Radioactivity – Edexcel GCSE Physics Revision Notes

What you'll learn

Radioactivity forms a substantial component of Edexcel GCSE Physics Paper 2, covering atomic structure, types of nuclear radiation, decay processes, and practical applications. This topic requires both theoretical understanding and mathematical skills, particularly for half-life calculations. Exam questions frequently test your ability to balance nuclear equations, interpret decay graphs, and explain safety procedures.

Key terms and definitions

Radioactive decay — the random, spontaneous process by which unstable atomic nuclei emit radiation to become more stable.

Isotopes — atoms of the same element with the same number of protons but different numbers of neutrons in the nucleus.

Alpha particle (α) — a helium nucleus consisting of 2 protons and 2 neutrons, with a charge of +2 and represented as ⁴₂He or ⁴₂α.

Beta particle (β) — a high-energy electron emitted from the nucleus when a neutron converts to a proton, with a charge of -1 and represented as ⁰₋₁e or ⁰₋₁β.

Gamma ray (γ) — electromagnetic radiation of very short wavelength emitted from the nucleus, with no mass and no charge.

Half-life — the time taken for the number of radioactive nuclei in a sample to halve, or for the count rate to fall to half its initial value.

Activity — the rate at which nuclei decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second.

Background radiation — low-level ionising radiation that exists all around us from natural and artificial sources.

Core concepts

Atomic structure and isotopes

The nucleus contains protons (positive charge) and neutrons (no charge), with electrons orbiting in shells around the nucleus. The atomic number (Z) represents the number of protons, while the mass number (A) represents the total number of protons and neutrons.

Isotopes have identical chemical properties because they have the same number of electrons, but different physical properties due to varying nuclear stability. For example:

Carbon-12 (¹²₆C) — stable isotope with 6 protons and 6 neutrons
Carbon-14 (¹⁴₆C) — radioactive isotope with 6 protons and 8 neutrons

Unstable isotopes undergo radioactive decay to reach a more stable configuration. This process is completely random for individual atoms, meaning it is impossible to predict when a particular nucleus will decay. However, the behaviour of large numbers of nuclei follows predictable statistical patterns.

Types of ionising radiation

Alpha radiation (α):

Composition: 2 protons + 2 neutrons (helium nucleus)
Charge: +2
Mass: 4 atomic mass units (relatively heavy)
Penetration: stopped by paper or a few centimetres of air
Ionising power: highly ionising (causes most ionisation per cm of travel)
Range in air: approximately 3-5 cm
Deflection in magnetic/electric fields: small deflection towards negative plate

Beta radiation (β):

Composition: high-energy electron from nuclear decay
Charge: -1
Mass: 1/2000 atomic mass units (very light)
Penetration: stopped by thin aluminium (approximately 3-5 mm)
Ionising power: moderately ionising
Range in air: approximately 1 m
Deflection in magnetic/electric fields: large deflection towards positive plate

Gamma radiation (γ):

Composition: electromagnetic wave
Charge: 0
Mass: 0
Penetration: reduced by thick lead or several metres of concrete (never completely stopped)
Ionising power: weakly ionising
Range in air: several kilometres
Deflection in magnetic/electric fields: no deflection

Edexcel GCSE Physics exams frequently test your understanding of how to select appropriate materials for shielding different types of radiation based on their penetrating properties.

Nuclear decay equations

Nuclear equations must balance for both mass number (top) and atomic number (bottom).

Alpha decay: When a nucleus emits an alpha particle, the mass number decreases by 4 and the atomic number decreases by 2.

Example: Radium-226 decays to Radon-222 ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α

Beta decay: A neutron converts to a proton and an electron. The electron is emitted as a beta particle. The mass number stays the same while the atomic number increases by 1.

Example: Carbon-14 decays to Nitrogen-14 ¹⁴₆C → ¹⁴₇N + ⁰₋₁β

Gamma emission: Gamma rays carry away excess energy from an excited nucleus but do not change the mass number or atomic number.

Example: Technetium-99m decays to Technetium-99 ⁹⁹ₘ₄₃Tc → ⁹⁹₄₃Tc + γ

Half-life calculations

Half-life remains constant for a particular isotope regardless of external conditions such as temperature or pressure. Different isotopes have vastly different half-lives, from fractions of a second to billions of years.

Using the half-life equation: To calculate remaining quantity after n half-lives:

Remaining amount = initial amount × (½)ⁿ
Where n = total time ÷ half-life

Graphical method: Activity or count rate graphs show exponential decay. To find half-life from a graph:

Identify the initial count rate
Calculate half the initial value
Draw a horizontal line at this value until it intersects the curve
Drop a vertical line to the time axis to read the half-life
Repeat from different starting points to verify consistency

Examples of isotopes and their half-lives tested in Edexcel GCSE Physics:

Iodine-131: 8 days (used in thyroid treatment)
Cobalt-60: 5.3 years (used in radiotherapy)
Carbon-14: 5,730 years (used in radiocarbon dating)
Uranium-238: 4.5 billion years (nuclear fuel)

Background radiation

Background radiation comes from both natural and artificial sources:

Natural sources (approximately 85% of total):

Radon gas from rocks (largest contributor in UK, ~50% of background)
Cosmic rays from space
Rocks and soil containing radioactive isotopes
Food and drink containing naturally occurring isotopes (e.g., potassium-40)
Living organisms

Artificial sources (approximately 15% of total):

Medical procedures (X-rays, CT scans, radiotherapy)
Nuclear power generation
Nuclear weapons testing (historical)
Nuclear accidents (e.g., Chernobyl, Fukushima)

Background radiation varies with location. Areas with granite bedrock have higher radon levels, while high-altitude locations receive more cosmic radiation. When measuring radioactivity in experiments, background radiation must be subtracted from readings to obtain the true count rate from the source.

Corrected count rate = measured count rate - background count rate

Uses and dangers of radioactivity

Medical applications:

Radiotherapy: Gamma rays (from Cobalt-60) or beta particles target and destroy cancer cells
Medical tracers: Gamma emitters (e.g., Technetium-99m) injected into patients to diagnose problems; detected externally by gamma cameras
Sterilisation: Gamma rays sterilise surgical equipment and medical supplies

Industrial applications:

Thickness monitoring: Beta sources measure paper, foil or metal thickness in manufacturing
Smoke detectors: Alpha sources (Americium-241) ionise air; smoke disrupts the current, triggering the alarm
Carbon dating: Carbon-14 content determines the age of organic archaeological specimens

Safety precautions:

Store sources in lead-lined containers
Use tongs or robotic handling equipment to maximise distance
Minimise exposure time
Wear protective clothing and dosimeter badges
Never point sources at people
Wash hands thoroughly after handling
Use appropriate shielding (paper for α, aluminium for β, lead for γ)

Health risks: Ionising radiation damages living cells by ionising atoms within them, potentially causing:

Immediate tissue damage at high doses (radiation burns)
Increased cancer risk at lower doses
Genetic mutations affecting future generations
Damage to rapidly dividing cells (bone marrow, digestive system, reproductive organs)

Alpha sources are particularly dangerous if ingested or inhaled because their high ionising power causes extensive damage inside the body, despite being harmless externally.

Worked examples

Example 1: Nuclear decay equation (2 marks)

Complete the nuclear equation for the alpha decay of Polonium-210: ²¹⁰₈₄Po → ? + ⁴₂α

Solution: Mass number: 210 - 4 = 206 ✓ Atomic number: 84 - 2 = 82 (which is lead, Pb) ✓

Answer: ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂α

Example 2: Half-life calculation (4 marks)

A radioactive source has an initial activity of 800 Bq. The half-life of the isotope is 6 hours. Calculate the activity after 18 hours.

Solution: Number of half-lives = total time ÷ half-life n = 18 ÷ 6 = 3 half-lives ✓

After 1st half-life (6 hours): 800 ÷ 2 = 400 Bq After 2nd half-life (12 hours): 400 ÷ 2 = 200 Bq ✓ After 3rd half-life (18 hours): 200 ÷ 2 = 100 Bq ✓

Or using the formula: 800 × (½)³ = 800 × ⅛ = 100 Bq ✓

Answer: 100 Bq

Example 3: Correcting for background radiation (3 marks)

A Geiger counter measures 180 counts per minute near a radioactive source. When the source is removed, the background count is 30 counts per minute. Calculate the corrected count rate from the source.

Solution: Corrected count rate = measured count rate - background count rate ✓ Corrected count rate = 180 - 30 ✓ Corrected count rate = 150 counts per minute ✓

Answer: 150 counts per minute

Common mistakes and how to avoid them

Mistake: Confusing the properties of alpha, beta and gamma radiation, particularly their penetrating abilities. Correction: Remember the penetration order: alpha (paper) < beta (aluminium) < gamma (lead/concrete). Alpha is most ionising but least penetrating; gamma is least ionising but most penetrating.
Mistake: Incorrectly balancing nuclear equations, especially forgetting that mass numbers and atomic numbers must balance separately. Correction: Always check both numbers independently. Top numbers (mass) must equal on both sides; bottom numbers (atomic) must equal on both sides.
Mistake: Dividing by the half-life value instead of dividing the activity by 2 for each half-life period. Correction: The half-life tells you the time interval; halve the activity/count/mass for each complete half-life that passes. If 3 half-lives pass, divide by 2 three times (or multiply by ½³).
Mistake: Stating that radioactive decay can be affected by external conditions such as temperature or pressure. Correction: Radioactive decay is a nuclear process that occurs spontaneously and randomly. External conditions affect electrons in chemical reactions but cannot influence the nucleus.
Mistake: Forgetting to subtract background radiation from experimental measurements. Correction: All radioactivity measurements include background radiation. Always subtract the background count to find the true count rate from your source: corrected = measured - background.
Mistake: Claiming alpha radiation is harmless because it cannot penetrate skin. Correction: Alpha sources are extremely dangerous if ingested or inhaled because alpha particles cause intense ionisation inside the body where they cannot escape and directly contact internal tissues.

Exam technique for Radioactivity

"Describe" questions require you to state observable features or processes. For decay types, mention the particle emitted and changes to mass/atomic number. Typically worth 2-3 marks requiring 2-3 distinct points.
"Explain" questions demand reasons or mechanisms. Use linking words like "because," "this causes," or "therefore" to show causal relationships. For safety questions, explain the risk and how the precaution reduces exposure. Usually 3-4 marks.
Calculation questions always show your working clearly. For half-life problems, explicitly state the number of half-lives and show each division by 2. Write the formula if using one. Even with wrong arithmetic, method marks are available.
Nuclear equation questions require precise notation. Write mass numbers as superscripts and atomic numbers as subscripts. Check both numbers balance. Common errors lose marks even if you understand the concept.

Quick revision summary

Radioactive decay is the random emission of alpha particles (⁴₂He, stopped by paper), beta particles (⁰₋₁e, stopped by aluminium), or gamma rays (electromagnetic waves, reduced by lead) from unstable nuclei. Nuclear equations must balance mass and atomic numbers. Half-life is the time for activity to halve; calculate by dividing the original amount by 2 for each half-life period. Background radiation exists from natural (radon, cosmic rays) and artificial sources (medical, nuclear power). Always subtract background counts. Radioactivity has medical and industrial uses but requires strict safety precautions due to ionisation damage to living cells.