Alpha, beta and gamma properties
Alpha (α): a helium nucleus (2p + 2n), charge , mass 4; stopped by paper or a few cm of air; most ionising, shortest range. Beta-minus (β⁻): a fast electron , charge , negligible mass; stopped by ~3 mm aluminium; moderately ionising, medium range. Gamma (γ): high-frequency EM (electromagnetic) radiation, no charge or mass; reduced (not stopped) by thick lead or concrete; least ionising, longest range.
Decay is random and spontaneous
Spontaneous: a nucleus decays with no external trigger — not affected by temperature, pressure or chemical reactions. Random: you cannot predict which nucleus decays next or when, only the probability per unit time. Both words are needed — "unpredictable" or "cannot be controlled" alone drops a mark. Because decay is random, a Geiger counter's measured count rate fluctuates irregularly even from a steady source.
Nuclear equations: balance top and bottom
Balance both numbers across the arrow: nucleon number (top) and proton number (bottom) are conserved. Alpha: (top ; bottom ). Beta-minus: (top ; bottom ). Gamma emission changes neither number. Method: fill in the knowns, then subtract to find the unknown, and check both totals.
Drawn from real examiner reports.
Most penetrating ≠ most dangerous
Do not assume more penetrating means more dangerous. Ionising and penetrating power are oppositely ordered: alpha is most ionising but least penetrating (stopped by paper); gamma is the reverse. Externally, gamma is most dangerous (reaches internal organs); internally (inhaled or ingested), alpha is most dangerous due to intense close-range ionisation.
November 2024 Paper 2P Q1 — candidates frequently confused the properties of the three radiation types, especially which type is stopped by which material.
Alpha is not simply "safe"
"Alpha is harmless because skin stops it" is penalised in hazard questions. External alpha is low-risk (stopped by the dead outer skin), but if ingested, inhaled or entering a cut it irradiates living tissue at close range with very high ionising power — polonium-210 is acutely toxic this way. Always distinguish external exposure from internal contamination.
November 2024 Paper 2P Q1 — candidates who said alpha was safe because paper/skin stops it scored zero on follow-up hazard questions about ingested sources.
Balance BOTH nuclear numbers
Many reduce the proton number by 2 for alpha decay but forget to reduce the nucleon number by 4 (or balance one number and then pick the wrong element symbol). The examiner needs both the top (nucleon) and bottom (proton) totals correct on each side. Safe method: fill in the knowns, subtract to find the missing number, then check the top and bottom totals separately.
November 2024 Paper 2P Q1 — many candidates only partially balanced nuclear equations, getting the proton-number change right for alpha decay but not the nucleon-number change.
Gamma is reduced, not stopped
Alpha and beta are effectively stopped by paper and aluminium, but gamma is never completely stopped — it is attenuated (its intensity is reduced) by thick lead or concrete. The mark scheme accepts "thick lead reduces gamma" but rejects "lead stops gamma completely". Use "reduced" or "attenuated" for gamma shielding.
Name neutrons, not "particles"
Nuclear questions demand precise particle names. Writing "particles", "electrons", "atoms", "molecules" or "ions" instead of neutrons (or the correct sub-particle) loses marks even when the physics is right. This precision applies to reactor and decay descriptions alike — always name the exact particle involved.
June 2024 Paper 2PR Q2(b) — candidates regularly used imprecise particle names ("particles", "electrons", "atoms", "molecules", "ions") instead of "neutrons" in nuclear reactor descriptions.
Subtract background radiation
A Geiger counter always records background radiation (cosmic rays, radon from rocks, food, medical and nuclear sources) as well as the source. To find the corrected count rate you must subtract the background count from the measured count. Forgetting this overstates the source's activity. Background is also why the readings fluctuate.
Run the five-property checklist
For any radiation-type question, answer with the same five properties in order: (1) composition/particle; (2) charge (, , 0); (3) penetration / what stops it; (4) ionising power; (5) range in air (cm, m, km). Covering all five keeps comparison answers complete.
"Explain why" = property → consequence
For "explain why" questions, chain the property to the consequence: alpha's charge and large mass → strong interaction with matter → many ionisations per cm → energy lost quickly → short range, stopped by paper. State the property, then the mechanism, then the result.
Balance equations by subtraction
Write the equation, fill in the known top and bottom numbers, then subtract to find the unknown. Finally check that the top numbers sum equally on both sides AND the bottom numbers sum equally. Use the periodic table to name the daughter element from its proton number.
Match the source to the job
Match penetration to the task: a paper or metal thickness gauge needs beta (small thickness changes alter the count rate); a medical tracer needs gamma (penetrates out of the body, least ionising); a smoke detector uses alpha (ionises air, sealed inside).
This topic is primarily qualitative. There are no calculation formulas in 7.2, but nuclear equations use conservation laws:
| Property | Alpha () | Beta-minus () | Gamma () |
|---|---|---|---|
| Composition | nucleus (2p + 2n) | Fast electron () | Electromagnetic (EM) radiation |
| Charge | |||
| Mass (relative) | 4 | 0 | |
| Penetrating power | Lowest — stopped by paper or ~5 cm air | Medium — stopped by ~3 mm aluminium | Highest — reduced by thick lead or concrete |
| Ionising power | Highest | Medium | Lowest |
| Range in air | A few centimetres | Several metres | Very large (kilometres) |
Memory trick — penetration order: P-A-L (Paper stops Alpha; Aluminium stops beLta; Lead reduces Gamma).
Define alpha (α) radiation. State its composition, charge, and what it is stopped by.
