Ionizing Radiation

Ionizing radiation is any form of electromagnetic or particle radiation that can cause ionization of a substance through which it passes. As the radiation passed through the substance, it detaches one or more electrons from one or more atoms or molecules, leaving the atom or molecule with an excess positive charge. This charged particle is called a positive ion.

To remove an electron from an atom or molecule, the ionizing particles (photons or other) must have a kinetic energy exceeding the binding energy of the target species, typically a few electron volts. (An electron volt is the work it takes one electron to move across a voltage difference of one volt.) Ionizing radiation is commonly produced in nature by cosmic sources and by the breakdown of unstable elements such as uranium.

Some common ionizing charged species are highspeed electrons, positrons, protons, and beta particles (Helium nuclei). Electrons, positrons, and beta particles are emitted by radioactive elements. Photons, the uncharged particles of light, can be emitted by radioactive nuclides but can also be generated by x-ray devices. All the charged species, as well as neutrons, are currently produced at human-made particle accelerators, and lasers now generate photons of sufficient energy to exceed the binding energy of many atoms and molecules.

Most elements formed during the very early expansion of the universe were radioactive in the past, but over time became stable. Some, such as uranium, thorium, radium, and radon are still unstable, and spontaneously emit ionizing radiation. Here on Earth many rocks and minerals emit radon gas, a radioactive gas formed by the decay of radium. Other radioactive elements (³H [Tritium] and ¹⁴C) can be produced by atmospheric interactions with cosmic rays (energetic particles continuously entering Earth’s atmosphere from outer space).

Ionizing radiation is more damaging to human tissue than non-ionizing, thermal-type radiation. The damage is initialized by the ionizing particle when it knocks an electron off an atom or molecule in a living system, leaving an unpaired electron behind. The target atom or molecule is then a free radical, a highly reactive type that can spawn many more free radicals in the body. These induced chemical changes have been shown to cause cancer and genetic damage.

A unit called the rem (roentgen equivalent man) is used to measure the absorbed dose of ionizing radiation. An absorption of 0.5 rem annually was previously considered safe for a humans (exposure of 0.1 to 0.2 rem per year from natural sources is considered normal, about 0.002 rem with each dental x ray, and about 0.05 rem from a chest x ray). In 2005, however, a National Academies report (Biological Effects of Ionizing Radiation VII) stated that “the smallest dose of low-level ionizing radiation has the potential to cause a small increase in health risks to humans.” According to the chair of the committee producing the report, there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial”