The average energy absorbed by a molecule interacting with radiation is a function of the ionisation energy of the molecule and is not related to the energy of the particular radiation. The average energy absorbed is approximately twice the ionisation energy and it is referred to as the W-Value for that particular substance. It is measured and reported in units of electron volts. A knowledge of W values permits us to calculate the number of ions produced by a particular radiation source. Dividing the W-value into the value for the energy of the radiation will give us the number of ions produced by the passage of a single particle or photon.
Positive ions produced by radiation are extremely unstable.
In the solid phase free radicals and positive ions may be trapped in the crystal structure. This trapping of species with unpaired electrons means that the solid substance becomes coloured after irradiation. For example glass becomes brown, while a crystal of sodium chloride becomes violet. In gases and in liquids the chemistry which follows the passage of radiation is that of the free radicals produced. In aqueous solutions, for example, the radiation will leave in its track H and OH radicals.