Types of Radiation

Alpha Radiation

Alpha particles are helium nuclei, consisting of 2 protons and 2 neutrons. They have a mass number of 4 and a charge of +2 because of the two protons.

Properties of alpha radiation include:

• Low penetration power: stopped by a sheet of paper or a few centimetres of air.
• High ionising power: they can easily knock electrons off atoms, creating ions.
• Positive charge (+2) and relatively large mass (mass number 4).

Because alpha particles are heavy and charged, they interact strongly with atoms in materials, losing energy quickly and so cannot penetrate far.

For instance, alpha particles cannot penetrate the outer dead layer of skin, so they are mainly hazardous if alpha-emitting materials are ingested or inhaled.

Example: If an alpha particle passes through air, it will be stopped within a few centimetres because its large mass and charge cause many collisions with air molecules, ionising them.

Beta Radiation

Beta particles are high-speed electrons emitted from the nucleus during radioactive decay (see Beta Decay topic for details).

Properties of beta radiation include:

• Moderate penetration power: stopped by a few millimetres of aluminium.
• Moderate ionising power: less ionising than alpha particles but more than gamma rays.
• Negative charge (-1) and very small mass (almost negligible compared to alpha particles).

Beta particles are much lighter than alpha particles and carry a single negative charge, so they penetrate further but ionise less.

Example: A beta particle can pass through paper but will be stopped by a 5 mm thick aluminium sheet.

Gamma Radiation

Gamma rays are electromagnetic waves emitted from the nucleus after alpha or beta decay, carrying away excess energy.

Properties of gamma radiation include:

• High penetration power: requires thick lead or several centimetres of concrete to be stopped.
• Low ionising power: they tend to pass through atoms without ionising them directly.
• No mass and no charge, as they are pure energy waves.

Gamma rays can travel long distances through materials and air, making them the most penetrating type of nuclear radiation.

Example: Gamma rays from a radioactive source can pass through the human body and require lead shielding in hospitals and nuclear facilities to protect people.

Comparison of Radiation Types

Radiation Type	Particle/Wave	Charge	Mass	Penetration	Ionising Power	Shielding Needed
Alpha	Helium nucleus	+2	4 (mass number)	Low (stopped by paper)	High	Paper, skin
Beta	Electron	-1	Negligible	Moderate (stopped by aluminium)	Moderate	Aluminium sheet (~5 mm)
Gamma	Electromagnetic wave	0	0	High (stopped by thick lead/concrete)	Low	Lead, concrete

Penetration and Ionisation Summary

The penetration ability of radiation is inversely related to its ionising power:

• Alpha particles ionise strongly but penetrate very little.
• Beta particles have moderate ionisation and penetration.
• Gamma rays penetrate deeply but ionise weakly.

This is because alpha particles are heavy and charged, losing energy quickly through collisions, while gamma rays are uncharged waves that interact less with matter.

Learning Example

If a radioactive source emits alpha particles, beta particles, and gamma rays, what shielding would be needed to stop each type?

Answer:

• Alpha particles: stopped by a sheet of paper or skin.
• Beta particles: stopped by a few millimetres of aluminium.
• Gamma rays: require thick lead or concrete shielding.