Identify alpha (α), beta (β) and gamma (γ) emissions from the nucleus by recalling: (a) their nature (b) their relative ionising effects (c) their relative penetrating abilities (β+ are not included, β-particles will be taken to refer to β−)

5.2.2 The Three Types of Nuclear Emission

Objective: Students must be able to identify alpha (α), beta‑minus (β⁻) and gamma (γ) emissions from the nucleus and recall:

• their nature (what the radiation actually is)
• their relative ionising effects
• their relative penetrating abilities
• how they behave in electric and magnetic fields

Note: The Cambridge IGCSE 0625 syllabus does not require knowledge of β⁺ (positron) emission. If you encounter β⁺ in other texts, remember that it is excluded from this topic.

1. Alpha (α) Emission

• Nature: A helium‑4 nucleus, ⁴₂He²⁺ (2 protons + 2 neutrons).
• Typical nuclear reaction

  \$^{A}{Z}\mathrm{X}\;\rightarrow\;^{A-4}{Z-2}\mathrm{Y}+α\$

• Key properties

  • Mass ≈ 4 u
  • Charge = +2 e
  • Typical speed ≈ 5 % c (≈ 1.5 × 10⁷ m s⁻¹)

• Ionising effect: Very high – the double positive charge and relatively large mass produce a dense track of ion pairs.
• Penetrating ability (air): A few centimetres (≈ 3–5 cm). Stopped by a sheet of paper or ≈ 0.5 mm of skin.
• Deflection in fields

  • Electric field: Deflected toward the negative plate (positive charge).
  • Magnetic field: Curves according to the right‑hand rule for a positive charge (clockwise when the field points into the page).

• Typical uses / hazards

  • Smoke detectors (e.g., ²⁴¹Am).
  • Can be stopped by the outer dead layer of skin but is extremely hazardous if inhaled or ingested.

2. Beta (β⁻) Emission

• Nature: A high‑energy electron emitted when a neutron converts to a proton.
• Typical nuclear reaction

  \$^{A}{Z}\mathrm{X}\;\rightarrow\;^{A}{Z+1}\mathrm{Y}+β^{-}+\barν_{e}\$

• Key properties

  • Mass ≈ 5.5 × 10⁻⁴ u (≈ 1/1836 of a proton)
  • Charge = –1 e
  • Typical speed ≈ 0.5–0.99 c (up to 3 × 10⁸ m s⁻¹)

• Ionising effect: Moderate – single negative charge gives a less dense ion‑track than α‑particles.
• Penetrating ability (air): Several metres (≈ 3–5 m for typical energies). Stopped by 1–2 mm of aluminium or ≈ 5 mm of plastic.
• Deflection in fields

  • Electric field: Deflected toward the positive plate (negative charge).
  • Magnetic field: Curves opposite to an α‑particle (right‑hand rule for a negative charge – counter‑clockwise when the field points into the page).

• Typical uses / hazards

  • Medical imaging and radiotherapy (e.g., ⁹⁰Sr/⁹⁰Y sources).
  • Can penetrate the outer dead skin layer; thin metal shielding or protective gloves are required.

3. Gamma (γ) Emission

• Nature: High‑energy photons – electromagnetic radiation emitted when an excited nucleus drops to a lower energy state.
• Typical nuclear reaction (following α or β decay)

  \$^{A}{Z}\mathrm{X}^{*}\;\rightarrow\;^{A}{Z}\mathrm{X}+γ\$

• Key properties

  • Mass = 0 (photon)
  • Charge = 0
  • Speed = c (≈ 3 × 10⁸ m s⁻¹)

• Ionising effect: Low – interaction occurs only via photoelectric absorption, Compton scattering or pair production.
• Penetrating ability (air): Many kilometres; essentially unattenuated in ordinary laboratory distances.
• Deflection in fields

  • Neutral – not deflected by either electric or magnetic fields.

• Typical uses / hazards

  • Food sterilisation, industrial radiography, cancer treatment (e.g., ⁶⁰Co sources).
  • Can pass through the whole human body; dense shielding such as several centimetres of lead or ≈ 30 cm of concrete is required.

Ionising‑Effect Ranking

Comparison of α, β⁻ and γ Radiation