Ionizing Radiation - Ionization and The Definition Problem

Ionization and The Definition Problem

The boundary between ionizing and non-ionizing radiation is fuzzy. In general, if the characteristic energy (kinetic energy in the case of massive particles, photon energy in the case of electromagnetic radiation, or mass energy in the case of antimatter) of the radiation particles is greater than the ionization energy of the target material, then each particle collision can be expected to ionize a target atom, no matter how low the power of the beam. This is an appealing bright line between ionizing and non-ionizing radiation, but it is subject to several caveats:

1. Most targets are composed of a variety of atoms which will have a range of ionization energies.
2. Particles can impart glancing blows that will only transfer part of their energy to the target.
3. Neutron capture can produce much higher energies than the neutron's original kinetic energy.
4. Full ionization and ion-pair production may not be necessary to trigger chemical reactions; the activation energy may be more relevant.
5. Radiation intensity may increase the frequency of collisions to the point that atoms may ionize from multiple excitations.

The boundary of greatest interest is for low intensity photon radiation striking organic material. Since the first ionization energy of hydrogen and oxygen are both 14 eV, the spectrum of ionizing radiation is commonly defined to start at approximately 10 eV (equivalent to a far ultraviolet wavelength of 124 nanometers). Some sources use the ionization energy of air to define the boundary at 33.97 eV (36.50 nm). Since the energy of a carbon-carbon bond is 4.9 eV (250 nm), it might be just as reasonable to draw a conservative boundary there. (Although cleavage of such bonds would produce free radicals, not ions.) All of these figures lie partway within the spectrum of ultraviolet light. X-rays and gamma rays are above all of these definitions and are always considered ionizing radiation.

When considering high-intensity long exposure scenarios, as in suntanning, the probability of multiphoton ionization increases. However, ultraviolet light even of low intensity can cause radiation burns similar to those produced by x-ray or gamma radiation. This is the result of UV's ability to produce free radicals and reactive oxygen species in skin, even at photon energies that do not produce ionization or significant heating. Ultraviolet can also cause damage to skin as a result of photoreactions in collagen, which are non-ionizing but cause similar types of single-molecule damage. DNA molecules may be directly or indirectly damaged by UV radiation carrying enough energy to excite certain molecular bonds to form thymine dimers (pyrimidine dimers) (this causes sunburn). The major difference in the effect of ultraviolet and X-rays is that skin is largely opaque to ultraviolet, and therefore protects internal tissues from ultraviolet damage.

Even microwave radiation, which has a photon energy well below that of visible light and is usually considered non-ionizing, can be considered ionizing if it is intense enough.

Truly non-ionizing radiation can still heat materials, cause ordinary burns, and even raise materials to their ionization temperature. Such heating does not produce free radicals until higher temperatures (for example, flame temperatures or "browning" temperatures, and above) are attained. In contrast, ionizing radiation produces free radicals, such as reactive oxygen species, even at room temperatures and below. Free radical production is a primary basis for the particular danger to biological systems of relatively small amounts of ionizing radiation that are far smaller than needed to produce significant heating. Free radicals easily damage DNA, and ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.

Free neutrons are able to cause many nuclear reactions in a variety of substances no matter their energy, because in many substances they give rise to high-energy nuclear reactions, and these (or their products) liberate enough energy to cause ionization. For this reason, free neutrons are normally considered effectively ionizing radiation, at any energy (see neutron radiation). Examples of other ionizing particles are alpha particles, beta particles, and cosmic rays. These charged particles are produced by nuclear forces, so their kinetic energy inevitably exceeds the ionization threshold of 10 or 33 eV, and commonly exceed thousands or even millions of eV of energy.