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Ionizing radiation is radiation with sufficient energy to ionize neutral atoms or compounds. The term is used regarding the effect of electromagnetic radiation (EMR) on human safety and is also used in astronomy and planetary science. Sometimes the term is meant to refer to EMR with photons with sufficient energy (ionizing photons), but other times is also meant to include particles with analogous kinetic energy, such as cosmic rays and products of radioactivity, i.e., alpha particles and beta particles. EMR considered ionizing radiation includes the short-wavelength end of ultraviolet and all of X-rays and gamma rays: the danger of these to human health is well-known.
10 eV is sometimes cited as the low-end limit, e.g., for safety purposes. For astronomy purposes, 10 eV is within the order-of-magnitude of enough to ionize atomic hydrogen from its ground state and would ionize any hydrogen atom above that state. Given the great variety of compounds and the possible states of excitation of atoms and molecules that may require little energy to free an electron, there is no straightforward cut-off regarding how much energy will free an electron. At high photon energy (thousands or millions times the 10 eV minimum), the freed electron has enough energy to trigger many additional ionizations.
In astrophysics, the term is used regarding the stellar radiation producing HII regions, and regarding the epoch of reionization (EOR), including the high-z stars and galaxies that produced the ionizing radiation that carried out the reionization. Early stars (e.g., OB stars) produce significant such radiation, which is a factor in their effect on surrounding clouds, such as is illustrated by the Strömgren sphere model. Active galactic nuclei (AGNs) also produce such energetic photons and affect surrounding interstellar medium (ISM).
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