In which of the following emission of electrons does not take place

Options

A. Thermionic emission
B. X-rays emission
C. Photoelectric emission
D. Secondary emission

Explanation

To solve the question "In which of the following emissions does not take place the emission of electrons?", we will analyze each option provided.

### Step-by-Step Solution:

1. **Understand Thermionic Emission**:
- Thermionic emission occurs when a metal is heated to a high temperature. The thermal energy allows electrons to gain enough energy to overcome the work function of the metal and be emitted from its surface.
- **Conclusion**: This process involves the emission of electrons.

2. **Understand X-ray Emission**:
- X-ray emission involves the transition of electrons between different energy levels within an atom. When an electron transitions from a higher energy state to a lower energy state, it emits a photon (X-ray).
- **Conclusion**: In this process, electrons are not emitted from the atom; they merely transition between states. Therefore, this does not involve electron emission.

3. **Understand Photoelectric Emission**:
- In photoelectric emission, high-energy photons strike a metal surface, imparting energy to electrons. If the energy is sufficient, electrons are ejected from the metal.
- **Conclusion**: This process also involves the emission of electrons.

4. **Understand Secondary Emission**:
- Secondary emission occurs when primary electrons bombard a metal surface, causing the ejection of additional electrons from that surface.
- **Conclusion**: This process involves the emission of electrons as well.

### Final Conclusion:
Based on the analysis:
- **Thermionic emission**: Involves electron emission.
- **X-ray emission**: Does NOT involve electron emission (correct answer).
- **Photoelectric emission**: Involves electron emission.
- **Secondary emission**: Involves electron emission.

Thus, the correct answer is **X-ray emission**.