What Is Radioactive Decay?
Some atomic nuclei are inherently unstable. When a nucleus has too many protons, too many neutrons, or simply too much energy, it will spontaneously release energy and particles in a process called radioactive decay. The original atom (the "parent") transforms into a different atom (the "daughter") in the process. This is the foundation of nuclear science and has wide-ranging applications in medicine, energy, and geology.
The Three Main Types of Radiation
Alpha (α) Decay
In alpha decay, the nucleus emits an alpha particle — which is identical to a helium-4 nucleus: 2 protons and 2 neutrons bound together. When an atom undergoes alpha decay, its atomic number decreases by 2 and its mass number decreases by 4.
Example: Uranium-238 decays into Thorium-234 by emitting an alpha particle.
- Penetrating power: Very low — stopped by a sheet of paper or a few centimetres of air
- Ionising power: Very high — causes dense ionisation in tissue if inhaled or ingested
- Range in tissue: Extremely short
Beta (β) Decay
Beta decay comes in two forms. In beta-minus (β⁻) decay, a neutron in the nucleus converts into a proton, releasing a high-energy electron (beta particle) and an antineutrino. The atomic number increases by 1.
In beta-plus (β⁺) decay, a proton converts into a neutron, releasing a positron (an antimatter electron) and a neutrino. The atomic number decreases by 1.
- Penetrating power: Moderate — stopped by a few millimetres of aluminium
- Ionising power: Moderate
- Medical use: Beta-emitting isotopes are used in cancer radiotherapy
Gamma (γ) Radiation
Gamma rays are not particles — they are high-energy electromagnetic radiation (photons) emitted when a nucleus releases excess energy after undergoing alpha or beta decay. Gamma decay does not change the atomic number or mass number; it simply releases energy.
- Penetrating power: Very high — requires thick lead or concrete shielding
- Ionising power: Lower than alpha or beta per unit path
- Medical use: Used in gamma-ray imaging, cancer treatment (gamma knife surgery), and sterilisation
Comparison Table
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (2p + 2n) | Electron or positron | Electromagnetic wave |
| Charge | +2 | −1 or +1 | 0 |
| Penetration | Very low | Moderate | Very high |
| Stopped by | Paper, skin | Aluminium sheet | Thick lead or concrete |
Half-Life: How Decay Is Measured
Radioactive decay is a statistical process — you can't predict when any single atom will decay, but you can predict how long it takes for half of a large sample to decay. This time period is called the half-life. Half-lives range from fractions of a second (for highly unstable isotopes) to billions of years (for uranium-238, which has a half-life of about 4.5 billion years).
Half-life is extremely useful in radiocarbon dating, where the decay of carbon-14 (half-life ~5,730 years) allows scientists to estimate the age of organic materials.
Practical Applications of Radioactive Decay
- Nuclear medicine: Diagnostic imaging (PET scans, gamma camera scans)
- Cancer treatment: Targeted radiation therapy
- Archaeology and geology: Radiometric dating
- Nuclear energy: Controlled fission of radioactive isotopes
- Smoke detectors: Americium-241 (an alpha emitter) is used in most household smoke detectors