# Alpha, beta and gamma radiation

> CIE A-Level Physics · Particle physics (AS)
> Source: https://www.owlsprep.com/study/cie-9702-u11-alpha-beta-and-gamma-radiation/

This sub-topic introduces the nature, key properties, and behavior of alpha, beta, and gamma ionizing radiation emitted by unstable atomic nuclei. You will compare their penetration, ionizing power, and interactions with electric and magnetic fields.

**Prerequisites:** Basic nuclear structure and isotopes; Force on charged particles in electric fields

## Learning objectives

- Describe the nature and composition of alpha, beta and gamma radiation
- Compare the ionizing power and penetrating power of each radiation type
- Predict deflection of each radiation in electric and magnetic fields
- Identify unknown radiation from its observed properties

## Nature and Composition of Each Radiation

Unstable nuclei undergo radioactive decay to reach a more stable energy state, emitting ionizing radiation in the process. There are three main types of radiation you need to know for CIE AS Physics:

**Ionizing Radiation** — Radiation that carries enough energy to eject electrons from neutral atoms, producing charged ions. Ionization can cause damage to biological DNA and materials.

*Example:* All three radiation types studied here are forms of ionizing radiation.

| Radiation | Composition | Relative Mass | Relative Charge |
| --- | --- | --- | --- |
| Alpha | Helium nucleus (2p + 2n) | 4 u | +$2e$ |
| Beta-minus | High-energy electron (from nucleus) | $\frac{1}{1840}$ u | -$1e$ |
| Gamma | High-energy electromagnetic photon | 0 | 0 |

**Worked example:** A radioactive source emits a particle with a relative mass of 4 and charge +2e. Identify the type of radiation.

1. Recall the composition and properties of each common radiation type from the table above.
2. Match the given properties: a particle with mass 4 and charge +2e matches the composition of a helium nucleus, which is an alpha particle.
3. Conclusion: The radiation is alpha.

*Calculator:* forbidden

## Ionizing Power and Penetrating Power

Ionizing power is the ability of radiation to ionize atoms per unit distance travelled. Penetrating power is the ability of radiation to pass through matter before being fully absorbed. These two properties are inversely related: higher ionizing power means radiation loses energy faster, so it has lower penetrating power.

**Inverse Relationship** — The order of ionizing power from highest to lowest is: $\alpha > \beta > \gamma$. The order of penetrating power from highest to lowest is: $\gamma > \beta > \alpha$.

**Worked example:** A type of radiation passes through thin paper but is stopped by a 5 mm thick sheet of aluminium. Identify the radiation.

1. Recall the typical absorption of each radiation: Alpha is stopped by ~0.1 mm of paper or 5 cm of air. Gamma is only attenuated (reduced) by several cm of lead or metres of concrete.
2. Beta radiation has intermediate penetrating power: it passes through paper, but is fully absorbed by a few mm of aluminium.
3. Conclusion: The unknown radiation is beta.

> **Exam tip:** CIE almost always asks to compare the ionizing and penetrating power of the three radiation types — remember the inverse relationship to avoid mixing up the order.

*Calculator:* forbidden

## Deflection in Electric and Magnetic Fields

Only charged particles experience a force in electric and magnetic fields, so only alpha and beta are deflected. Gamma has no charge, so it never deflects. The direction and magnitude of deflection depends on the sign of charge and mass of the particle.

- Alpha ($+2e$ charge): Deflects towards the negative plate (electric field) or according to Fleming's Left Hand Rule (magnetic field)
- Beta ($-1e$ charge): Deflects towards the positive plate, in the opposite direction to alpha
- Beta has ~8000 times less mass than alpha, so its deflection is much larger than alpha's deflection
- Gamma: No deflection at all

**Worked example:** A mixed beam of alpha, beta and gamma enters a uniform electric field between a positively charged top plate and negatively charged bottom plate. Describe the deflection of each component.

1. Alpha particles are positively charged, so they are attracted to the negative bottom plate. Their large mass gives them a small curvature of deflection.
2. Beta particles are negatively charged, so they are attracted to the positive top plate, in the opposite direction to alpha. Their very small mass gives them a much larger deflection than alpha.
3. Gamma rays are uncharged photons, so they experience no electric force. They travel in a straight line with no deflection.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Claiming beta particles are orbital electrons ejected from the atom's electron shell.
  - Why it fails: Beta particles are produced inside the nucleus when a neutron decays into a proton and an electron, which is immediately emitted.
  - Correct: State that beta particles are high-energy electrons emitted from the nucleus during decay.
- **Wrong:** Stating gamma radiation has higher ionizing power than alpha radiation.
  - Why it fails: Alpha particles are more massive and have higher charge, so they transfer more energy per unit distance, ionizing more atoms.
  - Correct: Order of ionizing power (highest to lowest): alpha > beta > gamma.
- **Wrong:** Claiming alpha particles have a larger deflection than beta in an electric field.
  - Why it fails: Deflection depends on acceleration, which is inversely proportional to mass for a given force. Beta has much smaller mass so acceleration is much larger.
  - Correct: Beta particles have a much larger deflection than alpha particles.
- **Wrong:** Mixing up the order of penetrating power, stating alpha > beta > gamma.
  - Why it fails: Higher ionizing power means radiation loses energy faster, so it is stopped more quickly, giving lower penetration.
  - Correct: Order of penetrating power (highest to lowest): gamma > beta > alpha.

## Cheatsheet

| Radiation | Composition | Ionizing Power | Penetration | Deflection |
| --- | --- | --- | --- | --- |
| Alpha | He nucleus (2p 2n) | Very High | Stopped by paper / 5cm air | Small deflection to negative |
| Beta | High energy electron | Moderate | Stopped by 3mm aluminium | Large deflection to positive |
| Gamma | EM photon | Very Low | Reduced by thick lead/concrete | No deflection |

## What's next

Understanding the properties of alpha, beta and gamma radiation is the foundational knowledge for all further topics in radioactivity for CIE A-Level Physics. This content is regularly tested in both multiple choice and structured written questions, and forms the basis for understanding radioactive decay processes, half-life, and safety applications of radiation in medicine and industry. Mastering the core properties of each radiation type will make it much easier to tackle nuclear equations and more advanced nuclear physics topics.

- [Radioactive decay](https://www.owlsprep.com/study/cie-9702-u11-radioactive-decay/)
- [Practical circuits (AS)](https://www.owlsprep.com/study/cie-9702-u12-overview/)

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