The best-known application of measuring methods based on neutron scattering and attenuation is moisture content determination, which relies on the special character of hydrogen as a neutron-scattering medium. When high-energy neutrons collide with hydrogen atoms, the former lose their energy and slow down. The rate of slowing down is great since the masses of the neutron and the hydrogen nucleus are similar.

If an Am-241/Be radiation source with activity of some GBq emitting fast neu­trons and a detector sensitive only to slow neutrons are placed into the medium to be tested, the intensity of low-energy neutrons detected by the sensor will be pro­portional to the hydrogen content of the material tested (Figure 11.24).

The fact that hydrogen, but not water content, is determined must be taken into consideration because this method also measures, for example, chemically bound hydrogen in organic compounds or crystal water. The measurement is also dis­turbed by the presence of other neutron-absorbing elements (B, Cd) and modifica­tion of the composition with elements that have low atomic numbers (Cl, O, S).

For reliable measurement, a permanent consistency must be maintained. If the consistency of the material is changing, the density must be measured separately. Today, moisture-measuring instruments with two separate radiation sources (one is gamma, and the other is a neutron emitter) and two detectors, already performing density compensation, are available (Figure 11.25).

Nuclear moisture-measuring instruments operated with a neutron source are mostly used in foundries and in producing materials in industry for the sampling — free, quick, and accurate measurement of the moisture content of various mixtures and the consistency of additives.

Examples of industrial applications include the following:

• Water addition is determined in concrete panel factories by the continuous measurement of the moisture content of river sand (this is the component that has the highest moisture content, as compared to dry components).

Intensity on the opposite side

• For determining the moisture content of soils, portable soil moisture-measuring instru­ments have been developed.

• Moisture content measurement is very important in road, railway, and basement construction.