutrons emitted from a radioactive source, such as americium–beryllium or radium–beryllium are slowed and changed in direction by elastic collisions with atomic nuclei of hydrogen atoms in soil. This process is called “thermalization,” and the low‐energy (about 0.03 eV) neutrons, after collision, are called “thermal/thermalized neutrons.” The density of the thermal neutrons (thermal cloud of neutrons) depends on the soil–water content; the vast majority of hydrogen in soil is associated with water. Higher water content in soil leads to increased thermalization and, thus, a denser thermal cloud. A “slow neutron” detector is installed adjacent to the source (emitter) to measure the cloud density. The measurement is usually in the form of a “count ratio,” a higher count ratio denotes higher water content and vice versa. Thus, the density of thermal neutrons can be calibrated against the water content volumetrically basis (2).