|Ionization is the ejection of one or more electrons from
an atom or molecule to produce a fragment with a net positive
charge (positive ion). The classification of radiation as
"ionizing" is essentially a statement that it has enough quantum energy to eject an electron. This is a crucial distinction, since "ionizing radiation" can produce a number of physiological effects, such as those associated with risk of mutation or cancer, which non-ionizing radiation cannot directly produce at any intensity. ||
Although the precise ionization energy differs with the atom or molecule involved, a general statement is any radiation with quantum energy above a few electron volts is considered to be ionizing radiation. The threshold for ionization lies somewhere in the ultraviolet region of the electromagnetic spectrum, so all x-rays and gamma-rays are ionizing radiation. All forms of nuclear radiation are also ionizing radiation because of their extremely high energies.