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Science Terms Definitions - G

Gamma Multihit Model - A generalization of the one-hit dose-response model which provides a better description of dose-response data.

Gamma Radiation - High-energy, short wavelength, electromagnetic radiation emitted from the nucleus. Gamma radiation frequently accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead or depleted uranium. Gamma rays are similar to x-rays.

Gaseous Diffusion - A method of isotopic separation based on the fact that gas atoms of molecules with different masses will diffuse through a porous barrier (or membrane) at different rates, according to their weight. This method is used to separate the lighter uranium-235 element from the heavier uranium-238 element. It requires large gaseous diffusion plants (GDPs) and enormous amounts of electric power.

Gaussian Distribution Model - A commonly used assumption about the distribution of values for a parameter, also called the normal distribution. For example, a Gaussian air dispersion model is one in which the pollutant is assumed to spread in air according to such a distribution and described by two parameters, the mean and standard deviation of the normal distribution.

Geographic Information System (GIS) - A computer system designed for storing, manipulating, analyzing, and displaying data in a geographic context.

Grab Sample - A single sample collected at a particular time and place that represents the composition of the water only at that time and place.

Gray (Gy) - The new international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and replaces the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad.

Greenhouse Effect - The warming of the Earth's atmosphere attributed to a build-up of carbon dioxide or other gases; some scientists think that this build-up allows the sun's rays to heat the Earth, while infra-red radiation makes the atmosphere opaque to a counterbalancing loss of heat.

Gross Alpha / Beta Particle Activity - The total radioactivity due to alpha or beta particle emission as inferred from measurements on a dry sample.

Groundwater - The supply of fresh water under the Earth's surface that forms a natural reservoir.

Groundwater Discharge - Groundwater entering near coastal waters which has been contaminated by landfill leachate, deep well injection of hazardous wastes, septic tanks, etc.

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