Nuclear Physics

Summary: Covers atomic structure (linking to Chemistry), isotopes, radioactivity (alpha, beta, gamma), half-life calculations, background radiation, uses and dangers of ionising radiation, nuclear fission (chain reactions, nuclear reactors), and nuclear fusion. This topic ties together physics, chemistry, and real-world energy applications. Tags: igcse physics nuclear radioactivity radiation fission fusion Created: 2026-07-16 Last Updated: 2026-07-16

Atomic Structure (Brief Recap)

An atom consists of:

• Atomic number (Z) = number of protons
• Mass number (A) = protons + neutrons
• In a neutral atom: number of protons = number of electrons

For a more detailed treatment of atomic structure, electron configurations, and nuclide notation, see the IGCSE Chemistry Atomic Structure page.

Isotopes

Definition: Isotopes are atoms of the same element (same number of protons/atomic number) with different numbers of neutrons (different mass numbers).

• Same chemical properties (same electron configuration)
• Different physical properties (different mass)
• Some isotopes are radioactive (unstable nucleus — will decay)

Radioactivity

Radioactivity is the spontaneous emission of radiation from an unstable nucleus as it decays to become more stable. This is a random process — it is not possible to predict when an individual nucleus will decay, but the behaviour of a large number of nuclei can be described statistically.

Three types of nuclear radiation:

Nuclear equations — when a nucleus decays:

• Alpha decay: mass number decreases by 4, atomic number decreases by 2
  • Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (α particle)
• Beta decay: mass number stays the same, atomic number increases by 1 (a neutron changes into a proton + electron)
  • Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e (β particle)
• Gamma decay: no change to mass number or atomic number. The nucleus just loses excess energy.

Example 1: Radium-226 (²²⁶₈₈Ra) decays by alpha emission. Identify the daughter nucleus.

²²⁶₈₈Ra → ⁴₂He + ²²²₈₆X
X has Z = 86, which is Radon (Rn). So the equation is: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Example 2: Carbon-14 (¹⁴₆C) decays by beta emission. Identify the daughter nucleus.

¹⁴₆C → ⁰₋₁e + ¹⁴₇X
X has Z = 7, which is Nitrogen (N). So the equation is: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e

Half-Life

Definition: The half-life of a radioactive isotope is the time taken for half the radioactive nuclei in a sample to decay. Alternatively: the time for the count rate (activity) of a sample to fall to half its initial value.

Key points:

• Half-life is constant for a given isotope — it cannot be changed by temperature, pressure, or chemical reactions
• Half-lives range from fractions of a second to billions of years

Half-life from a graph (decay curve):

1. Read the initial count rate or number of nuclei at time = 0
2. Find half of this value on the y-axis
3. Read across to the curve and down to the x-axis — this time is the half-life
4. Repeat for a second half-life to check consistency

Example 3: A radioactive sample has an initial activity of 800 Bq. Its half-life is 3 hours. What is the activity after 12 hours?

Number of half-lives = 12 / 3 = 4
After 1 half-life: 800 → 400 Bq
After 2 half-lives: 400 → 200 Bq
After 3 half-lives: 200 → 100 Bq
After 4 half-lives: 100 → 50 Bq
Answer: 50 Bq

Example 4: A sample decays from 1000 counts per minute to 125 counts per minute in 9 days. Find the half-life.

1000 → 500 → 250 → 125  (3 half-lives)
3 half-lives = 9 days → half-life = 3 days

Background Radiation

We are constantly exposed to low levels of radiation from natural and artificial sources:

Measuring background radiation: When performing experiments with radioactive sources, always measure and subtract the background count. Use a Geiger-Muller (GM) tube and counter.

Uses of Radioactivity

Dangers of Ionising Radiation

Ionising radiation (alpha, beta, gamma, X-rays) can damage cells and DNA:

• Low doses: DNA mutations → potentially cancer (leukaemia, thyroid cancer, etc.)
• High doses: radiation sickness (nausea, vomiting, hair loss, organ failure) → death
• Alpha is most dangerous if ingested/inhaled: Inside the body, alpha’s high ionising power causes concentrated damage to a small area of tissue. Alpha cannot penetrate dead skin, so external alpha sources are relatively safe
• Gamma is the most dangerous externally: It can penetrate deep into the body and reach vital organs

Safety precautions when handling radioactive sources:

• Minimise exposure time
• Maximise distance from source (inverse square law — doubling distance reduces exposure to 1/4)
• Use appropriate shielding (lead for gamma, perspex for beta)
• Never point a source directly at anyone
• Handle sources with tongs/forceps (never bare hands)
• Store sources in lead-lined containers when not in use
• Wash hands after handling sources

Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two (or more) smaller nuclei, releasing energy and neutrons.

Process:

1. A large unstable nucleus (e.g., Uranium-235 or Plutonium-239) absorbs a slow/thermal neutron
2. The nucleus becomes highly unstable and splits into two smaller daughter nuclei (fission fragments)
3. 2 or 3 neutrons are also released (typically 2.5 on average)
4. A large amount of energy is released (as kinetic energy of the fission fragments and neutrons, and as gamma radiation)

Example fission equation:

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3(¹₀n) + energy

Chain Reaction: The neutrons released by one fission event can be absorbed by other U-235 nuclei, causing them to fission and release more neutrons. This creates a self-sustaining chain reaction.

• Uncontrolled chain reaction → nuclear bomb (all neutrons cause further fissions)
• Controlled chain reaction → nuclear reactor (some neutrons absorbed to maintain a steady rate)

Nuclear Reactor

A nuclear power station uses controlled fission to produce heat → steam → turbine → generator → electricity.

Advantages of nuclear power:

• No CO₂ or SO₂ emissions (does not contribute to global warming or acid rain)
• Very high energy density — small amounts of fuel produce huge amounts of energy
• Reliable baseload power (not intermittent like wind/solar)

Disadvantages of nuclear power:

• Radioactive waste products — remain dangerous for thousands of years; need secure long-term storage
• Risk of catastrophic accidents (Chernobyl 1986, Fukushima 2011) — release of radioactive material into environment
• High decommissioning costs — reactors cannot simply be demolished; must be carefully dismantled
• Uranium is a finite (non-renewable) resource
• Public opposition due to safety concerns

Nuclear Fusion

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing a very large amount of energy.

Example: In the Sun, hydrogen nuclei (protons) fuse to form helium:

⁴(¹₁H) → ⁴₂He + 2(⁰₊₁e) + energy

(This is a simplified version — the actual process in stars is the proton-proton chain)

Conditions required for fusion:

• Very high temperature (~10-100 million °C) — so nuclei have enough kinetic energy to overcome their mutual electrostatic repulsion (both nuclei are positively charged)
• Very high pressure — to bring nuclei close enough for the strong nuclear force (attractive) to take over from the electrostatic force (repulsive)

Nuclear fusion as an energy source on Earth:

• Advantages: virtually unlimited fuel (deuterium and tritium from seawater), no long-lived radioactive waste (main product is helium), no CO₂, safer than fission (no chain reaction to run out of control — if conditions fail, fusion simply stops)
• Disadvantages: currently, a sustained fusion reaction producing net energy output has not yet been achieved on a commercial scale. Temperatures and pressures needed are extremely difficult to achieve and contain (no known material can withstand the temperature). Research continues (ITER project, tokamaks using magnetic confinement; laser-driven inertial confinement)

Fusion in stars: The immense gravity in stars provides the necessary temperature and pressure naturally. This is the process powering the Sun and all stars.

Sources

• BBC Bitesize GCSE Physics — Nuclear/atomic physics guide, BBC (free educational resource)
• OpenStax College Physics — Nuclear Physics chapter, Rice University (free, CC BY 4.0)
• Cambridge IGCSE Physics 0625 — Nuclear physics section, Cambridge Assessment International Education
• CK-12 Physics for High School — Nuclear Physics chapter, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• Atomic Structure (IGCSE Chemistry) — Detailed treatment of atomic structure, electron configuration, and isotopes
• Energy Resources and Transfer — Nuclear power as an energy resource, comparison with fossil fuels and renewables
• Waves — Gamma rays as part of the electromagnetic spectrum
• Space Physics — Nuclear fusion in stars as their energy source
• IGCSE-Phys-Index — Full IGCSE Physics index

Keywords

isotope, radioactivity, alpha, beta, gamma, ionising radiation, half-life, decay curve, background radiation, Geiger-Muller tube, nuclear fission, chain reaction, moderator, control rod, coolant, nuclear reactor, nuclear fusion, plasma, strong nuclear force, electrostatic repulsion

Common Misconceptions