||Radiation (Latin radiare, â€˜to emit beamsâ€™) is the emission of waves or particles. The three types of radiation are alpha ray particles, beta ray particles and gamma rays emitted from the nuclei of radioactive elements.
Alpha rays consist of a stream of helium nuclei, that each contain two neutrons and two protons. The penetrating power of alpha ray is small and it can be stopped by thin sheets of paper.
Beta rays consist of emitted electrons or positrons (which are identical to the electron but with a positive charge). The penetrating power of beta particles is greater than alpha particles, and it may need several millimetres of aluminium to stop such particles. Both beta and alpha rays can be deflected by a magnetic field.
Gamma rays on the other hand are electromagnetic waves (photons), and can only be stopped using many centimetres of lead. Since gamma rays have no charge they are not deflected by a magnetic field.
The time for half the atoms initially present in a sample to decay is known as the half life. Decay can result in a series of new elements being produced which may themselves in turn decay. Half lifes can last from seconds to years (protactinium has a half life of 72 seconds, while carbon has a half life of 5,730 years). It is carbon\'s half life that is used to date organic material, using the process of carbon dating. AA
Further reading Harald Enge, Introduction to Nuclear Physics.