Radioactivity
Specification: OCR A H556 | Section: 5.4.2 | Teaching time: ~4 hours
- Describe the nature and properties of α, β⁻, β⁺ and γ radiation
- Write and balance nuclear equations for radioactive decay
- Use the exponential decay equation and calculate half-life
- Evaluate applications and hazards of ionising radiation
Types of Nuclear Radiation
Radioactivity is the spontaneous emission of particles or energy from an unstable nucleus. The nucleus becomes more stable by losing energy through radioactive decay. This process is random and unaffected by external conditions (temperature, pressure, chemical bonding).
There are four main types of nuclear radiation you need to know:
| Type | Symbol | Composition | Charge | Mass (u) | Penetrating power |
|---|---|---|---|---|---|
| Alpha | α | Helium nucleus (2p + 2n) | +2e | 4 | Stopped by paper, ~5 cm air |
| Beta-minus | β⁻ | Electron (from neutron decay) | −e | 1/1836 ≈ 0 | Stopped by ~3 mm aluminium |
| Beta-plus | β⁺ | Positron (from proton decay) | +e | 1/1836 ≈ 0 | Annihilates with electrons |
| Gamma | γ | EM photon (high energy) | 0 | 0 | Reduced by lead, not stopped |
Alpha particles are the most ionising but least penetrating. Gamma rays are the least ionising but most penetrating. Ionising ability and penetrating power are inversely related.
Ionisation occurs when radiation removes electrons from atoms, creating ions. The greater the charge and mass of the particle, the more ionising it is. Ionisation damages cells by breaking chemical bonds and damaging DNA.
Nuclear Equations
All nuclear equations must conserve mass number (A = nucleons) and atomic number (Z = protons).
Alpha decay: The nucleus emits an α particle (helium nucleus).
Example: Radium-226 decays by alpha emission:
Beta-minus decay: A neutron converts to a proton, emitting an electron and antineutrino.
Example: Carbon-14 decays by beta-minus emission:
Beta-plus decay: A proton converts to a neutron, emitting a positron and neutrino.
Gamma emission: The nucleus loses energy by emitting a gamma photon. Mass number and atomic number do not change.
In nuclear equations, always check that the top numbers (mass) and bottom numbers (atomic/proton) balance on both sides. For β⁻, the electron is written as 0−1e (mass 0, charge −1).
Knowledge Check
23892U → 23490Th + 42α (2 marks for correct equation)
Daughter nucleus: Thorium-234 (Th-234) (1 mark)
- Alpha particles have +2 charge and greater mass (1 mark)
- They interact more strongly with electrons, removing them from atoms more effectively (1 mark)
Radioactive Decay and Half-Life
Activity (A) is the rate at which a source decays, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Half-life (T½) is the time taken for half the unstable nuclei in a sample to decay, or for the activity to halve.
Radioactive decay follows an exponential pattern:
Where λ is the decay constant (probability of decay per nucleus per second).
The number of undecayed nuclei after n half-lives is:
Half-life is constant for a given isotope, regardless of the starting amount. After 1 half-life, 50% remains. After 2 half-lives, 25% remains. After 3 half-lives, 12.5% remains.
Radioactive Decay Simulation
Background Radiation and Safety
Background radiation is the low-level ionising radiation present everywhere. Sources include:
- Radon gas (~50%) — from rocks and soil
- Medical (~14%) — X-rays, radiotherapy
- Ground and buildings (~14%) — rocks, building materials
- Cosmic rays (~10%) — high-energy particles from space
- Food and drink (~10%) — potassium-40 in bananas, etc.
- Nuclear industry (~1%) — weapons testing, power stations
Safe handling of radioactive sources:
- Keep sources in lead-lined containers
- Use tongs and keep at arm's length
- Minimise exposure time
- Maximise distance from source (inverse square law)
- Use shielding appropriate to radiation type
Always measure and subtract background count rate before calculating activity. Typical background: 15–40 counts per minute depending on location.
Exam-Style Questions
(a) Calculate the decay constant λ. (2 marks)
(b) Calculate the activity after 24 days. (2 marks)
(c) State why the activity never reaches zero. (1 mark)
(a) λ = ln(2) / T½ = 0.693 / (8 × 24 × 3600) = 1.00 × 10−6 s−1 (2 marks)
(b) After 24 days = 3 half-lives. A = 640 × (½)³ = 640 × 0.125 = 80 Bq (2 marks)
(c) Exponential decay approaches zero asymptotically / there will always be some undecayed nuclei remaining (1 mark)
(a) Describe how you would demonstrate that the source emits both types of radiation. (3 marks)
(b) Explain which type of radiation would be most dangerous if the source were: (i) ingested, (ii) held 1 m from the body. (4 marks)
(a) Use a Geiger-Müller tube to measure count rate. Place paper in front of source — count should drop (alpha blocked). Then place aluminium sheet — count should drop further (any beta blocked). Remaining count is from gamma (passes through). (3 marks)
(b) (i) Alpha is most dangerous if ingested because it is highly ionising and can damage cells directly. Gamma would pass through with minimal interaction. (2 marks)
(ii) Gamma is most dangerous at 1 m because alpha is stopped by air (~5 cm). Gamma has high penetrating power. (2 marks)
Topic Summary
α (He nucleus), β⁻ (electron), β⁺ (positron), γ (photon) — differ in charge, mass, penetration, ionisation
T½ = 0.693/λ, constant for each isotope, exponential decay
A = λN measured in Bq (decays per second), A = A₀e−λt