Cognito · AQA Physics 2026/27Radioactivity and half-life
Key facts
Atomic structure and isotopes
Atom = nucleus (protons + neutrons) + electrons in shells.
Atomic number (Z) = proton number. Mass number (A) = protons + neutrons.
Isotopes = atoms of the same element with different numbers of neutrons.
Unstable nuclei emit radiation to become more stable — radioactive decay.
Activity = number of decays per second. Unit: becquerel (Bq).
Types of radiation — fill in the table
Type
What it is
Stopped by
Ionising power
Alpha (α)
Beta (β)
Gamma (γ)
Alpha is most dangerous inside the body (most ionising). Gamma is most dangerous outside (most penetrating).
Half-life
Half-life = time taken for half the radioactive nuclei in a sample to decay.
Also = time for activity to halve.
After n half-lives: activity = initial activity × (½)ⁿ.
Half-life is constant for a given isotope — not affected by temperature, pressure or chemical state.
Read from graph: find when activity = half its initial value → that time is one half-life.
Nuclear equations
Alpha decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. Mass number drops by 4, atomic number drops by 2.
Beta decay: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Mass number unchanged, atomic number increases by 1.
In all nuclear equations: mass numbers must balance, atomic numbers must balance.
Fission — large unstable nucleus splits into two smaller nuclei + neutrons + energy. Used in nuclear reactors.
Fusion — two light nuclei join to form a larger nucleus + energy. Powers the Sun. Very high temp/pressure needed.
Exam questions — 4 questions · 13 marks
⚡ Extended questions included. Show all working for calculation questions. The AI marks against the real AQA mark scheme.
1 markAlpha propertiesAQA 8463 P1 style
Why is alpha radiation the most ionising but least penetrating type of radiation?
A Alpha particles have a negative charge so they attract electrons from atoms but cannot travel far
B Alpha particles are large and slow — they interact strongly with atoms (high ionisation) but are absorbed after a short distance
C Alpha particles travel faster than gamma rays so they collide more often
D Alpha particles are electrically neutral so they only interact through gravity
3 marksHalf-life calculationAQA 8463 P1 style — real exam style
A radioactive source has an initial activity of 800 Bq. Its half-life is 3 hours. Calculate the activity after 12 hours. Show your working. (3 marks)
Hint: Calculate how many half-lives fit into 12 hours. Then halve the activity that many times.
+30 XP
3 marksNuclear equationAQA 8463 P1 style
Uranium-238 undergoes alpha decay. Write the nuclear equation for this decay, identifying the daughter nucleus. Show that both mass number and atomic number are conserved. (3 marks)
Hint: Alpha particle = ⁴₂He. Subtract 4 from mass number and 2 from atomic number. Check both balance.
+30 XP
1 markFission vs fusionAQA 8463 P1 style
Which statement correctly compares nuclear fission and fusion?
A Both fission and fusion are used in current nuclear power stations
B Fission splits heavy nuclei releasing energy; fusion joins light nuclei releasing even more energy per kg of fuel
C Fusion is safer than fission because it produces no radiation
D Fission requires very high temperatures; fusion does not
3 marksUses and dangers of radiationAQA 8463 P1 style
Explain why gamma radiation is used in medical imaging but alpha radiation is not. (3 marks)
Hint: Cover why gamma works (penetrating, detected outside body) AND why alpha cannot (stopped by skin, too ionising).