Question

The effective soil volume of measurement through neutron probe, is known as

• A. Depth of soil water content
• B. Representative volume of soil
• C. Sphere of measurement
• D. Sphere of influence

Hint:

The "Sphere of Influence" of a neutron probe is inversely proportional to soil water content. It is small in wet soils and large in dry soils because fast neutrons must travel further in dry soil to be slowed down by hydrogen atoms.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A neutron probe measures soil moisture by emitting fast neutrons into the soil.
These fast neutrons collide with hydrogen atoms (primarily present in soil water), which slows them down into "thermal" (slow) neutrons.
The probe detects these slow neutrons to estimate soil water content.

Step 2: Detailed Explanation:
The fast neutrons emitted from the probe's radioactive source scatter in all directions, creating an approximately spherical zone of moderation.
The volume of soil within which fast neutrons are slowed down and detected is called the Sphere of influence (or sphere of moderation).
The radius of this sphere of influence is not constant; it depends heavily on soil moisture content:
1. In wet soils, fast neutrons collide with abundant hydrogen atoms near the source, resulting in a small, dense sphere of influence (radius of about $15\text{ cm}$).
2. In dry soils, fast neutrons must travel further to encounter enough hydrogen atoms to be thermalized, resulting in a larger sphere of influence (radius of up to $50\text{ cm}$).
Understanding this sphere of influence is critical to avoid placing the probe too close to the soil surface, where neutrons could escape into the air.

Step 3: Final Answer:
The effective measurement volume of a neutron probe is called the Sphere of influence, which corresponds to option (D).