Part IV

Glossary

Terms in italics appear in the Glossary as separate entries.

Alpha radiation

See Ionizing radiation.

Association

In statistics, the correlation or relationship between one factor and one or more other pertinent factors as demonstrated by experimental data.

Atomic bomb

An explosive device in which a large amount of energy is released through the nuclear fission of uranium or plutonium. The first atomic bomb test, known as the Trinity Shot, took place in the desert north of Alamogordo, New Mexico, on July 16, 1945. Several weeks later, an atomic bomb was used for the first time as an instrument of war, detonating over the Japanese cities of Hiroshima (August 6) and Nagasaki (August 9).

Atomic pile

See Nuclear reactor.

Becquerel

See Units of radioactivity.

Beta radiation

See Ionizing radiation.

Biodistribution

The pattern and process of a chemical substance's distribution through the body.

Biological dosimeter

See Dosimeter.

Biopsy

The removal and/or examination of tissues, cells, or fluids from a living body for the purposes of diagnosis or experimental tests.

Biophysics

The application of physical principles and methods to the study of the structures of living organisms and the mechanics of life processes.

Body burden

The amount of a radioactive material present in a body over a long time period. It is calculated by considering the amount of material initially present and the reduction in that amount due to elimination and radioactive decay. It is commonly used in reference to radionuclides having a long biological half-life. A body burden that subjects the body's most sensitive organs to the highest dose of a particular radionuclide that regulators allow is known as a maximum permissible body burden (MPBB).

Bone marrow infusion

The injection of bone marrow (an essential tissue producing red and white blood cells and platelets) into the body, used primarily to replace bone marrow destroyed by disease or in the course of radiation and other therapies for certain types of cancer.

Carcinogen

A material that can initiate or promote the development of cancer. Well-known carcinogens include saccharine, nitrosamines found in cured meat, certain pesticides, and ionizing radiation.

Chain reaction

The process by which the fission of a nucleus releases neutrons, causing other nuclei to undergo fission in turn. Both the atomic bomb and the nuclear reactor use a chain reaction to generate energy.

Clinical trial

A research study involving human subjects, designed to evaluate the safety and effectiveness of new therapeutic and diagnostic treatments.

Common Rule

The 1991 federal regulation that provides the basic procedures and principles that are to be followed in the conduct of human subject research sponsored by federal agencies.

Critical mass

The amount of fissionable material (uranium 235 or plutonium 239) sufficient to sustain a nuclear chain reaction.

Curie

See Units of radioactivity.

Cyclotron

A device that uses alternating electric fields to accelerate subatomic particles (a particle smaller than an atom, such as an alpha particle or a proton). When these particles strike ordinary nuclei, radioisotopes are formed. For his work in developing the cyclotron in the early 1930s, Ernest Lawrence of the University of California received the 1939 Nobel Prize in Physics.

Deterministic effect

An effect, such as kidney damage, whose severity increases with increasing dose of radiation or other agent.

Diagnostic procedure

A method used to identify a disease in a living person.

Dosage

The prescribed amount of medicine or other therapeutic agent administered to treat a given illness.

Dose

In radiology, a measure of energy absorbed in the body from ionizing radiation, measured in rad.

Dose reconstruction

The process of using information about an individual's past exposures to ionizing radiation as well as general knowledge about the behavior of radioactive materials in the human body and in the environment to estimate the dose of radiation that someone has received.

Dosimeter

An instrument that measures the dose of ionizing radiation. A biological dosimeter is a biological or biochemical indicator of the effects of exposure, such as a change in blood chemistry or in blood count. A highly accurate biological dosimeter has yet to be found.

Dosimetry

The measurement and calculation of radiation doses.

Endocrinology

The study of the body's hormone-producing glands, such as the thyroid, pituitary, and adrenal glands, and the functions of the hormones they synthesize and secrete.

Epidemiology

The study of the determinants (risk factors) and distribution of disease among populations.

Fallout

Radioactive debris that falls to earth after a nuclear explosion.

Fission

The division of an atomic nucleus into parts of comparable mass. Generally speaking, fission may occur only in heavier nuclei, such as isotopes of uranium and plutonium. Atomic bombs derive energy from the fission of uranium or plutonium.

Fission product

An atom or nucleus that results from the fission of a larger nucleus.

Fusion

The combining of two light atomic nuclei to form a single heavier nucleus, releasing energy. Hydrogen bombs derive a large portion of their energy from the fusion of hydrogen isotopes.

Gamma radiation

See Ionizing radiation.

Genetic effects

Changes in a person's germ calls (sperm or ova) that are transmissible to future generations. Such changes result from mutations in genes within the germ cells.

Gray

see Units of radioactivity.

Half-life

The average time required for one-half of the amount of radioactivity of a radionuclide to undergo radioactive decay. For material with a half-life of one week, half of the original amount of activity will remain after one week; half of that (one-quarter of the original amount) will remain after two weeks; and so on.

Health physics

A branch of physics specializing in accurate measurement of agents, such as ionizing radiation, which can have effects on human health.

Hydrogen bomb (also known as a thermonuclear weapon)

An explosive weapon that uses nuclear fusion to release energy stored in the nuclei of hydrogen isotopes. The high temperatures essential to fusion are attained by detonating an atomic bomb placed at the H-bomb's structural center. The United States tested the first hydrogen bomb in 1954 at the Pacific Test Site.

Institutional review board (IRB)

Under the Common Rule, a local review board convened by any institution conducting federally sponsored human subject research, vested with the responsibility to review research proposals to ensure compliance with federal research regulations.

Internal emitter

A radioisotope incorporated into a tissue in the body that decays in place and continuously exposes that tissue to ionizing radiation.

Ionization

The process by which a neutral atom or molecule loses or gains electrons, thereby acquiring a net electrical charge. When charged, it is known as an ion.

Ionizing radiation

Any of the various forms of radiant energy that causes ionization when it interacts with matter. The most common types are alpha radiation, made up of helium nuclei; beta radiation, made up of electrons; and gamma and x radiation, consisting of high-energy particles of light (photons).

Irradiation

Exposure to radiation of any kind, especially ionizing radiation.

Isotope

A species of nucleus with a fixed number of protons and neutrons. The term isotope is usually used to distinguish nuclear species of the same chemical element (i.e., those having the same number of protons, but different numbers of neutrons), such as iodine 127 and iodine 131.

Latency period

The time between when an exposure occurs and when its effects are detectable as an injury or illness.

Maximum Permissible Body Burden (MPBB)

see Body burden

Metabolism

The manner in which a substance is acted upon (taken up, converted to other substances, and excreted) by various organs of the body.

Natural background radiation

Ionizing radiation that occurs naturally. Its principal sources are cosmic rays from outer space, radionuclides in the human body, and radon gas (a decay product of natural uranium in the earth's crust).

Nuclear medicine

A branch of medicine specializing in the use of radionuclides for diagnostic and therapeutic purposes.

Nuclear reactor

A device containing fissionable material in sufficient quantity and suitable arrangement to maintain a controlled, self-sustaining nuclear chain reaction.

Nuclide

A type of nucleus with a fixed number of protons and neutrons. The term nuclide is usually used to distinguish nuclear species of different chemical elements (i.e., those having different numbers of protons and neutrons), such as iodine 127 and uranium 235.

Partial-Body Irradiation (PBI)

Exposure of part of the body to external radiation.

Permissible dose

In the judgment of a regulatory or advisory body, such as the National Committee on Radiation Protection, the amount of radiation that may be received by an individual within a specified period.

Principal investigator

The scientist or scholar with primary responsibility for the design and conduct of a research project.

Protocol

The formal design or plan of an experiment or research activity; specifically, the plan submitted to an institutional review board for review and to a government agency for research support. Protocols include a description of the research design or methodology to be employed, the eligibility requirements for prospective subjects and controls, the treatment regimen(s), and the methods of analysis to be performed on the collected data.

Rad

See Units of radiation

Radiation

The emission of waves transmitting energy through space or a material medium, such as water. Light, radio waves, and x rays are all forms of radiation.

Radiation biology

See radiobiology.

Radiation oncology

A branch of medicine specializing in the treatment of cancer with radiation. Radiation therapy and radiotherapy are equivalent terms.

Radiation sickness

Acute physical illness caused by exposure to doses of ionizing radiation large enough to cause toxic reactions. This can include symptoms such as nausea, diarrhea, headache, lethargy, and fever.

Radioactive decay

The process by which the nucleus of a radioactive isotope decomposes and releases radioactivity. For example, carbon 14 (a radioisotope of carbon) decays by losing a beta particle, thereby becoming nitrogen 14, which is unstable.

Radioactivity

The decay of unstable nuclei through the emission of ionizing radiation. The resulting nucleus may itself be unstable and undergo radioactive decay. The process stops only when the decay product is stable.

Radiobiology

Branch of biology specializing in the study of the effects of radiation on biological molecules, cells, tissues, and whole organisms, including humans. Radiobiology seeks to discover the molecular changes responsible for radiation effects such as cancer induction, genetic changes, and cell death.

Radiogenic

A term used to identify conditions observed to be caused by exposure to ionizing radiation, such as certain kinds of cancer.

Radioisotope

A radioactive isotope. Radioisotopes are used in medical research as tracers. See also isotope, nuclide, and radionuclide.

Radiological weapons

Weapons that use radioactive materials to cause radiation injury.

Radionuclide

A radioactive nuclide. Often used to distinguish radioisotopes of different chemical elements, such as iodine 131 and uranium 239.

Radiopharmaceuticals

Drugs (compounds or materials) that may be labeled or tagged with a radioisotope. In many cases, these materials function much like materials found in the body and do not produce special pharmacological effects. The principal risk associated with these materials is the consequent exposure of the body or certain tissues to radiation.

Radioresistance

The degree of resistance of organisms or tissues to the harmful effects of ionizing radiation.

Radiosensitivity

The degree of sensitivity of organisms or tissues to the harmful effects of ionizing radiation.

Radiotherapy

See radiation oncology.

Rem

See Units of radiation.

Rep

See Units of radiation.

Roentgen

See Units of radiation.

Tolerance dose

See Permissible dose.

Total-Body Irradiation (TBI)

Exposure of the entire body to external radiation.

Tracer

A distinguishable substance, usually radioactive, administered to determine the distribution and/or metabolism of materials in the body. In 1923, George Hevesy was the first investigator to use an isotope (radioactive thorium) in metabolic studies, exploring lead transport in the bean plant. Metabolic studies proliferated after World War II, when with the development of the cyclotron, radioisotopes of various atoms became more widely available. Isotopes commonly used as tracers today include carbon 14, iodine 131 and phosphorus 32.

Transuranic elements

Radioactive elements with atomic numbers (i.e., the number of protons in the nucleus) greater than 92. Only two of these elements (plutonium in minute amounts and neptunium) occur in nature; the others are produced in minute amounts through the radioactive decay of uranium. The first transuranic elements were discovered as synthetic radioisotopes at the University of California at Berkeley and the Argonne National Laboratory in the 1930s and 1940s.

Units of radiation

The basic unit of radiation exposure is the roentgen, named after Wilhelm Roentgen (discoverer of x rays). It is a measure of ionization in air, technically equal to one ESU (electrostatic unit) per cubic centimeter, due to radiation. A rep (roentgen equivalent physical) is an archaic measure of skin exposure to a dose of beta radiation having an effect equivalent to 1 roentgen of x rays. The basic unit of radiation absorbed by the body is the rad, technically equal to 100 ergs (energy unit) per gram of exposed tissue. One roentgen corresponds to roughly 0.95 rad. The rem (roentgen equivalent in man) is a unit of effective dose, a dose corrected for the varying biological effectiveness of various types of ionizing radiation. The currently accepted unit of radiation is the gray (Gy), the International System unit of absorbed dose, equal to the energy imparted by ionizing radiation to a mass of matter corresponding to one joule per kilogram.

Units of Radioactivity

The becquerel (Bq), named after the physicist Henri Becquerel (the discoverer of radioactivity), is a measure of radioactivity equal to one atomic disintegration per second. The curie (Ci), whose name honors the French scientists Marie and Pierre Curie (the discoverers of radium), is a standard based on the radioactivity of 1 gram of radium. It is equal to 3.7 x 1010 becquerels.

X rays

Invisible, highly penetrating electromagnetic radiation of a much shorter wavelength than visible light, discovered in 1895 by Wilhelm C. Roentgen. Most applications of X rays are based on their ability to pass through matter. They are dangerous in that they can destroy living tissue, causing severe skin burns on human flesh exposed for too long a time. This property is applied in x-ray therapy to destroy diseased cells. See Ionizing radiation.