In nuclear physics, nuclear fission in reactors is usually caused by the impact of which particle on a heavy nucleus?

Introduction / Context:
Nuclear fission is the process in which a heavy nucleus, such as uranium or plutonium, splits into two or more lighter nuclei, releasing a large amount of energy. In nuclear reactors and atomic bombs, fission is typically triggered by bombarding the heavy nuclei with suitable particles. This question tests whether you know which particle is commonly used to initiate fission in practical nuclear systems.

Given Data / Assumptions:
- The context is nuclear fission of heavy nuclei like uranium-235 or plutonium-239.
- Options include electron, neutron, proton and deuteron.
- We consider typical thermal reactors and chain reactions used in power and weapons applications.

Concept / Approach:
In most practical fission processes, slow (thermal) neutrons are used to induce fission. Neutrons are neutral particles, so they are not repelled by the positive charge of the nucleus and can easily penetrate. When such a neutron is absorbed by a fissile nucleus, the resulting compound nucleus becomes unstable and splits into smaller fragments, releasing more neutrons and energy. Charged particles like protons, electrons and deuterons experience strong Coulomb repulsion from the positively charged nucleus, making it harder for them to trigger fission in a controlled reactor setting.

Step-by-Step Solution:
Step 1: Recall that uranium-235 and plutonium-239 are typical fissile materials in reactors. Step 2: Recognise that slow neutrons are commonly used to strike these nuclei and produce fission. Step 3: Understand that neutrons are electrically neutral and therefore not repelled by nuclear charge. Step 4: When a neutron is captured by a fissile nucleus, the nucleus becomes unstable and splits into lighter nuclei plus more neutrons. Step 5: These freed neutrons can go on to cause further fission events, leading to a chain reaction. Step 6: Therefore, the correct particle responsible for causing fission by impact is the neutron.

Verification / Alternative check:
Descriptions of nuclear reactors emphasise moderators that slow down neutrons to thermal energies, making them more likely to induce fission in uranium-235. Control rods then adjust the number of neutrons to maintain a steady chain reaction. None of these designs rely on beams of electrons, protons or deuterons for routine fission events, confirming the central role of neutrons as the fission-triggering particles.

Why Other Options Are Wrong:
Electron: Electrons are light and negatively charged; they interact mainly with the electron cloud and are not used to induce nuclear fission in typical reactors.
Proton: Protons are positively charged and are strongly repelled by the positively charged nucleus, making it difficult for them to approach and be captured under normal reactor conditions.
Deuteron: A deuteron is a nucleus of deuterium (one proton and one neutron). It is charged and also experiences Coulomb repulsion; while it can be used in some nuclear reactions, it is not the standard particle used to cause fission in power reactors.

Common Pitfalls:
Some students think that because fission involves splitting a nucleus, any high-energy particle will do. They overlook the importance of electrostatic repulsion for charged particles. Another error is confusing fusion and fission processes, where deuterons appear in fusion reactions rather than in inducing fission. Remembering that neutrons are neutral and easily captured by heavy nuclei helps keep the distinction clear.

Final Answer:
Nuclear fission is most commonly caused by the impact of a neutron on a heavy fissile nucleus.