Radiation

Summary: Radiation, as a physical, electrodynamic process, is the propagation of energetic particles or electromagnetic waves within a space or a medium.

There are two distinct types of radiation: ionizing and nonionizing. Many phenomena occur as a result of distinct forms of radiation, including light, heat, and radioactivity. The physical unit used to measure all types of radiation is the joule.

Electromagnetic radiation (EMR) is characterized by both electric and magnetic fields, oscillating perpendicular to each other. All kinds of electromagnetic radiation are based on photons. The electromagnetic spectrum has been divided into certain ranges of frequencies that classify the associated radiation, in order from longer wavelength and lower frequency to shorter wavelength and higher frequency, as radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. The human eye is capable of seeing EMR in only a small range of frequencies, called the visible light spectrum.

Radiation with sufficiently high energy, such as alpha particles, beta particles, gamma rays, and X-rays, can ionize atoms and molecules and, thus, fall into the category of ionizing radiation. Other forms of radiation, such as radio waves, microwaves, and light waves, are not energetic enough and thus are categorized as nonionizing forms of radiation. All forms of radiation, ionizing and nonionizing, can be harmful to organisms and the natural environment. Nonetheless, medicine has found various forms of radiation and radioactive substances to have many useful applications. For instance, the imaging technologies that produce X-ray and magnetic resonance images enable doctors to diagnose conditions and illnesses, and radionuclides can be used not only diagnostically but also therapeutically, to treat disease, as well as in research. Since radiation disturbs cell division, radiotherapy is a way to treat cancer and prevent tumors from growing. In the geological sciences, radiocarbon dating is used to determine the age of organic materials. Tracer atoms help identify the pathways taken by pollutants through the environment.

Nonionizing Radiation

Although by definition nonionizing radiation is relatively low energy, it can modify the rotational, vibrational, or electronic valence configurations of molecules and atoms.

Visible light falls between wavelengths of about 380 and 760 nanometers on the EMR spectrum, with a frequency range of about 405 terahertz (THz) to 790 THz. Its speed, about 186,282 miles (299,792,458 meters) per second in a vacuum, is one of the fundamental constants of nature.

Infrared (IR) radiation is EMR with a longer wavelength than light, between 0.7 and 300 micrometers, and frequencies between approximately 1 and 430 THz. Thermal radiation, which includes infrared radiation as well as some wavelengths of microwaves and visible light, results from the movement of charged particles within matter; the kinetic energy of the particles is converted into electromagnetic energy and radiated from the surface of an object as heat. Corresponding physical laws are Wien’s law (giving the most likely frequency of the emitted radiation) and the Stefan-Boltzmann law (giving the heat intensity).

Microwaves are electromagnetic waves longer than infrared light, with frequencies between 300 megahertz (0.3 gigahertz) and 300 gigahertz, and wavelengths between 1 meter and 1 millimeter.

Radio waves occur naturally, from lightning, and are emitted from bodies in space. Artificially generated radio waves are used for communications systems, including telecommunications and cell phones, broadcasting, radar, satellite communication, and Wi-Fi networks. Nikola Tesla invented a prototype of wireless telephone by tuning a transmitter and a receiver to the same frequency.

Ionizing Radiation

When highly energetic electromagnetic waves or subatomic particles detach electrons from atoms, those atoms become ionized. Alpha (α) particles are emitted during the alpha decay of large nuclei. Consisting of two neutrons and two protons, these particles have a doubly positive charge and a high atomic mass, and they propagate slowly. Alpha particles can be stopped with a sheet of paper. They are unable to penetrate human skin, thus causing no external hazardous effects, but they may be dangerous upon ingestion.

Beta (β) particles are produced by beta decay. When those particles are electrons, the radiation is called beta-minus (β–) radiation and requires a few centimeters of metal to be stopped. Beta-plus (β+) radiation is the emission of positrons. Because these are antimatter particles, they annihilate any matter nearby, releasing gamma photons.

Gamma (γ) photons have a frequency higher than 10¹⁹ hertz. During atomic decay, gamma photons follow alpha particles and beta particles, to release so-called excess energy. Different from alpha and beta particles, photons have neither mass nor electric charge. Gamma radiation penetrates deeply through matter, stopped only by lead or depleted uranium shields.

The smaller the wavelength, the higher the energy, according to the equation E = h × c/λ, where h is Planck's constant, c is the speed of light, and λ is wavelength. Wavelengths shorter than 10 nanometers describe the range of X-rays. X-ray photons are absorbed by atoms, because of energy differences between orbital electrons.

Neutrons, from spontaneous or induced nuclear fission or fusion, are categorized according to their speed. They require hydrogen-rich shielding, such as concrete or water, as used in nuclear reactors, to block them. Neutrons do not ionize atoms, because they have no charge, but they do create unstable isotopes, thus inducing radioactivity in previously nonradioactive material.

In the International System of Units (SI), the physical unit of radioactive decay is the becquerel (Bq): 1 Bq = 1 decay per second. The gray (Gy) is the unit used to measure the absorbed dose of ionizing radiation, while the sievert (Sv) measures the equivalent, or biologically effective, absorbed dose; both 1 Gy and 1 Sv are equal to 1 joule per kilogram (J/kg). Non-SI units include the rad (absorbed radiation dose, 1 rad = 0.01 Gy) and the roentgen equivalent in man/mammal, or rem (1 rem = 0.01 Sv).

Health Concerns

Infrared and ultraviolet radiation may cause burns. Air travel exposes passengers to increased radiation from cosmic rays. For passengers, the recommended exposure from the International Commission on Radiological Protection (ICRP) is no more than 1 mSv per year.

Exposure to radiation causes changes in the chemical composition of gases and liquids, mainly as a result of radiolysis, and leads to the formation of free radicals. Water subjected to ionizing radiation forms free radicals of hydrogen and hydroxyl. This leads to oxidative stress in the cells of living organisms, because they are composed primarily of water. Excessive exposure to ionizing radiation results in tissue damage and radiation poisoning, also called acute radiation syndrome (ARS) or radiation sickness. A chronic radiation syndrome typically appeared in workers in early uranium and radium mines. Potential hazards include the development of cancer, tumors, and DNA damage.

Typical symptoms of radiation poisoning are nausea, bloody vomiting, bloody stools, headache, weakness, high fever, permanent hair loss, skin damage (called cutaneous radiation syndrome), and poor wound healing. Intense irradiation of the whole body causes immunodeficiency resulting from destruction of bone marrow.

To avoid radiation damage, the amount of energy deposited in sensitive material should be reduced by shielding or distance from the source. Some materials can be modified to be less sensitive to radiation damage by adding antioxidants or stabilizers.

Bibliography

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