GAMMA RAYS: The electromagnetic radiation of high frequency or short wavelength emitted by the nucleus of an
atom during a nuclear reaction. Gamma rays are undeflected by electric or magnetic fields. They are identified in
nature and properties to X-rays of the same wavelength, and differ only in their manner of production.
GAS HOLES: Blow holes, channels, or porosity produced by gas evolution, usually during solidification.
GAS HOLES (RT) (ON RADIOGRAPH): Appear as round or elongated, smooth-edged dark spots, occurring
individually, in clusters, or distributed throughout the casting.
GAS POROSITY: Refers to porous sections in metal that appear as round or elongated dark spots corresponding to
minute voids usually distributed through the entire casting.
GAS POROSITY (RT) (ON RADIOGRAPH): Represented by round or elongated dark spots corresponding to minute
voids usually distributed through the entire casting.
GATE (UT): Electronic device to monitor signals in a selected segment of the distance trace on an A-scan display.
GAUSS: This is the unit of flux density or induction. The strength of field induced in a ferromagnetic body is
described as being so many Gausses. It is usually designated by the letter B. Numerically, one Gauss is one line of
flux per square centimeter of area.
GEIGER COUNTER: A radiation detection and measuring instrument. It contains a gas-filled tube that discharges
electrically when ionizing radiation passes through it. Discharges are counted to measure the radiations intensity.
GENETIC EFFECTS OF RADIATION: Effects that produce changes in those cells of organisms which give rise to egg
or sperm cells and therefore affect offspring of the exposed individuals.
GEOMETRIC FACTORS (RT): General term used to describe the factors in radiographic exposures that account for
distortion and/or enlargement. Some of the more common geometric factors include focal spot size, specimen
thickness, and source-to-film distance.
GEOMETRIC UNSHARPNESS (RT): See UNSHARPNESS.
GHOST (UT): An indication that has no direct relation to reflected pulses from discontinuities in the materials being
GRADIENT (RT): The slope of a characteristic curve at a specified density. Symbol: G. Note: The term gamma is
used for the slope of the approximately straight portion of the curve.
GRAININESS (RT): A film characteristic which consists of the grouping or clumping together of the countless small
silver grains into relative large masses visible to the naked eye or with slight magnification.
GRAIN BOUNDARY: An interface separating two grains when the orientation of the lattices changes from that of one
grain to that of another. When the orientation change is very small, the boundary is sometimes referred to as sub-
GRAIN FLOW: See FLOW LINES.
GRAINS: Individual alloy crystals that form the structure of the metal.
GRAIN SIZE: Size of the crystals in metal when compared with a standard. Usually referred to as being fine, medium
GRAIN SIZE (RT): The average size of the silver halide particles in a photographic emulsion.