8.2 Radioactivity
Isotopes are atoms of the same element (same number of protons) but with a different number of neutrons.
For example, Carbon-12 (6 protons, 6 neutrons) is stable. Carbon-14 (6 protons, 8 neutrons) is unstable.
An unstable nucleus will try to become stable by emitting radiation. This is called radioactive decay. It is a random process.
There are three main types of radiation:
| Radiation | What is it? | Penetration | Ionising Power |
|---|---|---|---|
| Alpha (α) | A helium nucleus (2 protons, 2 neutrons) | Low (stopped by paper/skin) | Very High |
| Beta (β) | A high-speed electron | Medium (stopped by thin aluminium) | Medium |
| Gamma (γ) | A high-energy EM wave | High (stopped by thick lead/concrete) | Low |
Ionisation is the ability of radiation to knock electrons off atoms, creating ions. Alpha is the most ionising, which makes it the most dangerous *inside* the body. Gamma is the most penetrating, making it the most dangerous *outside* the body.
The activity of a radioactive source is the number of decays that happen every second. It is measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Activity decreases over time as the unstable nuclei decay.
The half-life of a source is the time it takes for the number of unstable nuclei to halve (or, the time it takes for the activity to halve).
Example: A source has an activity of 800 Bq. Its half-life is 2 days.
- After 2 days (1 half-life), activity = 400 Bq.
- After 4 days (2 half-lives), activity = 200 Bq.
- After 6 days (3 half-lives), activity = 100 Bq.
Background radiation is the low-level radiation that is all around us from natural sources (like rocks, food, and cosmic rays from space) and man-made sources (like medical scans).
Dangers: High doses of radiation are ionising. They can damage or kill living cells, which can lead to mutations or cancer.
Uses:
- Medical: Gamma rays are used as tracers to see inside the body and in radiotherapy to kill cancer cells.
- Industrial: Alpha particles are used in smoke detectors. Beta radiation is used to check the thickness of paper.