Radioactive DecayAQA GCSE Physics: Revision notes
Section 1
Why Do Nuclei Decay?
Some atomic nuclei are unstable. To become more stable, they undergo radioactive decay — a random process in which the nucleus emits radiation.
- Radioactive decay is random: it is impossible to predict exactly when any individual unstable nucleus will decay.
- The activity of a source is the rate at which unstable nuclei decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Section 2
What Are the Four Types of Nuclear Radiation?
| Radiation | What it is | Penetration | Range in air | Ionising power |
|---|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons (helium nucleus) | Stopped by paper/skin | A few cm | Strongly ionising |
| Beta (β) | High-speed electron from a neutron turning into a proton | Stopped by a few mm of aluminium | ~1 m | Moderately ionising |
| Gamma (γ) | Electromagnetic radiation from the nucleus | Reduced by thick lead/concrete | Long range | Weakly ionising |
| Neutron (n) | A neutron ejected from the nucleus | Stopped by materials rich in hydrogen (e.g. water, wax) | Variable | Weakly ionising directly |
There is a general pattern: the more strongly ionising a radiation is, the less penetrating it is, and vice versa.
Do not say alpha radiation is 'the most dangerous' in general — outside the body its short range makes it low-risk, but if inhaled or swallowed its strong ionising power makes it very dangerous.
Section 3
How Do You Write and Balance Nuclear Equations?
Nuclear equations represent radioactive decay and must balance on both sides: total mass number and total atomic number must be equal before and after decay.
- Alpha decay: mass number decreases by 4, atomic number decreases by 2 (the nucleus emits a helium nucleus).
- Beta decay: atomic number increases by 1, mass number stays the same (a neutron becomes a proton and an electron is emitted).
- Gamma emission: no change in mass number or atomic number — only energy is released.
Uranium-238 (atomic number 92) undergoes alpha decay to form thorium-234 (atomic number 90): mass number falls by 4 (238 → 234) and atomic number falls by 2 (92 → 90).
Section 4
Contamination and Irradiation — What Is the Difference?
These two hazards are often confused but are different:
- Contamination — the unwanted presence of radioactive atoms on or in another material (e.g. on skin, clothing, or in the body). The hazard comes from the decay of the contaminating atoms themselves, which may continue for a long time.
- Irradiation — exposing an object to nuclear radiation from a source. The irradiated object does not itself become radioactive, and the hazard stops as soon as the source is removed.
Suitable precautions (shielding, distance, limited exposure time, protective clothing) must always be taken to protect against the hazard a radioactive source presents.
Section 5
Where Does Background Radiation Come From, and How Is It Used Safely?
Background radiation is the radiation that is around us all the time, from:
- Natural sources: rocks and soil (radon gas), cosmic rays from space, food and drink.
- Man-made sources: nuclear weapons testing fallout, nuclear accidents, and medical procedures.
A person's radiation dose depends on their occupation (e.g. airline crew, nuclear industry workers) and location (e.g. areas with granite rock have higher radon levels).
Radioactive sources are used carefully in medicine — for example, tracers to explore internal organs, and targeted radiation to control or destroy unwanted tissue (such as tumours). The benefits and risks must be weighed against each other, using data on doses and outcomes, before radiation is used.
When asked to 'evaluate' a use of radiation, always weigh a specific benefit against a specific risk — don't just say 'it's risky' or 'it's useful'.
Must Know
- Radioactive decay is random; activity is the rate of decay, measured in becquerels (Bq).
- Alpha (helium nucleus), beta (fast electron), gamma (EM wave) and neutron radiation differ in penetration, range and ionising power — more ionising means less penetrating.
- Alpha decay: mass number −4, atomic number −2. Beta decay: atomic number +1, mass number unchanged. Gamma: no change to either.
- Contamination is radioactive material on/in an object; irradiation is exposure to radiation without becoming radioactive.
- Background radiation comes from natural (rocks, cosmic rays) and man-made (medical, nuclear industry) sources.
- Uses of radiation in medicine must be evaluated by weighing risks against benefits.
That's the notes covered.
Carry on to the next subtopic.