Thursday 11 March 2004
Definition : Radiation in physics is a process of emission of energy or particles. Various forms of radiation may be distinguished, depending on the type of the emitted energy/matter, the type of the emission source, properties and purposes of the emission, etc.
Radiation is energy distributed across the electromagnetic spectrum as waves (long wavelengths, low frequency) or particles (short wavelengths, high frequency).
Approximately 80% of radiation is derived from natural sources, including cosmic radiation, ultraviolet light, and natural radioisotopes, especially radon gas.
The remaining 20% is derived from manufactured sources that include instruments used in medicine and dentistry, consumer products that emit radio waves or microwaves, and nuclear power plants. The potentially catastrophic effects of radiation are most vividly illustrated by the effects of nuclear explosions.
The atomic bombs dropped on Hiroshima and Nagasaki in 1945 not only caused acute injury and death but also increased incidence of various cancers among the survivors. Numerous historical incidents document the deleterious effects of therapeutic radiation.
For example, early in the 20th century, American radiologists experienced an increased incidence of aplastic anemia and neoplasms of the skin, brain, and hematopoietic system.
Children who were treated with radiation for an enlarged thymus or benign skin lesions between 1910 and 1959 suffered from an increased incidence of thyroid abnormalities, thyroid tumors, and leukemias and lymphomas.
Exposure of the fetus to radiation can produce mental retardation, congenital anomalies, leukemia, and solid tumors. Investigation of these deliberate or accidental exposures to radiation led to an understanding of the relationship between the dose and timing of radiation and the acute and chronic health effects.
However, in general, these historical exposures were higher than radiation currently received by the general population from natural and manufactured sources, by patients undergoing diagnostic procedures such as mammography or chest radiography, and by nuclear power plant workers.
Unfortunately, fear of widespread radiation exposure following a terrorist attack reinforces the importance of understanding the mechanisms and clinical manifestations of radiation injury.
Despite our understanding of the health effects of high doses of radiation, the potential adverse effects of low doses are controversial. Furthermore, accidents at nuclear power plants in Windscale, England, in 1957, at Three Mile Island in Pennsylvania in 1979, and at Chernobyl in the former Soviet Union in 1986 perpetuate public anxiety about excess cancers associated with the medical, commercial, and military uses of radioactivity.
Electromagnetic radiation characterized by long wavelengths and low frequencies is described as nonionizing radiation. Electric power, radio waves and microwaves, infrared, and ultraviolet light are examples of nonionizing radiation. They produce vibration and rotation of atoms in biologic molecules.
Radiation energy of short wavelengths and high frequency can ionize biologic target molecules and eject electrons. X-rays, gamma rays, and cosmic rays are forms of ionizing radiation. Ionizing radiation can be in the form of electromagnetic waves, such as x-rays produced by a roentgen tube or gamma rays emitted from natural sources, or particles that are released by natural decay of radioisotopes or by artificial acceleration of subatomic particles.
Particulate radiation is classified by the type of particles emitted: alpha particles, beta particles or electrons, protons, neutrons, mesons, or deuterons. The energy of these particles is measured in million electron volts (MeV). Radioisotopes decay by emission of alpha or beta particles or by capture of electrons.
In the case of radon gas, unstable daughter nuclei are produced that subsequently disintegrate, releasing alpha particles. Alpha particles consist of two neutrons and two protons; they have strong ionizing power but low penetration because of their large size. In contrast, beta particles are electrons emitted from the nucleus of an atom; these have weaker ionizing power but higher penetration than alpha particles.
The decay of radioisotopes is expressed by the curie (Ci), 3.7 × 1010 disintegrations per second, or the becquerel (Bq), 1 disintegration per second. The rate of decay of radioisotopes is usually expressed as the half-life (t1/2) and ranges from a few seconds to centuries. Internal deposition of radioisotopes with long half-lives is especially dangerous because it results in continuous release of radioactive particles and gamma rays.
For example, radium was used to paint watch dials and treat cancer in the first half of the 20th century; its long half-life of 1638 years and ability to be concentrated in the skeleton result in delayed appearance of bone tumors.
Mothersill C, Seymour CB. Radiation-induced bystander effects—implications for cancer. Nat Rev Cancer. 2004 Feb;4(2):158-64. PMID: 14964312