What is the principle behind measuring moisture content in soil with a nuclear gauge?

The principle behind measuring moisture content in soil with a nuclear gauge is based on the behavior of fast neutrons in different moisture conditions. When fast neutrons are emitted from a source, they interact with hydrogen nuclei, which are abundant in water molecules. In soils with low moisture content, fast neutrons will travel farther because there are fewer hydrogen nuclei to slow them down through collisions. Conversely, in moist soil, the presence of more hydrogen atoms results in more interactions, which slows down the neutrons and reduces their travel distance.

This understanding allows the gauge to estimate the moisture content based on the degree of neutron moderation: the fewer the interactions with hydrogen nuclei, the less moderated the neutrons are, indicating drier soil. Thus, the correct choice accurately reflects the fundamental principle of how nuclear moisture measurements are conducted.