Alpha, beta and gamma radiation
CIE A-Level PhysicsΒ· 35 min read
1. Nature and Composition of Each Radiationβ βββββ± 8 min
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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 | + |
Beta-minus | High-energy electron (from nucleus) | u | - |
Gamma | High-energy electromagnetic photon | 0 | 0 |
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.
2. Ionizing Power and Penetrating Powerβ β ββββ± 10 min
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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: . The order of penetrating power from highest to lowest is: .
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 β remember the inverse relationship to avoid mixing up the order.
3. Deflection in Electric and Magnetic Fieldsβ β β βββ± 12 min
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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 ( charge): Deflects towards the negative plate (electric field) or according to Fleming's Left Hand Rule (magnetic field)
Beta ( 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
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.
4. Common Pitfalls
Wrong move:
Claiming beta particles are orbital electrons ejected from the atom's electron shell.
Why:
Beta particles are produced inside the nucleus when a neutron decays into a proton and an electron, which is immediately emitted.
Correct move:
State that beta particles are high-energy electrons emitted from the nucleus during decay.
Wrong move:
Stating gamma radiation has higher ionizing power than alpha radiation.
Why:
Alpha particles are more massive and have higher charge, so they transfer more energy per unit distance, ionizing more atoms.
Correct move:
Order of ionizing power (highest to lowest): alpha > beta > gamma.
Wrong move:
Claiming alpha particles have a larger deflection than beta in an electric field.
Why:
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 move:
Beta particles have a much larger deflection than alpha particles.
Wrong move:
Mixing up the order of penetrating power, stating alpha > beta > gamma.
Why:
Higher ionizing power means radiation loses energy faster, so it is stopped more quickly, giving lower penetration.
Correct move:
Order of penetrating power (highest to lowest): gamma > beta > alpha.
5. Quick Reference 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 |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 12
Compare penetration of radiations
- 2023 Β· 21
Deflection in electric field
- 2021 Β· 11
Identify radiation from properties
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.