describe the composition, mass and charge of α-, β- and γ-radiations (both β– (electrons) and β+ (positrons) are included)

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Physics 9702 – Atoms, Nuclei and Radiation

Atoms, Nuclei and Radiation

Objective

Describe the composition, mass and charge of the three main types of nuclear radiation:

\$\alpha\$‑radiation, \$\beta^{-}\$‑radiation, \$\beta^{+}\$‑radiation and \$\gamma\$‑radiation.

Key Concepts

  • Radioactive decay involves the transformation of an unstable nucleus into a more stable configuration.

  • During decay, particles or electromagnetic radiation are emitted. These emissions are called radiations.

  • The three principal radiations differ in their constituent particles, mass, charge and penetrating ability.

Composition, Mass and Charge

RadiationCompositionMass (relative to \$u\$)Charge (in units of \$e\$)Typical Penetration
\$\alpha\$Helium nucleus: 2 protons + 2 neutrons≈ 4 u (≈ 6.64 × 10⁻²⁷ kg)+2 \$e\$Stopped by a sheet of paper or a few cm of air
\$\beta^{-}\$High‑energy electron emitted from a neutron → proton conversion≈ 5.5 × 10⁻⁴ u (≈ 9.11 × 10⁻³¹ kg)–1 \$e\$Penetrates several mm of aluminium
\$\beta^{+}\$Positron (antielectron) emitted from a proton → neutron conversion≈ 5.5 × 10⁻⁴ u (≈ 9.11 × 10⁻³¹ kg)+1 \$e\$Similar to \$\beta^{-}\$; stopped by a few mm of aluminium
\$\gamma\$High‑energy photon (electromagnetic wave)0 (no rest mass)0 (electrically neutral)Highly penetrating; requires several cm of lead or metres of concrete

Detailed Description

Alpha (\$\alpha\$) Radiation

An \$\alpha\$ particle is essentially a \$^{4}_{2}\text{He}\$ nucleus. It carries a +2 \$e\$ charge and has a relatively large mass compared with other nuclear radiations. Because of its mass and charge, it interacts strongly with matter, losing energy quickly and therefore having low penetrating power.

Beta Minus (\$\beta^{-}\$) Radiation

In \$\beta^{-}\$ decay a neutron in the nucleus transforms into a proton, emitting an electron and an antineutrino:

\$ n \rightarrow p^{+} + e^{-} + \bar{\nu}_{e} \$

The emitted electron has a very small mass and a charge of –1 \$e\$. Its speed can approach a significant fraction of the speed of light, giving it moderate penetrating ability.

Beta Plus (\$\beta^{+}\$) Radiation

In \$\beta^{+}\$ decay a proton converts into a neutron, emitting a positron and a neutrino:

\$ p^{+} \rightarrow n + e^{+} + \nu_{e} \$

The positron is the antimatter counterpart of the electron, possessing the same mass but a charge of +1 \$e\$. It behaves similarly to \$\beta^{-}\$ particles in terms of penetration, but it can annihilate with electrons, producing \$\gamma\$ photons.

Gamma (\$\gamma\$) Radiation

\$\gamma\$ radiation consists of high‑energy photons emitted from an excited nucleus as it drops to a lower energy state. Because photons have no rest mass and no charge, they interact only via electromagnetic processes, giving them the greatest penetrating power among the four types of radiation.

Suggested diagram: Comparative illustration of \$\alpha\$, \$\beta^{-}\$, \$\beta^{+}\$ and \$\gamma\$ emissions from a nucleus, showing relative sizes, charges and typical shielding materials.

Summary Table (Compact Form)

RadiationParticleMass (kg)Charge (\$e\$)Penetration
\$\alpha\$He nucleus (2p + 2n)6.64 × 10⁻²⁷+2Paper, few cm air
\$\beta^{-}\$Electron9.11 × 10⁻³¹–1Few mm Al
\$\beta^{+}\$Positron9.11 × 10⁻³¹+1Few mm Al
\$\gamma\$Photon00Lead cm, concrete m

Common Misconceptions

  1. All radiation is “dangerous”. In fact, \$\alpha\$ particles cannot penetrate skin and are only hazardous if ingested or inhaled.

  2. Beta particles are “electrons”. While \$\beta^{-}\$ are electrons, \$\beta^{+}\$ are positrons, the antimatter counterpart.

  3. Gamma rays are “particles”. They are photons, i.e., packets of electromagnetic energy, not matter particles.

Key Equations

Mass–energy equivalence (relevant for decay energy calculations):

\$ E = mc^{2} \$

Charge conservation in decay processes:

\$ \sum Q{\text{initial}} = \sum Q{\text{final}} \$