Radioactive decay (AQA GCSE Physics Combined Science): Revision Notes
Radioactive decay
What is radioactive decay?
Radioactive decay happens when unstable atomic nuclei break down over time. This process is random - we cannot predict exactly when a particular nucleus will decay. When decay occurs, the nucleus releases energy in the form of radiation and may change into a different element.
Some elements are radioactive because their nuclei are unstable. These unstable nuclei will eventually decay by giving out different types of radiation.
The randomness of radioactive decay is a fundamental property of nature. While we can predict the average behaviour of large numbers of nuclei, individual decay events are completely unpredictable.
Types of nuclear radiation
There are four main types of radiation that can be emitted during radioactive decay:
Alpha particles (α)
Alpha particles are the largest and most massive type of nuclear radiation, making them unique in their behaviour compared to other radiation types.
- What it is: A helium nucleus containing two protons and two neutrons
- Charge: +2 (positive)
- Mass: 4 (heaviest of all radiation types)
- Ionising power: Very strong - easily removes electrons from atoms
- Penetrating power: Very weak - stopped by a thin sheet of paper or travels only about 5cm in air
Beta particles (β)
Beta particles represent high-speed electrons that are ejected from the nucleus during certain types of radioactive decay.
- What it is: A high-speed electron ejected from the nucleus
- Charge: -1 (negative)
- Mass: Very small (1/1840 compared to a proton)
- Ionising power: Moderate
- Penetrating power: Medium - stopped by a few millimetres of aluminium but travels several metres in air
Gamma rays (γ)
Gamma rays are electromagnetic waves similar to X-rays but with much higher energy levels.
- What it is: High-energy electromagnetic waves (like X-rays but more energetic)
- Charge: 0 (neutral)
- Mass: 0 (no mass)
- Ionising power: Weak
- Penetrating power: Very strong - needs thick lead or several metres of concrete to stop them
Neutrons
Neutrons are uncharged particles that can be released during certain nuclear reactions and decay processes.
- What it is: Particles with no electric charge from the nucleus
- Charge: 0 (neutral)
- Mass: 1 (similar to a proton)
- Ionising power: Low (they don't ionise directly)
- Penetrating power: Very high - can travel through buildings and humans for long distances
Neutron radiation is less common in everyday radioactive decay but becomes very important in nuclear reactors and nuclear weapons. Unlike other radiation types, neutrons can make other materials radioactive through neutron activation.
Key properties to remember
Understanding the relationship between ionising and penetrating power is crucial for radiation safety and protection planning.
Critical Inverse Relationship:
Ionising power decreases in this order: Alpha → Beta → Gamma → Neutron
Penetrating power increases in this order: Alpha → Beta → Gamma → Neutron
This means alpha particles cause the most damage to living tissue but are easiest to stop, while gamma rays and neutrons are hardest to stop but cause less immediate damage.
Safety considerations
Understanding the safety implications of each radiation type is essential for proper protection in any environment where radioactive materials are present.
Alpha particles: Not dangerous from outside the body (stopped by skin) but very dangerous if swallowed or breathed in
Beta particles: Can penetrate skin but stopped by thin metal sheets
Gamma rays: Very penetrating - need thick lead shielding for protection
Neutrons: Extremely penetrating and difficult to shield against
Key Points to Remember:
- Radioactive decay is a random process - we cannot predict when individual nuclei will decay
- Alpha particles are the heaviest and most ionising but least penetrating
- Beta particles are fast electrons with medium ionising and penetrating power
- Gamma rays have no mass or charge but are the most penetrating
- Neutrons are neutral particles that are very hard to stop
- The ionising power and penetrating power of radiation types work in opposite ways