Atoms and Radiation
Radioactive substances are substances that constantly give out radiation from their nuclei, no matter what is done to them.
Background radiation comes from various different sources, including natural ones such as rocks and cosmic rays from space, and man-made ones such as the fallout from nuclear accidents and weapons tests.
An alpha particle consists of two protons and two neutrons (the same as a helium nucleus).
A beta particle is an electron from the nucleus.
Gamma radiation is a form of electromagnetic radiation.
Alpha and beta particles are deflected by electric and magnetic fields; gamma radiation is not.
The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to decrease to half of its original number or for the rate of radiation from a sample to decrease to half of its original level.