Interactions With the Environment: Radiation and Risk — AQA Combined Science: Synergy
This topic covers infrared radiation, radioactive decay, half-life, types of nuclear radiation, contamination, irradiation and the link to cancer.
Absorption and emission of radiation
All objects emit and absorb infrared radiation; the hotter an object, the more it emits. Matt black surfaces are the best emitters and absorbers; shiny, light surfaces are good reflectors and poor absorbers. An object at constant temperature emits radiation at the same rate as it absorbs it.
Radioactive decay
Some atomic nuclei are unstable and decay randomly, emitting radiation. It is a random process — we cannot predict when a particular nucleus will decay. Activity (rate of decay) is measured in becquerels (Bq), and the count-rate is measured with a Geiger–Müller tube.
Types of nuclear radiation and penetration
| Radiation | Nature | Penetration | Ionising power |
|---|---|---|---|
| Alpha (α) | helium nucleus | stopped by paper/skin | most ionising, short range |
| Beta (β) | fast electron | stopped by a few mm aluminium | moderate |
| Gamma (γ) | EM wave | stopped by thick lead/concrete | least ionising, very penetrating |
Decay can be shown with nuclear equations, which must balance the mass numbers and atomic numbers. Alpha decay reduces the mass number by 4 and atomic number by 2; beta decay increases the atomic number by 1.
Half-life
The half-life is the time for the number of unstable nuclei (or the activity) to halve. After each half-life the activity halves again (½, ¼, ⅛ …).
Example: a source of half-life 2 hours starting at 800 Bq falls to 100 Bq after 6 hours (three half-lives).
Contamination and irradiation
- Irradiation — exposing an object to radiation. The object does not become radioactive.
- Contamination — radioactive atoms get onto or into an object, which then keeps emitting radiation.
Precautions: shielding, distance, limiting exposure time, and protective clothing. Findings on radiation are peer reviewed so they can be checked.
Ionising radiation and cancer
Ionising radiation can damage cells. At high doses it kills cells; at lower doses it can cause mutations that may lead to cancer. Radiation dose is measured in sieverts (Sv). Despite the risks, radiation is used in medicine (e.g. tracers and radiotherapy) where the benefit outweighs the small risk.
Exam tips
- Learn the three radiations with nature, penetration and ionising power.
- Balance simple alpha and beta nuclear equations.
- Define half-life carefully and practise calculations.
- Distinguish irradiation (exposure) from contamination (radioactive atoms present).
- Remember radioactive decay is random.