This page provides a list of general terms that are used to describe aspects of radiation science. Below is the radiation shielding terms and definitions.
Effective half-life
The time required for the amount of a radionuclide deposited in a living organism to be diminished 50 percent as a result of the combined action of radioactive decay and biological elimination.
Electromagnetic radiation
A traveling wave motion resulting from changing electric or magnetic fields. Familiar types of electromagnetic radiation range from x rays (and gamma rays) of short wavelength, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wavelength. Only the higher-energy (higher frequency/shorter wavelength) forms of electromagnetic radiation are ionizing. Radiation in the lower-energy ranges, such as visible, infrared, radar, and radio waves, are nonionizing.
Electron
An elementary particle with a negative charge and a mass 1/1837 that of the proton. Electrons surround the positively charged nucleus of the atom.
Element
One of the known chemical substances that cannot be broken down further without changing its chemical properties. Some examples include hydrogen, nitrogen, gold, lead, and uranium. See the periodic table of elements.
Exposure
A general term used loosely to express what a person receives as a result of being exposed to ionizing radiation.
External radiation
The situation in which the source of exposure is external to, that is, outside the body.
Extremities
The hands, forearms, elbows, feet, knees, legs below the knee, and ankles (permissible radiation exposures in these regions are generally greater than in the whole body because they contain less blood-forming organs and have smaller volumes for energy absorption).
Film badge
Photographic film used for measurement of ionizing radiation exposure for personnel monitoring purposes. The film badge may contain two or three films of differing sensitivities, and it may contain a filter that shields part of the film from certain types of radiation.
Fissile material
Although sometimes used as a synonym for fissionable material, this term has acquired a more restricted meaning. Namely, any material that is fissionable by thermal (slow) neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.
Fission (fissioning)
The splitting of the nucleus of an atom (generally of a heavy element) into at least two other nuclei and the release of a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation.
Fission gases
Those fission products that exist in the gaseous state. In nuclear power reactors, this includes primarily the noble gases such as krypton and xenon.
Fission products
The nuclei (fission fragments) formed by the fission of heavy elements, plus the nuclides formed by the subsequent decay products of the radioactive fission fragments.
Fissionable material
Commonly used as a synonym for fissile material, the meaning of this term has been extended to include material that can be fissioned by fast neutrons, such as uranium-238.
Fusion reaction
A reaction in which at least one heavier, more-stable nucleus is produced by the combination of two lighter, less-stable nuclei. Reactions of this type are responsible for enormous releases of energy, for example, the heat from the sun.
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