Gamma ray

After emitting an alpha or beta particle, the nucleus will often still be ‘excited’ and will need to lose energy.

It does this by emitting a high energy electromagnetic wave called a gamma ray.

Gamma radiation does not consist of particles but as short wavelength, high energy electromagnetic radiation emitted from unstable nuclei.

It is normally emitted alongside alpha or beta radiation.

Gamma ray emission causes no change in the number of particles in the nucleus meaning both the atomic number and mass number remain the same.

It can be written as γ or _{0}^{0}\textrm{γ}

Gamma decay

_{X}^{A}\textrm{X}\rightarrow_{Z}^{A}\textrm{Y}+_{0}^{0}\textrm{γ} (or γ).


Beta and gamma decay of cobalt-60


Properties of nuclear radiations

The different types of radiation are often compared in terms of their penetrating power, their ionising power and how far they can travel in the air.

SymbolPenetrating powerIonising powerRange in air
AlphaαSkin/paperHigh< 5 centimetre (cm)
Betaβ3 mm aluminium foilLow≈ 1 metre (m)
GammaγLead/concreteVery low> 1 kilometre (km)
Alpha, beta and gamma rays passing through a hand, beta and gamma rays passing through aluminium and gamma rays stopping at lead.

All types of radioactive decay can be detected by a Geiger-Muller tube, or G-M tube. The radiations ionise the gas inside the G-M tube and the resulting charged particles move across the chamber and get counted as charges rather like an ammeter.

Key points - alpha radiation

  • alpha particles are helium nuclei consisting of two protons and two neutrons emitted from unstable nuclei;
  • alpha radiation is stopped by a few centimetres of air or a thin sheet of paper;
  • alpha decay: _{Z}^{A}\textrm{X}\rightarrow_{Z-2}^{A-4}\textrm{Y}+_{2}^{4}\textrm{He} (or _{2}^{4}\textrm{α})
  • Alpha particles are relatively heavy and so produce the most ionisation.

Key points: beta radiation

  • beta particles are fast moving electrons emitted from the nucleus of an unstable atom;
  • beta radiation is stopped by several metres of air or a thin sheet of aluminium;
  • beta decay: _{X}^{A}\textrm{X}\rightarrow_{Z+1}^{A}\textrm{Y}+_{-1}^{0}\textrm{e} or ( _{-1}^{~0}\beta)
  • beta particles are much lighter than alpha particles and so produce a great deal less ionisation.

Key points: gamma radiation

  • gamma radiation is high energy electromagnetic waves emitted from unstable nuclei;
  • gamma radiation easily passes through air, paper, skin and aluminium but can be partly blocked by thick lead or concrete;
  • leaves mass number and atomic number unchanged;
  • gamma rays produce the least ionization.