Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: The probe of a neutron moisture meter is inserted into an access tube fixed vertically in the soil
Reason R: The probe of a neutron moisture meter contains a source of fast protons and a detector of slow electrons and helps in estimating soil moisture content
In light of the above statements, choose the most appropriate answer from the options given below

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Remember the physics of a neutron probe:
- Source \(\rightarrow\) Emits fast neutrons.
- Moderator \(\rightarrow\) Hydrogen atoms in soil water.
- Detector \(\rightarrow\) Detects slow neutrons.
Any statement referring to "protons" or "electrons" in a neutron probe is incorrect.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
A neutron moisture meter is an instrument used to measure the volumetric water content of soil in situ.
It operates on the principle of neutron moderation, where fast-moving neutrons are slowed down by collisions with hydrogen atoms (mostly present in soil water).

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion A: "The probe of a neutron moisture meter is inserted into an access tube fixed vertically in the soil."
This is correct.
To take measurements at different depths, an aluminum or plastic access tube is installed vertically in the soil. The probe containing the radioactive source and detector is lowered into this tube to record readings without disturbing the soil profile.
- Reason R: "The probe of a neutron moisture meter contains a source of fast protons and a detector of slow electrons..."
This statement is incorrect.
The probe contains a source of fast neutrons (such as americium-241/beryllium) and a detector of thermalized (slow) neutrons, not protons or electrons.
When fast neutrons are emitted into the soil, they collide with hydrogen nuclei, which have nearly the same mass as a neutron. This causes the neutrons to lose energy and slow down (thermalize).
The slow neutron detector counts these moderated neutrons. Since hydrogen in the soil is primarily found in water, the count of slow neutrons is directly proportional to the soil moisture content.
Therefore, Assertion A is correct, but Reason R is incorrect.

Step 3: Final Answer:
The correct option is (C).