The characteristic X-ray radiation is emitted when

A. the bombarding electrons knock out electrons from the inner shell of the target atoms and one of the outer electrons falls into this vacancy
B. the valance electrons are removed from the target atoms as a result of the collision
C. the source of electrons emits a mono energetic bean
D. the electrons are accelerated to a fixed energy

Options

A. the bombarding electrons knock out electrons from the inner shell of the target atoms and one of the outer electrons falls into this vacancy
B. the valance electrons are removed from the target atoms as a result of the collision
C. the source of electrons emits a mono energetic bean
D. the electrons are accelerated to a fixed energy

Answer

To solve the question regarding the emission of characteristic X-ray radiation, let's analyze the options step by step.

### Step-by-Step Solution:

1. **Understanding Characteristic X-rays**: Characteristic X-rays are produced when an electron from an inner shell of an atom is ejected, creating a vacancy. An electron from a higher energy level (outer shell) then falls into this vacancy, and the energy difference is emitted as X-ray radiation.

2. **Evaluating Option A**:
- Option A states: "The bombarding electrons knock out electrons from the inner shell of the target atoms and one of the outer electrons falls into this vacancy."
- This matches our understanding of characteristic X-ray production. The outer electron filling the vacancy results in the emission of X-rays.

3. **Evaluating Option B**:
- Option B states: "The valence electrons are removed from the target atoms as a result of the collision."
- This is incorrect because characteristic X-rays specifically involve inner shell electrons, not valence electrons.

4. **Evaluating Option C**:
- Option C states: "The source of electrons emits a mono energetic beam."
- While a mono energetic beam is important for the process, it does not directly describe the mechanism of X-ray emission.

5. **Evaluating Option D**:
- Option D states: "The electrons are accelerated to a fixed energy."
- This statement is also not directly related to the emission of characteristic X-rays, as it does not describe the interaction between the electrons and the inner shell electrons of the target atom.

6. **Conclusion**:
- The correct answer is Option A, as it accurately describes the process of characteristic X-ray emission.

### Final Answer:
**A. The bombarding electrons knock out electrons from the inner shell of the target atoms and one of the outer electrons falls into this vacancy.**