 |
 |
| NRIChEd | |
| Glossary | ||
|
|
|||||||||||||
|
absorbed dose: amount of radiation energy absorbed. It can be measured in rads (1 rad = 100 ergs of energy absorbed per gram of absorbing material) or grays (1 gray = 1 Gy = 1 Joule of energy absorbed per kilogram of absorbing material).
action level: 4 pCi/L, the concentration of 222-radon in indoor air at or above which the EPA recommends applying measures to reduce the 222-radon concentration.
activity: rate of decay, often expressed in units of disintegrations per second (dps).
alpha decay: type of radioactive decay in which an alpha particle, consisting of two protons and two neutrons, is emitted from the decaying nucleus. The product nucleus has a mass number four less than the original nucleus, and an atomic number two less.
alpha emitter: a nucleus that decays by emitting an alpha particle (two protons and two neutrons). Although alpha decay is observed for some rare earth elements, it occurs frequently only for elements heavier than 209 Bi.
alpha track detector: a passive method for measuring indoor airborne alpha activity, usually from radon. The underlying principles of the measurement are: (1) damage tracks in a plastic material are induced by the passage of alpha particles through the material, (2) damage tracks can be subsequently enlarged by exposure to potassium hydroxide solution, (3) enlarged tracts can be counted with a microscope, and (4) density (per unit area) of tracks is proportional to the alpha activity.
annihilation radiation: given off following emission of a positron from a nucleus. A positron slows to an energy comparable to that of surrounding electrons. The positron and a negative electron interact in such a way that both electron masses are converted to energy. Usually this energy is emitted in the form of two 0.511-MeV gamma rays emitted at 180† to each other.
antineutrino, see neutrino
artificial radiation, see manmade radiation
Auger electron: secondary electron emitted following electron capture decay. Electron capture results in an inner-shell vacancy. This vacancy is filled by an electron falling from a higher orbital shell. The energy given off in the process can be emitted in the form of an X ray or it can cause the ejection of an electron in a higher shell.
barn: unit used for cross section, where 1 barn = 10-24 cm2. The unit is based on the average cross-sectional area of a nucleus, which is about one barn. Nuclear reaction cross sections are normally in the range of 0.001-1 barn, but may be as low as 10-8 barn or as high as 104 barn.
becquerel: Si unit of radioactivity, equal to 1 disintegration per second (dps) and abbreviated Bq.
beta decay: any decay process that results in a change in Z (atomic number) of the nucleus with no change in A (mass number). The three main processes under this heading are negatron decay (b-), positron decay (b+), and electron capture decay (EC).
chain reaction: in the fission process, the condition in which, on the average, at least one neutron created in a fission process causes another fission.
charcoal canister detector: a container of activated charcoal which is exposed for several days to air which may contain radon. Radon gas accumulates on the charcoal and decays, leaving the longer-lived, gamma-emitting daughter nuclei. The count rate of gamma rays given off from the previously exposed canister is used to determine the radon level in the air to which the canister was exposed.
chart of the nuclides: a chart containing spaces for all the isotopes of the elements. Neutron number increases horizontally from left to right, and proton number increases upward vertically. Each horizontal row represents isotopes of an element, each vertical row represents isotones for a given neutron number, and isobars are found along diagonal lines.
Compton effect: interaction of a gamma ray with an atomic electron in such a way that it loses only part of its energy. In this process, an electron is scattered away from the atom and gains the energy lost by the gamma ray. The gamma ray, now with a lower energy, is deflected from its original path.
control rods: rods composed of a neutron-absorbing material that are placed above the nuclear reactor core and can be lowered into the core to regulate power fluctuations.
cosmic radiation: mainly high-energy protons that originate in outer space. When these particles encounter the earth's atmosphere, they induce nuclear reactions that produce other subatomic particles, such as electrons, muons, neutrinos, neutrons, and photons.
critical mass: minimum mass of 235-uranium of a certain isotopic purity that is required to produce a sustained chain reaction.
cross section: probability that a given nuclear reaction will occur. Expressed in units of barns, where 1 barn = 10-24 cm2.
curie: an older radioactivity unit still widely used. It is defined to be exactly 3.7 X 1010 disintegration per second (dps). The original definition was related to the activity of 1 gram of 226Ra, which has approximately the value in the definition of the curie.
cyclotron: a non-linear particle accelerator in which particles move in a spiral constrained to remain in a plane by magnets. The particles are accelerated by a potential difference imposed at the gaps between two semicircular, flat evacuated metallic cylinders called dees.
daughter nuclei: nuclei, stable or radioactive, which are the product of radioactive decay.
dose equivalent: absorbed dose multiplied by the quality factor; also called effective dose. It is expressed in units of rem, and is used by health physicists in discussing radiation effects.
E-PERM; passive integrating detector system for the measurement of 222-radon concentrations in air, consisting of an electret (charged Teflon disk) in an open-faced ionization chamber, and a voltage reader. When radon gas diffuses into the chamber containing the electret, the alpha particles emitted from the decay of radon ionize the air molecules; the ions are attracted to the charged surface of the electret and discharge it. The electret charge is measured before and after the exposure, and the rate of change of the charge is proportional to the concentration of radon in the area.
effective dose equivalent: dose equivalent, weighted for the differing susceptibilities of specific types of tissue to radiation.
electron capture decay: a beta decay mode in which an orbital atomic electron is captured by the nucleus. X rays or Auger electrons and a neutrino are subsequently given off. The daughter nucleus has a Z one unit less than that of the parent, and is the same daughter that would be produced by positron decay.
electron volt: the energy necessary to raise one electron through a potential difference of one volt, abbreviated eV. One eV is equivalent to 1.6 X 10-12 erg. In units of eV, one atomic mass unit (amu) is 931.5 MeV (1 MeV = 106 eV) and the electron mass is 0.511 MeV.
EOB: end of bombardment. A reference time used for calculating radiochemical yields for synthesis of radiolabeled compounds. The maximum amount of radioactive label is available at this time, when production of the label ceases. Reference to bombardment is used since most radiolabels are produced by bombardment with accelerated particles.
enriched uranium: uranium which contains a larger percent 235U than is found in nature. Natural uranium is 0.72%. Terms frequently used are “slightly enriched” (2-5% 235U), “highly enriched” (20-30%) and “fully enriched” (>90%).
fission: a decay mode of some heavy nuclei in which the nucleus breaks up into two intermediate-mass fragments and several neutrons.
fuel elements: fissile material (uranium or plutonium) shaped in a geometry suitable for assembling a critical mass in the reactor core. The elements are covered with a radiation-resistant cladding such as stainless steel or a zirconium alloy.
gamma camera: an array of gamma-ray detectors used to obtain geometric and coincidence information about gamma-ray emission. Gamma cameras are used in positron emission tomography, as well as other applications.
gamma rays: electromagnetic radiation with energy higher than X rays, typically associated with energies on the nuclear scale. Gamma rays with energies between a few thousand electron volts and about 7 MeV have been observed in radioactive processes.
gamma-ray catalysis: use of gamma rays to expedite chemical reactions. For instance, gamma rays can be used to initiate the polymerization process or to induce cross-linking in polymers. This type of catalysis occurs due to the formation of radicals.
grab sample: type of sampling method which collects a small sample of air in a scintillation cell over a short time span. The sample is analyzed for radioactive decay to determine the concentrations of radon or radon decay products.
gray: the amount of radiation that will deposit one joule of energy in one kilogram of absorbing material, abbreviated Gy. One gray equals 100 rad.
half-life: for a radioactive substance, the time needed for one half of a statistically large number of atoms to decay.
heavy water: water in which the majority of the molecules are composed of two deuterium atoms and one oxygen atom. D2O.
high-level nuclear waste: mainly, highly radioactive material resulting from the reprocessing of spent nuclear fuel. In general, it contains long-lived nuclei that need to be isolated from the environment for thousands of years or more.
ionizing radiation: any radiation which causes ionization, directly or indirectly, in its passage through matter. This includes gamma rays, alpha particles, beta particles, and neutrons with energies in excess of about 100 eV.
isobars: nuclei having the same mass number, but different numbers of protons (and therefore, neutrons).
isotones: nuclei with the same number of neutrons but different numbers of protons.
isotopes: nuclei with the same number of protons but different numbers of neutrons.
low-level nuclear waste: nuclear waste that is not classified as high-level, consisting mainly of waste from hospitals and research facilities that, after short-term storage, will no longer be significantly radioactive.
Lucas cell: ZnS-lined scintillation cell, often used to measure radon in air or radon emanating from a liquid or solid sample. Once the sample is collected in the Lucas cell, decays of radon or its daughters cause flashes of light to be emitted from the ZnS coating. A photomultiplier tube detects these flashes, which are counted and related to the concentration of radon present.
manmade radiation: radioactive nuclei produced by nuclear reactions, and not occurring in nature.
mass number: sum of the number of protons and the number of neutrons in the nucleus.
metastable state: nuclear energy state higher than the ground state. Eventually, such a state will decay to the ground state, with a characteristic half-life. Indicated by a lower-case “m” after the mass number in the symbol for the nuclide; for example, 99mTc, which is the most widely used radionuclide for diagnostic medical procedures.
microrisk: a risk that may kill one out of every million people exposed.
mill tailings: residue and waste left after processing of any ore. Mill tailings from uranium mining contain varying levels of radioactivity, which must be considered in disposal.
mitigation: a term used for any action taken to decrease indoor radon levels in buildings found to have concentrations above the action level.
moderator: a material used in a nuclear reactor to reduce fast neutrons, given off during the fission process,to thermal energies.
mrem, see rem
multiple-barrier system: use of two or more radon mitigation measures for the same building.
natural decay chain: a series of naturally occurring radioactive nuclei related to each other by decay. Three natural decay chains, originating from isotopes of uranium and thorium, account for all the natural activities in this region of the periodic table.
negatron decay: decay mode in which negative electrons are emitted from the nucleus, with an attendant increase of one unit in the value of Z. An antineutrino is also given off, and gamma rays may accompany negatron decay.
neutrino: a particle classified as a lepton that has no charge, no magnetic moment, zero rest mass, and a spin quantum number of 1/2. Because leptons must be conserved in nuclear reactions, positron decay is accompanied by emission of a neutrino and negatron emission is accompanied by emission of an antineutrino.
pair production: a process that involves the creation of a positron-electron pair by a gamma ray of at least 1.02 MeV (two electron masses).
photoelectric effect: the most important interaction of gamma rays with matter at low energies, in which all of the gamma ray energy goes into ejecting a bound electron from an atom or molecule and imparting kinetic energy to this electron.
positron: a positive electron.
positron decay: a type of beta decay in which positive electrons are emitted from the nucleus, with an attendant decrease of one unit in the value of Z. A neutrino is also given off, and gamma rays may accompany positron decay. Positron decay is possible only when the energy of the transition is greater than 1.02 MeV.
positron emission tomography: a medical imaging technique that relies on the use of positron-emitting radionuclides, given to a patient internally. External detection of the 0.511-MeV annihilation photons given off in positron decay can be used to obtain three-dimensional information of the internal region where the positron-emitter is localized.
quality factor: a weighting factor expressing the relative amount of energy deposited in matter by various types of radiation, ranging from one for gamma rays and beta particles to 20 for alpha particles.
rad: a unit used to measure radiation dose, defined as the dose of any type of radiation that will deposit 0.01 J of energy in one kilogram of absorbing material. 100 rad = 1 Gy.
radiolysis: decomposition of molecules caused by radiation.
radionuclide generator: any of a number of parent-daughter systems that can be packaged so that the daughter can be easily extracted for use. Generators allow for the use of radionuclides in locations where an accelerator is not available for their production by nuclear reaction. 99mTc, the single most used radionuclide for diagnostic nuclear medicine procedures, can be easily obtained from a generator containing its parent, 99Mo, which has a half-life of 65.9 hours. Radionuclide generators are sometimes called cows.
reactor core: portion of a nuclear reactor containing the fuel elements with cladding, moderator, coolant, and control rods.
rem: a unit used to express dose equivalent, which is obtained by multiplying the dose in rads by the quality factor. 100 rem = 1 sievert.
secular equilibrium: a limiting case of radioactive equilibrium in which the half-life of the parent nucleus is much larger than the half-life of the daughter. Under these circum-stances, the rates of production and decay of the daughter are equal, and the daughter is present at a constant concentration.
sievert: SI unit of dose equivalent, obtained by multiplying the dose in grays by the quality factor, and abbreviated as Sv. 1 Sv = 100 rem.
specific activity: activity per unit mass.
spent fuel: nuclear reactor fuel elements whose useful life is over due to depletion of fissile material, buildup of neutron-absorbing fission product poisons, or mechanical deterioration.
thermal neutrons: low energy, low velocity neutrons with energies approximately equal to that of gas molecules at room temperature (about 0.04 eV).
transient equilibrium: a limiting case of radioactive equilibrium in which the half-life of the parent is approximately 3-10 times longer than that of the daughter, but is short enough so there is a measurable decay of the parent during the experiment. After 3-5 half-lives of the daughter have elapsed, the ratio of daughter atoms to parent atoms becomes constant.
transuranic waste: radioactive waste containing elements heavier than uranium.
working level: the amount of potential alpha-particle energy dissipated in air by the daughters in equilibrium with 100 pCi/L of radon. 1 working level (WL) = 130,000 MeV of alpha-particle energy deposited per liter of air.
working-level month: A unit of exposure to short-lived radon daughters equal to the exposure to one WL for one working month of 170 hours.
zircaloy: one of the most commonly used fuel element claddings, an alloy of zirconium with varying amounts of nickel, chromium, iron, and tin.
| Last updated Tuesday, 20 June 2000 | Funded by a DOE/EPSCoR grant to Kentucky |
