............is a process in which a heavy nucleus breaks into two middle-weight nuclei.

Answer

**Step-by-Step Solution:**

1. **Understanding the Process**: The question asks for a process in which a heavy nucleus breaks into two middle-weight nuclei. This process is a fundamental concept in nuclear physics.

2. **Identifying the Process**: The process where a heavy nucleus splits into two lighter nuclei is known as **nuclear fission**.

3. **Example of Nuclear Fission**: A common example of nuclear fission involves the uranium nucleus, specifically Uranium-235 (U-235). When a neutron collides with this heavy nucleus, it can induce fission.

4. **Inducing Fission**: In nuclear fission, a low-energy neutron is absorbed by the heavy nucleus, making it unstable. This unstable state is often referred to as an intermediate state (for U-235, it becomes U-236).

5. **Breaking of the Nucleus**: The unstable intermediate nucleus (U-236) then undergoes fission, breaking apart into two smaller nuclei. For instance, it can split into Barium (Ba) and Krypton (Kr).

6. **Release of Neutrons and Energy**: During the fission process, additional neutrons (typically three) are released, along with a significant amount of energy. This energy release is what makes nuclear fission a powerful reaction.

7. **Final Answer**: Therefore, the process in which a heavy nucleus breaks into two middle-weight nuclei is called **nuclear fission**.

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