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: Identify alpha (α), beta (β) and gamma (γ) emissions from the nucleus by recalling:

1. their nature
2. their relative ionising effects
3. their relative penetrating abilities

1. Alpha (α) Emission

Nature: An alpha particle is a helium‑4 nucleus, consisting of 2 protons and 2 neutrons. It is denoted as \$^{4}_{2}\mathrm{He}\$ or simply \$α\$.

Typical nuclear reaction:

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

• Mass: ≈ 4 u (atomic mass units)
• Charge: +2 e
• Speed: \$10^{7}\$ m s⁻¹ (≈ 5 % of the speed of light)

Ionising effect: Very high. The +2 charge and relatively large mass cause dense ionisation along a short track.

Penetrating ability: Low. Stopped by a few centimetres of air or a sheet of paper (≈ 0.5 mm thick).

2. Beta (β) Emission (β⁻)

Nature: A beta particle is a high‑energy electron emitted when a neutron in the nucleus transforms into a proton. It is denoted as \$β^{-}\$.

Typical nuclear reaction:

\$^{A}{Z}\mathrm{X} \;\rightarrow\; ^{A}{Z+1}\mathrm{Y} + \beta^{-} + \bar{\nu}_{e}\$

• Mass: ≈ \$5.5\times10^{-4}\$ u (≈ 1/1836 of a proton)
• Charge: –1 e
• Speed: up to \$0.99c\$ (where \$c\$ is the speed of light)

Ionising effect: Moderate. The single negative charge produces ionisation, but because the particle is much lighter than an alpha particle, the ionisation is less dense.

Penetrating ability: Medium. Can travel several metres in air; stopped by a few millimetres of aluminium (≈ 1–2 mm).

3. Gamma (γ) Emission

Nature: Gamma radiation consists of high‑energy photons emitted from an excited nucleus as it drops to a lower energy state. It is denoted as \$γ\$.

Typical nuclear reaction (following an α or β decay):

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

• Mass: 0 (photon)
• Charge: 0
• Speed: \$c\$ (the speed of light)

Ionising effect: Low. Photons interact less frequently with matter, producing ionisation only when they undergo photoelectric absorption, Compton scattering or pair production.

Penetrating ability: High. Requires dense materials such as several centimetres of lead or several tens of centimetres of concrete to be significantly attenuated.

Comparison of the Three Emissions