• Radioactive decay is the process through which an unstable atomic nucleus loses energy by emitting radiation.
• In 1896 Henri Becquerel discovered radioactive decay. Works by Rutherford, Paul Villard, Pierre and Marie Curie , showed that there are three kinds of radiation through which nucleus releases energy : alpha , beta and gamma rays.
• Radioactive decay is also known as Radioactive transformation, because after decay,  the leftover atom is both chemically and physically different from the original atom .
• In radioactive decay, the alpha and beta particles are released from the nucleus.

Properties of radiation emitted:

• Penetrating power:
  • Alpha particles : It has the lowest penetrating power. It can be stopped by a few centimetres of air or just a sheet of paper.
  • Beta particles : Beta particles will be stopped after some metres of air or a few centimetres thick aluminium/copper sheet .
  • Gamma rays : Gamma rays have the highest penetrating power amongst the three. It can pass through a few centimetres of lead or many metres of concrete before being stopped.
• Ionising power : radiations are energetic enough to pull electrons away from atoms as they pass through matter. The atoms that have had electrons removed in this way are now charged particles, or ions.
  • Alpha particles : alpha particles interact with matter the most, and hence have the highest ionising power.
  • Beta particles : have ionisation power lower than alpha particles.
  • Gamma rays : gamma rays have the lowest ionisation power as they barely interact with matter.
• Deflection by electric and magnetic field :
  • Alpha particles : alpha particles are deflected by electric and magnetic field in the same way as positively charged particles. Hence we can conclude that alpha particles are positively charged.
  • Beta particles : beta particles are deflected by electric and magnetic field in the same way as negatively charged particles. Hence we can conclude that beta particles are negatively charged.
  • Gamma rays : gamma rays are not deflected by magnetic field. It is electrically neutral.
• Nature of radiation :
  • Alpha particles : it is equivalent to a Helium nucleus .
  • Beta particles: It is equivalent to an electron. But it originates from the nucleus of an atom.
  • Gamma rays : Gamma rays are high energy photons.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)