Introduction
Nuclear physics studies the atomic nucleus itself — its structure, its stability, and the enormous energies released when it changes. This chapter draws directly on special relativity’s mass–energy relation (Chapter I) and quantum tunnelling (Chapter II): both are needed to understand why nuclear reactions release so much more energy than chemical reactions, and how radioactive decay is possible at all.
1. Basic nuclear structure
The nucleus contains protons (positive charge) and neutrons (neutral), collectively called nucleons, bound together by the strong nuclear force — a short-range force stronger than the electrostatic repulsion between protons, but effective only over distances comparable to the nucleus itself. An element’s identity is fixed by its number of protons (atomic number, Z); the number of neutrons can vary, producing different isotopes of the same element with the same chemical behaviour but different nuclear stability and mass. Nuclear stability is governed by the balance between the attractive strong force and the repulsive electrostatic force among protons, which is why very heavy nuclei (with many protons) tend to be unstable.
2. Alpha, beta, and gamma radioactivity
Unstable nuclei spontaneously transform to reach a more stable configuration, emitting radiation in the process:
- Alpha decay emits a helium nucleus (2 protons + 2 neutrons), reducing both atomic number and mass number; it occurs via quantum tunnelling of the alpha particle through the nuclear potential barrier — classically the particle lacks the energy to escape, but its wavefunction has a small but nonzero amplitude beyond the barrier.
- Beta decay converts a neutron into a proton (emitting an electron and an antineutrino) or a proton into a neutron (emitting a positron and a neutrino), changing atomic number but not mass number; it is caused by the weak nuclear force.
- Gamma decay emits a high-energy photon as an already-formed nucleus drops from an excited energy state to a lower one, changing neither atomic number nor mass number.
Radioactive decay is statistical, not deterministic: for any individual nucleus, decay is random, but a large collection decays following the exponential law N(t) = N₀e^(−λt), characterised by a half-life — the time for half of a sample to decay — which is constant regardless of how much material remains, a distinctive signature of a memoryless random process.
3. Radiation detectors
Detectors convert the interaction between radiation and matter into a measurable (usually electrical) signal. A Geiger–Müller counter detects ionisation produced by radiation passing through a gas-filled tube, producing a pulse of current for each ionising event — simple and robust, but it cannot easily distinguish radiation type or energy. A scintillation counter uses a material that emits a flash of light when struck by radiation, detected and amplified by a photomultiplier tube, allowing energy measurement as well as detection. Semiconductor detectors use the ionisation produced within a semiconductor’s depletion region (related to the p–n junctions of Solid State Physics) to achieve high energy resolution, widely used in precision spectroscopy.
4. Mass spectrometers and particle accelerators
A mass spectrometer ionises a sample, then accelerates and deflects the ions using electric and magnetic fields; because the deflection depends on the ion’s mass-to-charge ratio, it can separate and precisely measure the masses of different isotopes — the primary tool for determining isotopic composition and exact atomic masses.
Particle accelerators use electric fields to increase charged particles’ kinetic energy (and magnetic fields to steer and focus their paths), reaching energies needed to probe nuclear and subatomic structure or to produce new particles (via E = mc² converting kinetic energy into mass). Linear accelerators accelerate particles along a straight path; cyclotrons and synchrotrons use magnetic fields to curve particles into repeated circular paths, allowing repeated acceleration within a smaller physical footprint.
5. Nuclear fission and reactor principles
Fission splits a heavy nucleus (such as uranium-235) into two lighter nuclei, releasing neutrons and a large amount of energy — a consequence of the fact that mid-mass nuclei are more tightly bound per nucleon than very heavy ones (binding energy per nucleon, when plotted against mass number, peaks near iron and decreases toward both very light and very heavy elements). If the released neutrons trigger fission in further nuclei, a chain reaction results. A nuclear reactor controls this chain reaction to release energy at a steady, usable rate, using key components: fuel (fissile material), a moderator (slows fast neutrons to increase the probability of causing further fission, since slow “thermal” neutrons are captured more effectively by uranium-235), control rods (absorb neutrons to regulate reaction rate, inserted or withdrawn to maintain a stable chain reaction), a coolant (removes heat to generate usable power and prevent overheating), and shielding (protects against escaping radiation).
6. Nuclear fusion and applications
Fusion joins light nuclei (such as isotopes of hydrogen) into a heavier nucleus, again releasing energy because light nuclei are less tightly bound per nucleon than mid-mass nuclei. Fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between positively charged nuclei — conditions found naturally in stellar cores (fusion is the energy source of the Sun) and pursued, with significant ongoing technical challenges around plasma confinement, in experimental fusion reactors on Earth as a potential future clean energy source.
7. Elementary particles
Beyond protons, neutrons and electrons, physics has identified a wider “particle zoo,” organised by the Standard Model into fermions (matter particles: quarks, which combine to form protons and neutrons, and leptons, including electrons and neutrinos) and bosons (force carriers: photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force, and the Higgs boson, associated with how particles acquire mass). At syllabus level, the essential requirement is recognising this basic classification (fermions vs. bosons, quarks vs. leptons) and each category’s general role, rather than the detailed theory of particle interactions.
Key Takeaways
- Nuclear stability balances the short-range attractive strong force against long-range electrostatic repulsion between protons; isotopes share proton number but differ in neutron number and stability.
- Alpha, beta, and gamma decay differ in what is emitted and what nuclear property changes; radioactive decay follows a statistical exponential law characterised by a constant half-life.
- Detectors (Geiger–Müller, scintillation, semiconductor) convert radiation interactions into measurable signals with varying degrees of energy resolution; mass spectrometers and accelerators use electric and magnetic fields to measure mass and probe nuclear structure respectively.
- Fission (splitting heavy nuclei) and fusion (joining light nuclei) both release energy because they move nuclei toward the maximum binding energy per nucleon near iron; reactors control fission chain reactions using fuel, moderator, control rods, coolant, and shielding.
- The Standard Model classifies elementary particles into matter particles (fermions: quarks and leptons) and force-carrying particles (bosons), providing the current deepest layer of the mechanics-to-nuclear-physics chain built through this subject.