What you'll learn
This revision guide covers radioactive decay and the three types of nuclear radiation tested in AQA GCSE Physics. You'll understand the nature of alpha, beta and gamma radiation, their properties including penetration and ionisation abilities, and how to write balanced nuclear equations. This topic appears in both Paper 1 and is essential for understanding atomic structure and nuclear physics applications.
Key terms and definitions
Radioactive decay — the random process by which unstable atomic nuclei emit radiation to become more stable, releasing energy in the form of particles or electromagnetic waves.
Alpha particle (α) — a type of nuclear radiation consisting of two protons and two neutrons (identical to a helium nucleus), carrying a charge of +2.
Beta particle (β) — high-speed electron emitted from the nucleus when a neutron converts into a proton, carrying a charge of -1.
Gamma ray (γ) — electromagnetic radiation of very short wavelength emitted from an unstable nucleus, carrying no charge or mass.
Ionising radiation — radiation that has sufficient energy to remove electrons from atoms, creating positive ions; all three types of nuclear radiation are ionising.
Penetrating power — the ability of radiation to pass through materials; depends on the type of radiation and energy of the particles or waves.
Nuclear equation — a balanced equation showing radioactive decay, where the total mass number and atomic number are conserved on both sides.
Background radiation — low-level ionising radiation present in the environment from natural and artificial sources, detectable at all times.
Core concepts
Nature and properties of alpha radiation
Alpha particles are relatively large and heavy compared to other types of nuclear radiation. Each alpha particle contains two protons and two neutrons, giving it a mass number of 4 and an atomic number of 2. Because alpha particles carry a positive charge of +2, they interact strongly with surrounding atoms.
Key properties of alpha radiation:
- Strongly ionising — alpha particles create many ions as they travel through materials, removing electrons from atoms they encounter
- Low penetration — stopped by a few centimetres of air, a thin sheet of paper, or human skin
- Short range — typically travel only 3-5 cm in air before losing all their energy
- Deflected by magnetic and electric fields — curved in the opposite direction to beta particles due to positive charge
- Relatively slow speed — travel at approximately 5% the speed of light
When an atom emits an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2. This transforms the atom into a different element.
Nature and properties of beta radiation
Beta particles are high-speed electrons ejected from the nucleus during radioactive decay. They originate when a neutron transforms into a proton, releasing an electron in the process. Despite being emitted from the nucleus, beta particles are not nuclear constituents — they are created during the decay process itself.
Key properties of beta radiation:
- Moderately ionising — less ionising than alpha but more than gamma radiation
- Moderate penetration — stopped by a few millimetres of aluminium or several metres of air
- Medium range — can travel up to a metre in air
- Deflected by magnetic and electric fields — curved in the opposite direction to alpha particles due to negative charge
- High speed — travel at up to 90% the speed of light
When beta decay occurs, a neutron converts to a proton (which remains in the nucleus) and an electron (which is emitted). The mass number stays the same, but the atomic number increases by 1, again creating a different element.
Nature and properties of gamma radiation
Gamma rays are electromagnetic waves, not particles. They belong to the electromagnetic spectrum alongside visible light, X-rays and microwaves, but have the shortest wavelengths and highest frequencies. Gamma emission usually follows alpha or beta decay when the nucleus has excess energy.
Key properties of gamma radiation:
- Weakly ionising — causes the least ionisation of the three radiation types
- High penetration — requires several centimetres of lead or metres of concrete to significantly reduce intensity
- Long range — follows the inverse square law; intensity decreases with distance but theoretically infinite range
- Not deflected by magnetic or electric fields — has no charge or mass
- Travels at the speed of light — 3 × 10⁸ m/s in a vacuum
Gamma emission does not change the mass number or atomic number of the nucleus — the element remains the same but moves to a lower energy state. Gamma rays are often emitted alongside alpha or beta particles.
Writing nuclear decay equations
Nuclear equations must balance for both mass number (top) and atomic number (bottom). The mass number represents the total number of protons and neutrons, while the atomic number represents the number of protons.
Alpha decay equation structure:
The parent nucleus loses 2 protons and 2 neutrons:
Mass number: decreases by 4 Atomic number: decreases by 2
Example: Radium-226 decaying to Radon-222
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α
Check: Mass numbers: 226 = 222 + 4 ✓ Atomic numbers: 88 = 86 + 2 ✓
Beta decay equation structure:
A neutron converts to a proton, emitting an electron:
Mass number: unchanged Atomic number: increases by 1
Example: Carbon-14 decaying to Nitrogen-14
¹⁴₆C → ¹⁴₇N + ⁰₋₁β
Check: Mass numbers: 14 = 14 + 0 ✓ Atomic numbers: 6 = 7 + (-1) ✓
Gamma emission:
No change in composition, only energy release:
Example: Technetium-99m emitting gamma radiation
⁹⁹ₘ₄₃Tc → ⁹⁹₄₃Tc + γ
The 'm' indicates a metastable (excited) state.
Comparing ionisation and penetration
The ionising and penetrating abilities of radiation types are inversely related. Alpha particles ionise strongly because they are large, slow-moving and highly charged, but this means they lose energy quickly and cannot penetrate far. Gamma rays barely interact with matter, so they ionise weakly but penetrate deeply.
Ionising power (strongest to weakest):
- Alpha — creates thousands of ion pairs per millimetre in air
- Beta — creates hundreds of ion pairs per millimetre in air
- Gamma — creates only a few ion pairs per millimetre in air
Penetrating power (strongest to weakest):
- Gamma — requires thick lead or concrete
- Beta — stopped by aluminium foil or thin metal
- Alpha — stopped by paper or dead skin cells
Materials that stop each type:
- Alpha: paper, card, skin, thin fabric
- Beta: aluminium (3-5 mm), perspex, wood
- Gamma: thick lead (several cm), thick concrete (metres)
These properties determine appropriate safety precautions and practical applications for each radiation type.
Sources and detection of background radiation
Background radiation is present everywhere and comes from various natural and artificial sources. It must be measured and subtracted when conducting radiation experiments to obtain accurate readings.
Natural sources (approximately 85% of total):
- Cosmic rays from space — intensity increases with altitude
- Rocks and soil — granite contains uranium and thorium
- Radon gas — seeps from underground rocks, especially in certain geographical areas
- Food and drink — potassium-40 in bananas, carbon-14 in all organic matter
- Living tissues — carbon-14 and potassium-40 in our bodies
Artificial sources (approximately 15% of total):
- Medical procedures — X-rays, CT scans, radiotherapy
- Nuclear weapons testing — historical atmospheric tests
- Nuclear power and accidents — Chernobyl, Fukushima
- Air travel — increased cosmic ray exposure at altitude
Background radiation varies by location. Areas with granite bedrock (like Cornwall in the UK) have higher radon levels. Urban areas near hospitals may show slightly elevated readings from medical isotopes.
Detection: Radiation is detected using Geiger-Müller tubes, which produce an electrical pulse when ionising radiation enters. The count rate (counts per second or minute) must have background radiation subtracted:
Corrected count rate = measured count rate - background count rate
Worked examples
Example 1: Identifying radiation types
Question: A laboratory has three unknown radioactive sources. Scientists conduct tests to identify the type of radiation each emits:
Source A: Radiation is stopped by paper Source B: Radiation passes through paper but is stopped by 3 mm aluminium Source C: Radiation passes through aluminium but is reduced by thick lead
Identify the type of radiation from each source. (3 marks)
Answer:
- Source A emits alpha radiation (1 mark) — only alpha is stopped by paper
- Source B emits beta radiation (1 mark) — beta passes through paper but is stopped by thin aluminium
- Source C emits gamma radiation (1 mark) — gamma penetrates aluminium and requires thick lead to reduce intensity
Example 2: Nuclear decay equations
Question: Polonium-210 is an alpha emitter.
(a) Complete the nuclear equation for this decay: ²¹⁰₈₄Po → _____ + ⁴₂α (2 marks)
(b) State what happens to a neutron during beta decay. (2 marks)
Answer: (a) ²⁰⁶₈₂Pb (1 mark for correct mass number 206, 1 mark for correct atomic number 82) Working: Mass number: 210 - 4 = 206 Atomic number: 84 - 2 = 82 (this is lead, Pb)
(b) A neutron converts/changes into a proton (1 mark) and an electron is emitted (1 mark) OR: A neutron splits into a proton and an electron (2 marks)
Example 3: Penetration and safety
Question: A hospital uses different radioactive sources for medical procedures:
- Americium-241 (alpha emitter) in smoke detectors
- Technetium-99m (gamma emitter) as a medical tracer
- Strontium-90 (beta emitter) for treating eye diseases
(a) Explain why the alpha source in smoke detectors is safe for home use. (2 marks)
(b) Suggest why gamma sources are used as medical tracers that can be detected outside the body. (2 marks)
Answer: (a) Alpha radiation is stopped by the plastic casing/dead skin cells (1 mark), so cannot reach living tissue/is not absorbed by the body (1 mark) OR: Alpha has very low penetrating power (1 mark) so cannot escape the detector housing to cause harm (1 mark)
(b) Gamma radiation is very penetrating/passes through body tissue (1 mark), so can be detected by external sensors/cameras outside the patient's body (1 mark)
Common mistakes and how to avoid them
Confusing mass and charge — Remember: mass number is the total of protons + neutrons (top number), atomic number is protons only (bottom number). Beta particles have essentially zero mass (⁰₋₁) but carry a -1 charge.
Incorrect balancing of nuclear equations — Always check both numbers balance independently. The top numbers must add to the same total on both sides, AND the bottom numbers must add to the same total. Work systematically: do mass first, then atomic number.
Thinking beta particles come from electron shells — Beta particles originate in the nucleus when a neutron decays, not from the electron shells surrounding the nucleus. This is why beta decay increases the atomic number by 1 (one more proton).
Reversing ionisation and penetration — Remember they are inversely related: alpha ionises STRONGLY but penetrates WEAKLY; gamma ionises WEAKLY but penetrates STRONGLY. Think of alpha particles as large and clumsy — they bump into everything (high ionisation) but can't get far (low penetration).
Forgetting to subtract background radiation — In any practical experiment, you must measure background count rate first, then subtract it from your readings. Examiners specifically look for this in practical questions.
Claiming radiation can be "used up" or "absorbed" to make other things radioactive — Materials don't become radioactive just by being exposed to radiation (except in very specific nuclear reactions beyond GCSE). A detector doesn't become radioactive by detecting radiation.
Exam technique for "Radioactive decay: alpha, beta and gamma radiation"
Command word "compare" requires you to give similarities AND differences, or advantages AND disadvantages. For example: "Compare alpha and beta radiation" needs statements like "Alpha is stopped by paper whereas beta penetrates paper but is stopped by aluminium" (a direct comparison), not just separate lists of properties.
Nuclear equation questions typically award 1 mark for mass number and 1 mark for atomic number — show your working by writing out the calculation (e.g., "210 - 4 = 206") to ensure you get method marks even if you make an arithmetic error.
Practical questions about detection require you to state that background radiation must be measured AND subtracted. Stating only "background radiation is present" without explaining what to do about it typically scores zero marks.
Extended response questions (6 marks) on radiation properties require structured answers covering ionising power, penetrating power, what stops each type, and often practical applications. Use the format: alpha/beta/gamma separately, giving 2-3 distinct points for each type to ensure comprehensive coverage.
Quick revision summary
Radioactive decay releases three types of radiation with different properties. Alpha particles (helium nuclei, +2 charge) ionise strongly but are stopped by paper. Beta particles (fast electrons, -1 charge) have moderate ionisation and penetration, stopped by aluminium. Gamma rays (electromagnetic waves, no charge) ionise weakly but penetrate deeply, requiring thick lead to reduce intensity. In nuclear equations, mass and atomic numbers must balance. Background radiation exists naturally and must be subtracted from measurements. Ionising power and penetrating power are inversely related.