NCRP Composite Glossary [updated July 2011] A absolute risk: Expression of excess risk due to exposure as the arithmetic difference between the risk among those exposed and that obtained in the absence of exposure. The resultant risk coefficient is normalized to a population base of 10,000 people and is expressed as the number of excess cases per 10,000 persons per gray per year at risk [i.e., (104 PY Gy)–1]. Absolute risk coefficients project can be modeled as a function of time since exposure (or attained age). [159] absorbed dose (D): The energy imparted to matter by ionizing radiation per unit mass of irradiated material at the point of interest. The SI unit is J kg–1 with the special name gray (Gy). The special unit previously used was rad. 1 Gy = 100 rad (see mean absorbed dose). [163] absorbed dose ( D): The energy imparted to matter by ionizing radiation per unit of irradiated material at the point of interest. Absorbed dose is the quotient of d∈ by dm, where d∈ is the mean energy imparted by ionizing radiation to matter in a volume element and dm is the mass of matter in that volume element: D = d∈/dm. The SI unit of absorbed dose is joule per kilogram (J kg –1), and its special name is gray (Gy). The special unit previously used was rad. 1 Gy = 100 rad (see mean absorbed dose). [158] absorbed dose (D): Quotient of d∈ by dm, where d∈ is the mean energy imparted by ionizing radiation to matter in a volume element and dm is the mass of matter in that volume element: D = d∈/dm. For purposes of radiation protection and assessing dose or risk to humans in general terms, the quantity normally calculated is the mean absorbed dose in an organ or tissue (T): DT = ∈T/mT, where ∈ is the total energy imparted in an organ or tissue of mass mT. The SI unit of absorbed dose is the joule per kilogram (J kg–1), and its special name is the gray (Gy). In previous units often used by federal and state agencies, absorbed dose is given in rad; 1 rad = 0.01 Gy. [155] absorbed-dose coefficient (d): In nuclear medicine, the absorbed dose per unit administered activity. [164] absorbed fraction (AF): In the MIRD schema, the fraction of radiation energy emitted within a source tissue that is absorbed in a target tissue. [164] absorbed fraction: The fraction of the photon energy emitted within a specified volume of material that is absorbed by the volume. The absorbed fraction depends on the source distribution; the photon energy; and the size, shape and composition of the volume. [163] absorbed fraction: The fraction of energy emitted as a specified radiation in a specified source tissue which is absorbed in a specified target tissue. [161] absorbed fraction: The fraction of ingested or inhaled material that is absorbed by an animal and transferred to the blood, body fluid, or a specific organ or tissue. [154] absorption: The fractional passage of material through a membrane, such as the fraction of intake that passes through the gut wall into the blood. [164] absorption: Movement of material to blood regardless of mechanism. Generally applies to the uptake into blood of soluble substances and material disassociated from particles. [161] absorption: To take in and make part of an existent whole; normally refers to the fractional passage of material through a membrane, such as the fraction of intake that passes through the gut wall into the blood. [154] absorption functions: Mathematical equations describing the rate of transfer of radionuclides into blood after deposition on skin, in wounds, in the gastrointestinal tract and in the respiratory tract (equations can be exponential, polynomial or constant relationships). [161] absorption functions [A(t)]: Dissolution and absorption functions of radionuclides found in the lung after deposition (functions can be exponential, polynomial or constant relationships). [125] accelerator: A device that accelerates charged particles (e.g., protons, electrons) to high speed in order to produce ionization or nuclear reactions in a target; often used for the production of certain radionuclides or directly for radiation therapy. The cyclotron and the linear accelerator are types of accelerators. [160] accelerator head: The part of the accelerator enclosing the x-ray target or source from which the useful beam emanates. The accelerator head contains shielding and may rotate about an axis. [151] accelerator-produced radioactive material (ARM): Any radioactive material that is produced by a particle accelerator. [141] accessible emission limit (AEL): The maximum accessible emission level permitted within a particular class of lasers. [148] accessible environment: The atmosphere, land surfaces, surface waters, oceans, and all of the lithosphere that is beyond a controlled area and is generally accessible to the public. [152] accident: An unintentional or unexpected happening that is undesirable or unfortunate, especially one resulting in injury, damage, harm or loss. [166] accuracy: A measure of the extent of agreement between the measured value and the true value. [164] accuracy: The extent of agreement between a measurement or prediction of a quantity and its true value. An accurate measurement or prediction should be precise and unbiased (see bias and precision). [163] acetabulum: The socket of the hip’s ball-and-socket joint, part of the pelvic bone. [164] acinus: Minute sac-like beginnings of the alveolar gland, an air cell of the lung. [125] acoustic cavitation: The mechanical response of one or more cavities to a sound field; it may be inertial or noninertial. [140] acoustic pressure: The excess of the instantaneous pressure at a point in a sound field over the pressure in the absence of sound. [140] acoustic pressure amplitude: For a sound field where the acoustic pressure varies sinusoidally with the time, the maximal value of the acoustic pressure. For a pulsed field of ultrasound, or for nonlinearly distorted waves. [140] acoustic radiation force: Time-averaged force on an object produced by a sound field. [140] acoustic radiation torque: Time-averaged torque on an object produced by a sound field. [140] acoustic streaming: Time-averaged flow of a liquid or gas produced by a sound field. [140] actinide: Element with atomic number from 90 through 103; a member of the actinide series of rare earths. [161] action levels: The inner-most level around the operating level on a control chart. When measured data reaches or exceeds the action level, the equipment operator must take corrective action (e.g., request service support from the contractor). Note that the equipment can continue to be used unless the measured values exceed the control limits. The corrective action is intended to bring the measured values back toward the operating level and within the action levels. [Comm20] activation: Production of radionuclides by absorption of radiations (e.g., photons, neutrons or alpha particles) by atomic nuclei. [164] activation: The process where an incident particle or x ray interacts with the nucleus of an atom changing its structural and energy stability such that it emits radiation in the form of waves or particles. [Comm20] active detector: A radiation detector which processes the signal at the time of detection (e.g., ionization chambers) and displays the measured value instantaneously. [Comm20] activity (A): Rate of transformation (or disintegration or decay) of radioactive material. The SI unit of activity is the reciprocal second (s–1) (meaning one transformation per second), and its special name is the becquerel (Bq). In previous units often used by state and federal agencies, activity is given in curies (Ci); 1 Ci = 3.7 x 1010 Bq. [166] activity: The average number of spontaneous nuclear transformations occurring in a radioactive material per unit time. The unit for activity in the SI system is reciprocal second (s–1) (i.e., one nuclear transformation per second), with the special name becquerel (Bq). The special unit previously used was curie (Ci); 1 Ci = 3.7 x 1010 Bq. Activity is also expressed as disintegrations per minute per unit area (dpm cm–2) with regard to surface contamination. [165] activity, cumulated (Ã): The time integral of the activity, [∫A(t)dt], which is proportional to the sum of all the nuclear transitions during a given time interval. [128] activity density: The density of contamination, express on a mass, volume, or area basis. [154] activity median aerodynamic diameter (AMAD): The diameter of a unit density sphere with the same settling velocity in air as that of an aerosol particle whose activity is the median for the entire aerosol. Fifty percent of the activity (aerodynamically classified) in the aerosol is associated with particles greater than the AMAD. A lognormal distribution of particle sizes is usually assumed. Used when deposition depends principally on impaction and sedimentation. [166] activity median aerodynamic diameter (AMAD): The diameter in an aerodynamic particle size distribution for which the total activity above and below this size are equal. A lognormal distribution of particle sizes is usually assumed. [164] activity median aerodynamic diameter (AMAD): Median diameter of airborne radioactive particles having the same aerodynamic properties as unit density spheres. Fifty percent of the of the activity (aerodynamically classified) in the aerosol is associated with particles greater than the AMAD. A lognormal distribution of particle sizes is assumed. The default values for environmental (public) and occupational exposures are 1 and 5 μm, respectively. [161] acute radiation exposure: Radiation exposure received during a short-time period (e.g., hours). [167] acute radiation syndrome (ARS): A broad term used to describe a range of signs and symptoms that reflect severe damage to specific organ systems that can lead to death within hours or several weeks. [166] additive (absolute risk) model: A model in which excess risk is expressed as a term to be added to the underlying natural or baseline risk (compare with the multiplicative model).