Explain their relative ionising effects with reference to: (a) kinetic energy (b) electric charge

5.2.2 The Three Types of Nuclear Emission

Unstable nuclei decay spontaneously and at random. The radiation is emitted isotropically (in all directions) and can be one of three types defined in the Cambridge IGCSE Physics (0625) syllabus.

1. Alpha (α) emission

• Nature: Helium‑4 nucleus (2 protons + 2 neutrons)
• Mass: ≈ 4 u ≈ 6.6 × 10⁻²⁷ kg
• Electric charge: +2 e
• Typical kinetic energy: 4 – 8 MeV
• Relative ionising effect: Very high – dense ionisation over a very short range
• Relative penetrating ability: Very low – stopped by a sheet of paper or a few centimetres of air

2. Beta (β) emission

• Nature (syllabus focus): High‑speed electrons (β⁻) emitted from the nucleus. (β⁺ – positron emission – is not examined for IGCSE.)
• Mass: ≈ 1 / 1836 u ≈ 9.1 × 10⁻³¹ kg
• Electric charge: –1 e (β⁻)
• Typical kinetic energy: up to 0.1 – 2 MeV; the energy spectrum is continuous from 0 up to a maximum value characteristic of the nuclide.
• Relative ionising effect: Medium – less dense than α but greater than γ
• Relative penetrating ability: Medium – stopped by a few millimetres of aluminium or a few centimetres of plastic

3. Gamma (γ) emission

• Nature: High‑energy photons emitted from an excited nucleus after an α or β transition
• Mass: 0
• Electric charge: 0
• Typical photon energy: 0.1 – 10 MeV (discrete energy lines for each transition)
• Relative ionising effect: Low – ionisation occurs only indirectly via secondary electrons (photoelectric effect, Compton scattering, pair production)
• Relative penetrating ability: High – requires several centimetres of lead or several metres of concrete to attenuate appreciably

Relative Ionising Effects and Penetrating Abilities

The three radiations follow the clear hierarchy required by the syllabus:

α > β > γ in ionising power, and the opposite order for penetrating ability.

Why the Differences Occur

• Kinetic energy: α particles carry a comparable or slightly higher kinetic energy to β particles but, because of their large mass, they lose energy rapidly through frequent collisions, producing a very dense ionisation track. γ photons, being massless, travel at the speed of light and interact only occasionally, so each interaction deposits relatively little energy.
• Electric charge: Charged particles interact directly with electrons in matter via Coulomb forces. The magnitude of the charge matters – a +2 e charge (α) exerts a stronger attractive/repulsive force than a ±1 e charge (β), giving α a higher ionising power. γ rays have no charge; ionisation occurs only via secondary processes, making their ionising effect the lowest per unit path length.

Deflection in Electric and Magnetic Fields (Supplementary Requirement)

• Alpha particles: Strongly deflected because of their +2 e charge and relatively low speed (compared with γ). In a uniform magnetic field they describe a tight circular path.
• Beta particles (electrons): Moderately deflected; the curvature is larger than for α particles because of the smaller charge‑to‑mass ratio and higher speed.
• Gamma rays: Not deflected at all – they are neutral photons and travel in straight lines regardless of electric or magnetic fields.

Typical Applications (Higher‑Order Learning)

• Alpha: Smoke detectors (americium‑241 source); static charge removal.
• Beta: Medical tracers (e.g., phosphorus‑32), thickness gauges in industry.
• Gamma: Radiography (X‑ray & gamma imaging), cancer radiotherapy, sterilisation of medical equipment.

Practical Experiment – Identifying the Three Radiations

A Geiger‑Müller (GM) tube connected to a counter can be used to demonstrate the differing penetrating abilities.

1. Place a weak source (e.g., ²⁴¹Am, ⁹⁰Sr, ⁶⁰Co) directly above the GM tube and record the count rate.
2. Insert a sheet of paper between source and detector:

   • If the count falls to background → the radiation is α.

3. Replace the paper with a few millimetres of aluminium:

   • If the count drops but remains above background → the radiation is β.

4. Finally, place several centimetres of lead:

   • If the count is reduced only slightly → the radiation is γ.

This activity satisfies the syllabus requirement to “identify α, β and γ emissions and describe their nature”.

Optional Higher‑Order Exploration

Using a uniform magnetic field apparatus, students can compare the curvature of α and β tracks. The radius of curvature, r, is given by r = mv/qB. Because α particles have a larger charge (q = +2e) and much greater mass, they follow a tighter curve than β particles for the same magnetic field strength, illustrating the charge‑to‑mass ratio concept.