Radiation - Ionizing Radiation

Ionizing Radiation

Radiation with sufficiently high energy can ionize atoms. Most often, this occurs when an electron is stripped (or "knocked out") from an electron shell, which leaves the atom with a net positive charge. Because cells and more importantly the DNA can be damaged, this ionization can result in an increased chance of cancer, and thus "ionizing radiation" is somewhat artificially separated out of particle and electromagnetic radiation, simply due to its larger potential for biological damage per unit of energy. An individual cell is made of trillions of atoms, only a small fraction of which will be ionized at low radiation powers. The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, as adjusted for the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the organism or tissue being irradiated (effective dose).

Roughly speaking, photons and particles with energies above about 10 electron volts (eV) are ionizing. Alpha particles, beta particles, cosmic rays, gamma rays, and X-ray radiation all carry energy high enough to ionize atoms. In addition, free neutrons are also ionizing, since their interactions with matter are inevitably more energetic than this threshold.

Ionizing radiation comes from radioactive materials, X-ray tubes, particle accelerators, and is present in the environment. It is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases, it may lead to secondary emission of visible light upon interaction with matter, as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue, resulting in skin burns, radiation sickness and death at high doses and cancer, tumors and genetic damage at low doses.

Electromagnetic radiation (sometimes abbreviated EMR) takes the form of self-propagating waves in a vacuum or in matter. EM radiation has an electric and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy propagation. Electromagnetic radiation is classified into types according to the frequency of the wave, these types include (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called visible spectrum or light, is sensed by the eye of various organisms.

Ionizing electromagnetic radiation is that for which the photons making up the radiation have energies larger than about 10 electron volts. The ability of an electromagnetic wave (photons) to ionize an atom or molecule thus depends on its frequency, which determines the energy of a photon of the radiation. An energy of 10 eV is about 1.6×10−18 joules, which is a typical binding energy of an outer electron to an atom or organic molecule. This corresponds with a frequency of 2.4×1015 Hz, and a wavelength of 125 nm (this is in far ultraviolet). Radiation on the short-wavelength end of the electromagnetic spectrum, and above 125 nm, is ionizing. This includes extreme ultraviolet, X-rays, and gamma rays.

Most of the ultraviolet spectrum (which begins above energies of 3.1 eV (400 nm)) is non-ionizing, but is still biologically hazardous due the ability of single photons of this energy to cause electronic excitation in biological molecules, and thus damage them by means of unwanted reactions. An example is formation of pyrimidine dimers in DNA. This property gives the ultraviolet spectrum some of the dangers of ionizing radiation in biological systems, without actual ionization occurring. In contrast visible light and longer-wavelength electromagnetic radiation, such as infrared, microwaves, and radio waves, consists of photons with too little energy to cause damaging molecular excitation, and thus this radiation is far less hazardous per unit of energy.