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Gamma ray: a penetrating, short-wavelength electromagnetic radiation emitted during the radioactive decay of many radioactive materials. Except for their origin (the nucleus of the atom, rather than the outer electron shells) and higher energy, their characteristics are similar to those of x-rays. Gamma radiation is the most penetrating of the common kinds of ionizing radiation and is of concern as an external and internal radiation hazard.
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Half-life: the time in which half a collection of atoms of a particular radioactive substance disintegrate into atoms of another element. Half-lives range from millionths of a second to billions of years.
High-level radioactive waste (HLW): (1) Irradiated (spent) reactor fuel. (2) Liquid waste resulting from the operation of the first-cycle solvent-extraction system, and the concentrated wastes from later extraction cycles, in a facility for reprocessing irradiated reactor fuel. (3) Solids into which such liquid wastes have been converted. HLW is primarily in the form of spent fuel discharged from commercial nuclear power reactors. It also includes some reprocessed material from defense activities and a small quantity of reprocessed commercial HLW (10 CFR Part 60) (USNRC, 1999b).
Isotopes: nuclides that have the same number of protons in their nuclei, and hence the same atomic number, but that differ in the numbers of neutrons, and therefore in the mass number; chemical properties of isotopes of a particular element are almost identical.
Low-level waste (LLW): includes wastes that are not high-level wastes (HLW), transuranic waste (TRU), naturally occurring radioactive materials (NORM), or source-material waste defined under the Atomic Energy Act Section 11.(e) (2). LLRW includes waste that come principally from nuclear power generation; hospitals and medical, educational, and research institutions; industries; private or government laboratories; and other nuclear fuel-cycle facilities (such as, fuel fabrication plants).
Mixed low-level radioactive and hazardous waste (mixed LLW): is defined as waste that satisfies the definition of low-level radioactive waste (LLRW) in the Low-Level Radioactive Waste Policy Amendments Act of 1985 (LLRWPAA) and contains hazardous waste that either (1) is listed as a hazardous waste in Subpart D of 40 CFR Part 261 or (2) causes the LLRW to exhibit any of the hazardous-waste characteristics identified in Subpart C of 40 CFR Part 261.
Positron emission tomography (PET): a non-invasive, diagnostic imaging technique for measuring the metabolic activity of cells in the human body. It is useful clinically in patients with conditions affecting the brain and the heart and in patients with particular types of cancer.
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