Soil Moisture Determination by a Portable Neutron Scattering Moisture Meter

Abstract

Abstract A portable, battery‐powered device for measurement of soil moisture by neutron scattering has been constructed and used for field measurements. The equipment, aside from being portable, differs from previously reported devices of this type as follows: (a) A fast neutron source in the form of an annulus is placed about the center of a slow neutron detecting tube; (b) Recently developed glow transfer tubes are used for absolute neutron count determinations; ( c ) A calibrating volume of paraffin, which is also used as a neutron shield, is incorporated as a part of the source‐detector carrying case, to permit simple field checking and standardization of the device. The detector tube is partially shielded with cadmium to reduce the vertical extent of the soil “sample” contributing to the neutron count. Field data are presented. It was found, by locating the source‐detector at various depths in pipes sunk vertically into the soil, that, except for the surface 6 to 9 inches, the equipment generally gave the soil moisture per unit soil bulk volume, within the range of the standard deviation of gravimetric determinations. A single calibration curve serves for all soils tested (sand, silt loam, silty clay loam). The equipment, complete with batteries, lead and paraffin shielding, weighs 45 pounds. The shielding protects the operator against radiation hazard and is sufficient for at least 8 hours per day use, 6 days a week.