Know that a β-particle is a high-speed electron emitted from the nucleus, formed when a neutron changes into a proton and an electron and a reduction in the number of excess neutrons; the following change in the nucleus occurs during β-emission neutr

5.2.3 Radioactive Decay – Beta (β) Emission

What is a β‑particle?

• High‑speed electron emitted from the nucleus of an unstable atom.
• Charge = –1 e.
• Mass ≈ 9.1 × 10⁻³¹ kg (≈ 1/1836 of a proton).
• Ionising power: moderate – can ionise gases and liquids but is stopped more easily than α‑particles.
• Typical shielding: a few mm of aluminium, plastic or glass; a sheet of paper is insufficient.

Fundamental nuclear change (β⁻‑decay)

The basic reaction is

• n – neutron (neutral).
• p⁺ – proton (positive charge).
• e⁻ – electron (the β‑particle).
• \(\bar{\nu}_{e}\) – antineutrino (neutral, extremely difficult to detect).

Effect on the nucleus

• The neutron‑to‑proton ratio is reduced, moving the nucleus toward the band of stability.
• Atomic number Z increases by 1 (one extra proton).
• Mass number A remains unchanged because a neutron is simply converted into a proton.
• The element changes to the next higher element in the periodic table.

General nuclear equation (nuclide notation)

where ^{A}{Z}X is the parent nuclide and ^{A}{Z+1}Y the daughter nuclide.

Safety note

β‑radiation is stopped by thin metal or plastic, but direct skin exposure should be avoided because the particles can cause superficial burns. Always handle β‑emitters with appropriate shielding (e.g., aluminium foil) and follow laboratory safety procedures.

Link to decay rate

The rate of β‑decay for a given isotope is characterised by its half‑life, which is covered in Section 5.2.4 – Radioactive decay – half‑life and decay equations. The same exponential decay law applies to β‑emitters as to α‑ and γ‑emitters.

Comparison with α‑decay

Worked examples

Example 1 – Carbon‑14

Carbon‑14 undergoes β‑decay to become nitrogen‑14:

Mass number stays 14; atomic number rises from 6 (C) to 7 (N).

Example 2 – Phosphorus‑32

Write the β‑decay equation for phosphorus‑32 (³²₁₅P):

The daughter nucleus is sulphur‑32; again A is unchanged and Z increases by 1.

Common misconceptions

• β‑particle = proton: It is an electron.
• Mass number changes: In β‑decay A does not change.
• Only an electron is emitted: An antineutrino is also emitted, carrying away part of the decay energy.
• β‑radiation makes a nucleus more unstable: It actually reduces the neutron‑to‑proton ratio, moving the nucleus toward greater stability.

Brief note on β⁺ (positron) decay (optional)

The syllabus for 5.2.3 focuses on β⁻‑decay, but many textbooks also mention β⁺‑decay, where a proton converts into a neutron, emitting a positron (e⁺) and a neutrino. This process is not required for the IGCSE exam and is therefore omitted from the core notes.

Summary

β‑decay removes excess neutrons from an unstable nucleus. A neutron converts into a proton, emitting a high‑speed electron (β‑particle) and an antineutrino. The atomic number increases by 1, the mass number remains unchanged, and the element changes to the next higher \(Z\). This reduces the neutron‑to‑proton ratio, bringing the nucleus closer to the band of stability. β‑particles have moderate ionising and penetrating abilities and are stopped by a few mm of aluminium, plastic or glass. Their decay rates are described by the isotope’s half‑life (see Section 5.2.4).