Ionising radiations differ in their physical nature, interactions with matter, and ability to penetrate materials. Understanding these differences is essential for interpreting absorption experiments and explaining how radiation behaves in practical contexts.

The Nature of α, β, and γ Radiation

The three main types of nuclear radiation—alpha (α), beta (β), and gamma (γ)—originate from unstable nuclei undergoing radioactive decay. Each radiation type has distinct physical characteristics that determine how it interacts with matter.

Alpha (α) Radiation

Alpha radiation consists of helium nuclei, each with two protons and two neutrons. Because an α-particle is relatively large and carries a +2 charge, it interacts strongly and frequently with surrounding matter.
These interactions cause α-particles to lose energy rapidly, giving α radiation a very short range in air and high ionising ability.

A single sentence helps to separate explanation blocks naturally. Alpha radiation is commonly emitted by heavy, unstable nuclei and is observed in many natural radioactive sources.

Beta (β) Radiation

Beta radiation consists of fast-moving electrons (β⁻) or positrons (β⁺) emitted when a nucleus undergoes a change in quark structure, though the details of quark processes fall outside this subsubtopic. Because β-particles have a much smaller mass than α-particles and carry a single charge, they interact less intensely with matter.

Beta radiation therefore penetrates further through air and thin materials than α-radiation, although it is less ionising.

Gamma (γ) Radiation

Gamma radiation consists of high-energy photons emitted when a nucleus relaxes from an excited state. Because γ photons have no mass and no charge, they interact only weakly with matter.

Gamma radiation is highly penetrating with a long range in air, though its ionising power is low compared to α and β radiation.

Comparative Penetration and Range

The differences in charge, mass, and energy lead to clear contrasts in how α, β, and γ radiations pass through materials.

Penetration Abilities

Students should be confident in recognising the characteristic penetration patterns:

Diagram comparing how alpha, beta, and gamma radiations are absorbed by paper, aluminium, and lead. Alpha is stopped first, beta penetrates further, and gamma requires thick shielding. Layout is simplified to emphasise qualitative differences. Source.

• Alpha radiation

  • Penetrates only a few centimetres of air.

  • Stopped by paper or the outer layer of human skin.

  • Highly ionising due to frequent collisions.

• Beta radiation

  • Penetrates several tens of centimetres of air.

  • Stopped by a few millimetres of aluminium.

  • Moderate ionising power.

• Gamma radiation

  • Penetrates many metres of air.

  • Reduced significantly only by thick lead or several centimetres of dense concrete.

  • Low ionising power, but potentially hazardous due to long range.

Range in Air

The range of each radiation type can be demonstrated using counter readings:

• α: typically 2–5 cm

• β: typically up to around 1 m

• γ: effectively unlimited for simple laboratory distances, although intensity decreases with distance following an inverse-square relationship.

Absorption Experiments

Absorption experiments provide essential, empirical evidence for the different properties of α, β, and γ radiation. Such experiments often use a Geiger–Müller tube to measure count rates as various materials are placed between the source and the detector.

Basic Experimental Approach

Students should be able to outline how absorption is investigated:

• Set up a radioactive source directed towards a Geiger–Müller detector.

Schematic of a Geiger–Müller counter including the end-window tube, high-voltage supply, and counting electronics. The mica window admits low-penetrating radiation while the gas avalanche produces detectable pulses. External absorber sheets are typically placed in front of the window during penetration experiments. Source.

• Measure a background count rate and subtract it from all readings to ensure accuracy.

• Insert materials of increasing thickness between the source and detector.

• Record changes in count rate to determine how each type of radiation is absorbed.

One sentence is included here to ensure spacing between definition and equation blocks, even if no equations are necessary for this topic.

Observations from Absorption Trials

Typical experimental findings include:

• Alpha radiation

  • Detectable only when no absorber or extremely thin materials are present.

  • Count rate drops to background once even thin paper is inserted.

• Beta radiation

  • Count rate decreases progressively as aluminium sheets increase in thickness.

  • Little reduction with paper or plastic.

• Gamma radiation

  • Count rate is reduced gradually but never reaches zero, regardless of absorber thickness.

  • Typical materials such as lead cause only partial attenuation.

Interpreting Results

Absorption experiments help students relate theoretical descriptions of radiation to measurable behaviours. Key interpretations include:

• Stronger absorption implies stronger interactions between radiation and matter.

• Incomplete absorption, especially for γ radiation, demonstrates the weak interaction of photons with electrons and nuclei.

• Sharp drops in count rate, as with α radiation, confirm its short range and high ionising power.

• Gradual attenuation curves for γ photons reflect probabilistic interactions described in exponential attenuation relationships, though detailed equations are beyond this subsubtopic.

Practical Considerations in Radiation Studies

Understanding absorption also supports awareness of safe handling practices. Key considerations include:

• Using appropriate shielding depending on the radiation type.

• Maintaining distance, particularly from γ sources, due to their long range.

• Minimising exposure time and using tongs or remote-handling tools.

• Monitoring radiation levels frequently during experimental work.

Key Features to Remember

To consolidate understanding of the core conceptual material for this subsubtopic, students should retain the following:

• α, β, and γ radiations differ fundamentally in nature, charge, mass, and energy.

• These intrinsic properties determine their ionising power, range, and penetration.

• Absorption experiments provide essential evidence for comparing these radiations and interpreting their physical behaviours.