4.2.1. Water content: nuclear radiation methods

The radiometric methods of measuring the soil water distribution are presented. The neutron method consists of measuring the thermal neutron density around a fast neutron source. Since the moisture in the soil is usually the principle hydrogen compound the thermal neutron density is a function of the water content. The neutron gauge may be of the subsurface type, placed in a vertical access tube, or of the surface type, resting on the soil surface. Cf 252 is a useful neutron source, having low mean energy and being cheap. Tritium-target deuterium bombarded neutron generators may be used in large volume single or dual tube measurements. The hydrogen content of the dry soil matrix and the dry density profile should be determined. Epithermal measurements eliminate the effect of thermal neutron absorbers. The ideal access tube is of thin-walled aluminium, but this in many cases lacks the required strength and durability, and iron or stainless steel may be used. The measured volume ranges from 20cm to 110cm radius, and the resolution is limited to 30cm layers, with measurement intervals of 15cm. Gamma ray sources may also be used, both in single-well density gauges in conjunction with a neutron gauge, and in a dual-tube arrangement, measuring the water content by attenuation, using a Cs 137 source. This can give a resolution of down to 0.5cm, and an accuracy of 0.0015g/cm3. Finally radiation dose calculations are briefly discussed. (JIW)