Artificial sources
- Related Topics:
- light
- luminescence
- X-ray
- photoelectric effect
- gamma ray
- On the Web:
- Massachusetts Institute of Technology - Radiation Chemistry (PDF) (Apr. 08, 2025)
In addition to natural background radiation, people are exposed to radiation from various man-made sources, the largest of which is the application of X rays in medical diagnosis. Although the doses delivered in different types of X-ray examinations vary from a small fraction of a mGy to tens of mGy (Table 7), the average annual dose per capita from medical and dental irradiation in developed countries of the world now approaches in magnitude the dose received from natural background radiation (Table 6). Less significant artificial sources of radiation include radioactive minerals in crushed rock, building materials, and phosphate fertilizers; radiation-emitting components of television sets, smoke detectors, and various other consumer products; radioactive fallout from nuclear weapons (Table 8); and radiation released in nuclear power production (Table 6).
| Worldwide dose commitment from radioactive fallout from nuclear tests prior to 1970* | |||
| source | isotope | half-life | due to bone surfaces (mGy) |
| external radiation | short lived (e.g., iodine-131) | 8 days | 360 |
| longer lived (e.g., cesium-137) | 30 years | 360 | |
| internal radiation | strontium-89 and -90 | 50 days | 1,310 |
| cesium-137 | 28 years | 210 | |
| carbon-14** | 5,730 years | 160 | |
| total | 2,400 | ||
| *North temperate zones; doses calculated for bone surface. **Calculated to year 2000 only. | |||
| Typical doses to exposed tissue received in routine X-ray diagnosis | |
| examination | dose per exposure in milligray (mGy)* |
| X-ray photograph | |
| chest | 0.4-10 |
| abdominal | 10 |
| extremities | 2.5-10 |
| fluoroscopy | 100-200 per minute |
| X-ray movies | 250 per examination |
| CAT scan | 50-100 per examination |
| *Milligray is a unit of absorbed radiation dose; it corresponds to 1/1,000 joule of radiation energy absorbed per kilogram of tissue. | |
Most of the radioactivity produced in nuclear power reactors is safely contained; however, a small percentage escapes as stack gas or liquid effluent and eventually may contaminate the atmosphere and water supply. (There are similar releases from nuclear-fuel reprocessing plants.) Though nuclear plants are basically clean sources of energy, they thus contribute to the worldwide background radiation level. This problem cannot be entirely avoided by using coal instead of nuclear fuel for power production, since many sources of coal contain natural radioactivity (e.g., radium) that is released in stack gases, along with chemical pollutants.
From Table 6 it is evident that the human population is now exposed to about twice as much radiation from all sources combined as it receives from natural sources alone. Hence, it is important to understand the possible consequences, if any, that may result from the additional exposure to radiation.
In comparison with the relatively small amounts of radiation described above, the dose typically administered to a patient in the treatment of cancer is thousands of times larger; i.e., a total dose of 50 Sv or more is usually delivered to a tumour in daily exposures over a period of four to six weeks. To protect the normal tissues of the patient against injury from such a large dose, as well as to protect medical personnel against excessive occupational exposure to stray radiation, precautions are taken to restrict exposure to the tumour itself insofar as possible. Comparable safeguards are utilized to minimize the exposure of workers employed in other activities involving radiation or radioactive material. Similarly, elaborate safety measures are required for disposal of radioactive wastes from nuclear reactors, due in part to the slow rate at which certain fission products decay. A given amount of plutonium-239, for example, still retains about one-half of its radioactivity after 25,000 years, so that reactor wastes containing this long-lived radionuclide must be safely isolated for centuries.
In the event of an atmospheric nuclear bomb explosion, large quantities of radioactivity are released, the dispersal of which depends on the prevailing weather conditions as well as on the height and nature of the blast. Although the level of contamination resulting from such an explosion or from a nuclear-power plant accident is generally highest in the immediate vicinity of the event itself, both radioactive gas and dust may be transported via air or water for many hundreds of kilometres and eventually contaminate the entire globe.
Mechanism of biologic action
As ionizing radiation penetrates living matter, it gives up its energy through random interactions with atoms and molecules in its path, leading to the formation of reactive ions and free radicals. It is the molecular alterations resulting from these ionizations and, in turn, the resultant biochemical changes that give rise to various types of injury. X rays and gamma rays, for example, impart their energy to “planetary” atomic electrons, which are thereby ejected from their orbits. Such an ejection of a planetary electron results in an ion pair consisting of a free electron and the electrically charged atom from which it was ejected. The ejected electron may give rise to a highly reactive free radical, which in turn may diffuse far enough to attack a biologically important target molecule in its vicinity. This so-called indirect action process, through which radiation causes damage via radiation-induced free radicals, may be envisioned as follows:
While the initial steps in the above process occur almost instantaneously, expression of the biologic effect may take years or decades, depending on the type of injury involved. The indirect action of radiation is more important in the biologic effects of low-LET radiations than in those of high-LET radiations (see above The passage of matter rays: Linear energy transfer and track structure), but the latter have a greater capacity to cause injury through direct interaction with biologic targets.
Direct biologic actions, studied in detail between 1927 and 1947, gave rise to a target theory of radiobiology that has provided a quantitative treatment of many of the biologic effects of radiation, particularly in the field of genetics. According to this theory, a tissue or cell undergoing irradiation is likened to a field traversed by machine-gun fire, in which the production of a given effect requires one or more hits by an ionized track on a sensitive target. The probability of obtaining the effect is thus dependent on the probability of obtaining the requisite number of hits on the appropriate target or targets.
The distribution of ionizing atomic interactions along the path of an impinging radiation depends on the energy, mass, and charge of the radiation. The ionizations caused by neutrons, protons, and alpha particles are characteristically clustered more closely together than are those caused by X rays or gamma rays. Thus, because the probability of injury depends on the concentration of molecular damage produced at a critical site, or target, in the cell (e.g., a gene or a chromosome), charged particles generally cause greater injury for a given total dose to the cell than do X rays or gamma rays; i.e., they have a high RBE. At the same time, however, charged particles usually penetrate such a short distance in tissue that they pose relatively little hazard to tissues within the body unless they are emitted by a radionuclide, or radioactive isotope, that has been deposited internally.
Radionuclides and radioactive fallout
Radionuclides emit various ionizing radiations (e.g., electrons, positrons, alpha particles, gamma rays, or even characteristic X rays), the precise types of which depend on the radionuclide in question. Exposure to a radionuclide and its emissions may be external, in which case the penetrating power of the radiation is an important factor in determining the probability of injury. Alpha particles, for example, do not penetrate deeply enough into the skin to cause damage, whereas energetic beta particles or X rays can be hazardous to the skin and deeper tissues.
Accumulation in critical organs
Radionuclides can enter the body by ingestion, inhalation, or injection. Once taken into the body, their radiation effects depend on their anatomic distribution, duration of retention in the body, and rate of radioactive decay, as well as on the energies of their emitted radiations. An internally deposited radioactive element may concentrate in, and thus irradiate, certain organs more than others. Radioiodine, for example, collects in the thyroid gland, whereas radium and strontium accumulate chiefly in the bones. Different radioelements also vary in their rates of removal. Radioiodine, for instance, is normally eliminated from the thyroid rapidly enough so that its concentration is halved within days. Strontium-90, on the other hand, is retained in high concentrations in the skeleton for years.
The term critical organ refers to the part of the body most vulnerable to a given isotope. The critical organ for plutonium, radium, strontium, and many other fission products is bone and the adjacent bone marrow. For iodine, the critical organ is the thyroid gland. Insoluble airborne radioactive dust often settles in the alveoli of the lungs, while small colloidal particles may become deposited in the bone marrow, liver, or spleen. Table 9 gives an abbreviated list of the maximum permissible concentrations (U.S. recommendations) of some radionuclides for humans. (The maximum permissible concentration is the largest amount of a radionuclide that can be accumulated in the body without producing undue risk of injury.)
| Values for the maximum permissible concentration (MPC) of certain radionuclides | |||
| isotope | chemical form | critical organ | mBq in body |
| tritium (hydrogen-3) | water | 7.4(10−3) | |
| carbon-14 | carbon dioxide | 1.5(10−5) | |
| strontium-90* | water-soluble salt | 1.5(10−6) | |
| bone | 1.5(10−7) | ||
| iodine-131 | water-soluble salt | 1.8(10−6) | |
| thyroid | 2.6(10−8) | ||
| cesium-137 | water-soluble salt | 1.1(10−6) | |
| radon-222** | gas | ||
| radium-226*** | water-soluble salt | 7.4(10−8) | |
| bone | 3.7(10−8) | ||
| uranium | water-soluble salt | 7.4(10−8) | |
| kidney | 1.8(10−10) | ||
| plutonium-239 | water-soluble salt | 1.5(10−8) | |
| bone | 1.5(10−9) | ||
| *MPC in drinking water: 3.7(10−9) micro Bq per litre. **MPC in air: 3.7(10−11) micro Bq per litre. ***MPC in drinking water: 3.7(10−10) micro Bq per litre. | |||
Since a radionuclide delivers radiation continuously to the surrounding tissue, the effect of such protracted continuous exposure must be distinguished from that of a single exposure or of periodically repeated exposures. From experiments with divided doses of gamma radiation or X radiation, it has been found that up to about 60 percent of the radiation effect from a single brief exposure is repaired within several hours. The body therefore is able to tolerate a larger total dose when the dose is accumulated slowly or when part of it is absorbed at a later time. There is less recovery with neutron and alpha radiation, however. (Neutrons are generally more effective agents of mutation than are X rays: for a single brief exposure, by a factor 1 to 8; for chronic irradiation, by a factor up to 100.)
Fallout is the deposition of airborne radioactive contaminants on Earth. Radioisotopes are produced naturally in the air by cosmic radiation, and they may enter the air in stack gases from nuclear power plants or be released through industrial accidents or nuclear explosions. After 1954, nuclear bomb tests carried out by several nations produced measurable fallout on the surface of the entire Earth, arousing great concern and controversy with respect to the resultant health effects. While much of the hazard from the detonation of a nuclear weapon is due to blast waves and heat, the radiation dose from fission products can be so intense that only persons remaining in underground shelters for some weeks could hope to survive. Usually the most prominent isotopes in fallout are fission products; however, all materials exposed to nuclear blasts may become radioactive.
The hazards of long-lived radioisotopes
Several of the radioisotopes contained in fallout are especially hazardous because they remain radioactive for relatively long periods. Cesium-137, strontium-90, and plutonium-239 may be the most significant among these. Fallout material can cover external surfaces and foliage and later be washed into the soil, from which plants may absorb strontium-90, along with the chemically similar calcium, and cesium-137 with potassium. Humans take in these radioactive materials chiefly from drinking water and from plant and animal foods, including milk. Many fallout isotopes that reach the sea and inland waterways eventually end up in concentrated form in the bodies of waterborne animals and plants, becoming a source of concern when they are part of the human food chain.
The most easily detectable fallout product in humans and other animals is iodine-131, an isotope that emits beta and gamma rays and is enriched about 100 times in the thyroid gland through selective accumulation. Because of its relatively short half-life (eight days), iodine-131 is probably not the most hazardous fallout isotope; yet, excessive amounts of radiation from this isotope can lead to metabolic disturbances and an increased incidence of thyroid cancer, especially in children.
A mixture of radioactive gases is discharged into the atmosphere in small amounts by nuclear power reactors. Reactors are thus generally placed at sites where atmospheric mixing and transport are such that the short-lived gases decay and are diluted before they can be inhaled in appreciable amounts by human populations.
Methods that have been developed for biologic protection against fallout range from measures designed to keep radioisotopes out of the body to biochemical means for rapidly eliminating such isotopes from tissues. At times of nuclear emergencies, airborne radioactive particles may be kept from the lungs by staying indoors or by wearing masks with suitable filtration. Absorption of ingested isotopes via the intestinal tract may be inhibited by certain mucoprotein substances that possess great surface affinity for adsorption of strontium and other substances; sodium alginate prepared from seaweed kelp is such a substance. It is possible with appropriate chemicals to remove virtually all radioactive strontium from cow’s milk without affecting its essential nutritive components. Certain chelates—for example, EDTA (ethylenediaminetetraacetic acid)—will react with strontium and “cover” this atom. As a result, the presence of EDTA in the blood reduces the deposition of strontium in bones (elimination of already deposited isotopes also is somewhat accelerated). Unfortunately, however, EDTA and most other chelating agents are not specific for strontium; they also chelate the closely related and important element calcium. Consequently, their use requires expert medical supervision and is limited in effectiveness. On the other hand, the uptake of radioactive iodine by the thyroid gland may be reduced by the ingestion of large amounts of stable iodine, which is relatively nontoxic except to those with special sensitivity.
