A : Fast moving neutrons do not cause fission of a uranium nucleus.
R: A fast moving neutron spends very little time inside the nucleus.

Options

A. if both assertion & reason are true and the reason is correct explanation of the assertion then mark (1)
B. if both assertion & reason are true and the reason is not correct explanation of the assertion then mark (2)
C. if assertion is true statement but reason is false then mark (3)
D. if both assertion & reason are false then mark (4)

Answer

To analyze the statements A and R, we need to understand the concepts of nuclear fission and the behavior of neutrons with respect to uranium nuclei.

### Step-by-Step Solution:

1. **Understanding Nuclear Fission**:
- Nuclear fission is a process where a heavy nucleus, such as uranium-235, splits into two or more smaller nuclei along with the release of energy.
- For fission to occur, the nucleus must absorb a neutron, which makes it unstable and leads to its splitting.

2. **Types of Neutrons**:
- Neutrons can be classified as fast or slow (thermal) neutrons.
- Fast neutrons have high kinetic energy and move quickly, while slow neutrons have lower kinetic energy and are more likely to be captured by a nucleus.

3. **Fast Neutrons and Uranium-235**:
- Fast-moving neutrons do not cause fission of uranium-235 because they do not spend enough time in the vicinity of the nucleus to be captured.
- The probability of a fast neutron being absorbed by uranium-235 is low due to its high velocity.

4. **Role of Slow Neutrons**:
- Slow neutrons are more effective in causing fission because they can be captured by the uranium-235 nucleus, leading to the formation of uranium-236, which is unstable and can undergo fission.

5. **Analyzing Statement A**:
- Statement A: "Fast moving neutrons do not cause fission of a uranium nucleus." This statement is true because fast neutrons are not effectively captured by the nucleus.

6. **Analyzing Statement R**:
- Statement R: "A fast moving neutron spends very little time inside the nucleus." This statement is also true because the high speed of the neutron means it does not linger long enough for the nucleus to capture it.

7. **Conclusion**:
- Both statements A and R are true, and R provides a valid explanation for A. Therefore, the correct conclusion is that both statements are true, and R is the reason for A.

### Final Answer:
Both A and R are true, and R is the reason for A.