Ionization is one of the very important phenomena of the production of ions that occurs in certain physical processes. For example, when a high energy particle or radiation passes through a material medium (might be solid, liquid or gaseous), the atoms or molecules in the medium on the radiation-path get struck so that one or some of the electrons get knocked out of the atoms or molecules leaving behind the residual positive ions. The knocked out electrons might in turn get attached to electronegative species of atoms or molecules in the medium, thus giving rise to the appearance of negative ions. Not only the radiation passage, but the heating to high temperatures, chemical reactions as well as electrical processes etc. may also produce ionizations in a given medium.