Radiation Safety Office


Absorbed Dose: The amount of energy imparted to matter by ionizing radiation per unit mass of irradiated material. (See Rand.)

Absorption: The phenomenon by which radiation imparts some or all of its energy to any material through which it passes.

Activity: The number of nuclear disintegrations occurring in a given quantity of material per unit time. (See Curie.)

ALARA: Acronym for the phrase "as low as is reasonably achievable," which embodies general regulatory philosophy pertaining to exposure of personnel to ionizing radiation.

Alpha Particle: A strongly ionizing particle emitted from the nucleus during radioactive decay having a mass and charge equal in magnitude to a helium nucleus, consisting of 2 protons and 2 neutrons with a double positive charge.

Annihilation (Electron): An interaction between a positive and negative electron (or other particle, anti-particle pair) wherein their energy, including rest energy, is converted into electromagnetic radiation (annihilation radiation).

Atom: Smallest particle of an element which is capable of entering into a chemical reaction.

Autoradiograph: Record of radiation from radioactive material in an object, made by placing the object in close proximity to a photographic emulsion.

Background Radiation: Ionizing radiation arising from radioactive material other than the one directly under consideration. Background radiation due to cosmic rays and natural radioactivity is always present. There may also be background radiation due to the presence of radioactive substances in other parts of the building, in the building material itself, etc.

Becquerel: SI unit of activity, abbreviated Bq. 1 Bq = 1 disintegration/sec.

Beta Particle: Charged particle emitted from the nucleus of an atom, having a mass and charge equal in magnitude to that of the electron.

Beta Ray: A stream of high speed electrons or positrons of nuclear origin more penetrating but less ionizing than alpha rays.

Bremsstrahlung: Electromagnetic (x-ray) radiation associated with the deceleration of charged particles passing through matter. Usually associated with energetic beta emitters, e.g., phosphorus-32.

Calibration: Determination of variation from standard, or accuracy, of a measuring instrument to ascertain necessary correction factors.

Contamination, Radioactive: Deposition of radioactive material in any place where it is not desired, and particularly in any place where its presence may be harmful. The harm may be in vitiating the validity of an experiment or a procedure, or in actually being a source of excessive exposure to personnel.

Carrier Free: An adjective applied to one or more radioactive isotopes of an element in minute quantity, essentially undiluted with stable isotope carrier.

Count (Radiation Measurements): The external indication in a device designed to enumerate ionizing events. It may refer to a single detected event or to the total registered in a given period of time. The term is often erroneously used to designate a disintegration, ionizing event, or voltage pulse.

Critical Organ: That organ or tissue, the irradiation of which will result in the greatest hazard to the health of the individual or his descendants.

Curie: The quantity of any radioactive material in which the number of disintegrations is 3.700 x 1010 per second. Abbreviated Ci. Millicurie: One-thousandth of a curie (3.7 x 107 disintegrations per second), abbreviated mCi. Microcurie: One millionth of a curie (3.7 x 104 disintegrations per second), abbreviated µCi. Picocurie: One millionth of a microcurie (3.7 x 10-2 disintegrations per second or 2.22 disintegrations per minute), abbreviated pCi.

Decay, Radioactive: Disintegration of the nucleus of an unstable nuclide by the spontaneous emission of charged particles and/or photons.

Dose: A general term denoting the quantity of radiation or energy absorbed in a specified mass. For special purposes it must be appropriately qualified (see absorbed dose and dose equivalent).

Dose Equivalent: The product of the absorbed dose in tissue and the quality factor (and all other necessary modifying factors at the location of interest). The SI unit of dose equivalent is the sievert (for absorbed dose in grays). The other unit is the rem (for absorbed dose in rads).

Efficiency (Counters): A measure of the probability that a count will be recorded when radiation is incident on a detector. Usage varies considerably so it is well to make sure which factors (e.g., window, transmission, sensitive volume, energy dependence, etc.) are included in a given case.

Electron: Negatively charged elementary particle which is a constituent of every neutral atom. Its unit of negative electricity equals 4.8 x 10-10 electrostatic units or 1.6 x 10-19 coulombs. Its mass is 0.00549 atomic mass units. The positively charged anti-particle of the electron is called the positron.

Electron Capture: A mode of radioactive decay involving the capture of an orbital electron by its nucleus. Capture from the particular electron shell is designated as "K-electron capture," "L-electron capture",etc.

Electron Volt: A unit of energy equivalent to the amount of energy gained by an electron in passing through a potential difference of 1 volt, abbreviated eV. Larger multiple units of the electron volt frequently used are keV for thousand or kiloelectron volts, MeV for million electron volts, and GeV for billion electron volts.

Exposure: A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electrical charges on all ions of one sig produced in air when all electrons liberated by photons in a volume element of air are completely stopped in air, divided by the mass of air in the volume element. The special unit of exposure is the roentgen.

Film Badge: A packet of photographic film used for the approximate measurement of radiation exposure for personnel monitoring purposes. The badge may contain two or more films of differing sensitivity, and it may contain filters which shield parts of the film from certain types of radiation.

Filter (Radiology), Primary: A sheet of material, usually metal, placed in a beam of radiation to remove, inasmuch as possible, the less penetrating components of the beam.

Secondary: A sheet of material of lower atomic number, relative to that of the primary filter, placed in the filtered beam of radiation to remove characteristic radiation produced by the primary filter.

Gamma Ray: Very penetrating electromagnetic radiation of nuclear origin. Except for origin, identical to x-ray.

Geiger-Mueller (G-M) Counter: Highly sensitive gas-filled detector and associated circuitry used for radiation detection and measurement.

Genetic Effect of Radiation: Inheritable changes, chiefly mutations, produced by the absorption of ionizing radiations. On the basis of present knowledge these effects are purely additive, and there is no recovery.

Gray: SI unit of absorbed dose, abbreviated Gy. 1 Gy = 100 rad.

Half-Life, Biological: The time required for the body to eliminate one-half of an administered dose of any substance by the regular processes of elimination. This time is approximately the same for both stable and radioactive isotopes of a particular element

Half-Life, Effective: Time required for a radioactive nuclide in a system to be diminished 50 percent as a result of the combined action of radioactive decay and biological elimination. Effective half-life = Biological half-life x Radioactive half-life Biological half-life + Radioactive half-life

Half-Life, Radioactive: Time required for a radioactive substance to lose 50 percent of its activity by decay. Each radionuclide has a unique half-life.

Half Value Layer (Half thickness): The thickness of any specified material necessary to reduce the intensity of an x-ray or gamma ray beam to one-half its original value.

Health Physics: A term in common use for that branch of radiological science dealing with the protection of personnel from harmful effects of ionizing radiation.

Inverse Square Law: The intensity of radiation at any distance from a point source varies inversely as the square of that distance. For example: If the radiation exposure is 100 R/hr at 1 inch from a source, the exposure will be 0.01 R/hr at 100 inches.

Ion: Atomic particle, atom, or chemical radical bearing an electrical charge, either positive or negative.

Ionization: The process by which a neutral atom or molecule acquires either a positive or a negative charge.

Ionization Chamber: An instrument designed to measure the quantity of ionizing radiation in terms of the charge of electricity associated with ions produced within a defined volume.

Ionization, specific: The number of ion pairs per unit path length for ionizing radiation in a medium; e.g., number per centimeter of air or per micron of tissue.

Ionizing Radiation: Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter.

Isotopes: Nuclides having the same number of protons in their nuclei, and hence having the same atomic number, but differing in the number of neutrons, and therefore in the mass number. Almost identical chemical properties exist among isotopes of a particular element.

Labeled Compound: A compound consisting, in part, of labeled molecules. By observations of radioactivity or isotopic composition this compound or its fragments may be followed through physical, chemical, or biological processes.

Milliroentgen (mR): A submultiple of the roentgen equal to one one-thousandth (1/1000th) of a roentgen. (See Roentgen.) ("mR" also is used for "millirad.")

Monitoring, Radiological: Periodic or continuous determination of the amount of ionizing radiation or radioactive contamination present in an occupied region as a safety measure for purposes of health protection. Area Monitoring: Routine monitoring of the level of radiation or of radioactive contamination of any particular area, building, room, or equipment. Personnel Monitoring: Monitoring any part of an individual, his breath, excretions, or any part of his clothing. (See Radiological Survey.)

Neutron: Elementary particle with a mass approximately the same as that of a hydrogen atom and electrically neutral. It has a half-life of about 10 minutes, and decays in a free state into a proton, an electron, and an anti-neutrino.

Nuclide: A species of atom characterized by its mass number, atomic number, and energy state of its nucleus, provided that the atom is capable of existing for a measurable time.

Protective Barriers: Barriers of radiation-absorbing material, such as lead, concrete, plaster, and plastic, that are used to reduce radiation exposure. Protective Barriers, Primary: Barriers sufficient to attenuate the useful beam to the required degree. Protective Barriers, Secondary: Barriers sufficient to attenuate stray or scattered radiation to the required degree.

Proton: Elementary stable particle with a positive charge of the same magnitude as the electron charge and a mass of 1837 times that of the electron.

Quality Factor: The modifying factor, Q, used to derive dose equivalent (H, in rems) from absorbed dose (D, in rads). H = QD. In SI units, if D is in grays then H is in sieverts.

Rad: Special unit of absorbed dose. One rad equals 0.01 joule per kilogram of material, i. e., 0.01 gray or 100 ergs per gram.

Radiation: 1. The emission and propagation of energy through space or through a material medium in the form of waves; for instance, the emission and propagation of electromagnetic waves, or of sound and elastic waves. 2. The energy propagated through a material medium of waves; for example, energy in the form of electromagnetic waves or of elastic waves. The term, "radiation" or "radiant energy," when unqualified, usually refers to electromagnetic radiation. Such radiation commonly is classified according to frequency as Hertzian, infrared, visible (light), ultra-violet, x-ray, and gamma ray. 3. By extension, corpuscular emissions, such as alpha and beta radiation, or rays of mixed or unknown type, such as cosmic radiation.

Radioactive Material: Any material (solid, liquid, or gas) which emits radiation spontaneously.

Radioactive Materials Laboratory: A volume bounded by a floor, a ceiling, and at least four floor-to-ceiling walls or partitions, in which radioactive materials are used or stored.

Radiological Survey: Evaluation of the radiation hazards incident to the production, use or existence of radioactive materials or other sources of radiation under a specific set of conditions. Such evaluation customarily includes a physical survey of the disposition of materials and equipment, measurements or estimates of the levels of radiation that may be involved, and a sufficient knowledge of processes using or affecting these materials to predict hazards resulting from expected or possible changes in materials or equipment.

Radiotoxicity: Term referring to the potential of an isotope to cause damage to living tissue by absorption of energy from the disintegration of the radioactive material introduced into the body.

Relative Biological Effectiveness (RBE): For a particular living organism or part of an organism, the ratio of the absorbed dose of a reference radiation that produces a specified biological effect to the absorbed dose of the radiation of interest that produces the same biological effect.

REM: The special unit of dose equivalent. The dose equivalent in rems is numerically equal to the absorbed dose in rads multiplied by the quality factor.

Roentgen (R): The quantity of x or gamma radiation such that the associated corpuscular emission per 0.001293 grams of dry air produces, in air, ions carrying one electrostatic unit of electrical charge of either sign. The roentgen is the special unit of exposure.

Scintillation Counter: A counter in which light flashes produced in a scintillator by ionizing radiation are converted into electrical pulses by a photomultiplier tube.

Shielding Material: Any material which is used to absorb radiation and thus effectively reduce the intensity of radiation, and in some cases, eliminate it. Lead, concrete, aluminum, water, and plastic are examples of commonly used shielding material.

Sievert: SI unit of dose equivalent, abbreviated Sv. 1 Sv = 100 rem.

Smear (Swipe or Wipe Test): A procedure in which a swab, e.g., a circle of filter paper, is rubbed on a surface and its radioactivity measured to determine if the surface is contaminated with removable radioactive material.

Specific Activity: Total radioactivity of a given nuclide per unit mass or per unit volume of a compound, element, or nuclide.

Tracer, Isotopic: The isotope or non-natural mixture of isotopes of an element which may be incorporated into a sample to make possible observation of the course of that element, alone or in combination, through a chemical, biological, or physical process. The observations may be made by measurement of radioactivity or of isotopic abundance.

Thermoluminescent Dosimeter: A dosimeter made of certain crystalline material which is capable of both storing a fraction of absorbed ionizing radiation and releasing this energy in the form of visible photons when heated. The amount of light released can be used as a measure of radiation exposure to these crystals.

X-rays: Penetrating electromagnetic radiations having wave lengths shorter than those of visible light. They are usually produced by bombarding a metallic target with fast electrons in a high vacuum. In nuclear reactions it is customary to refer to photons originating in the nucleus as gamma rays, and those originating in the extranuclear part of the atom as x-rays. These rays are sometimes called "roentgen rays" after their discoverer, W. C. Roentgen.


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