A `gamma`- ray photon is emitted

Options

A. After ionization of an atom
B. Due to conversion of a neutron into a proton in the nucleus
C. After de- excitation of a nucleus
D. Due to conversion of a proton into a neutron in the nucleus

Answer

To solve the question regarding the emission of a gamma-ray photon after ionization of an atom, we can break down the process into clear steps:

### Step-by-Step Solution

1. **Understanding the Process of Ionization**:
- Ionization occurs when an atom loses or gains electrons, resulting in a charged ion. In the context of nuclear reactions, ionization can also refer to changes in the nucleus, such as the conversion of neutrons to protons or vice versa.

2. **Nuclear Reactions**:
- In the nucleus, a neutron can be converted into a proton (beta decay) or a proton can be converted into a neutron (positron emission). These processes can lead to an excited state of the nucleus.

3. **Excited State of the Nucleus**:
- After a nuclear reaction, the nucleus may be left in an excited state, meaning it has excess energy. This energy needs to be released for the nucleus to return to a more stable (de-excited) state.

4. **Emission of Gamma Rays**:
- The excess energy from the excited nucleus is released in the form of gamma-ray photons. This emission occurs when the nucleus transitions from the excited state to the ground state, or a lower energy state.

5. **Conclusion**:
- Therefore, the emission of a gamma-ray photon is a result of the de-excitation of the nucleus after it has undergone a nuclear reaction that leaves it in an excited state. Gamma rays are typically emitted when there is leftover energy after other forms of decay (like alpha or beta decay).

### Final Answer
The emission of a gamma-ray photon occurs when an excited nucleus transitions to a lower energy state, releasing energy in the form of gamma radiation.