G

• Gadolinium 153 - A radioisotope of the element gadolinium.
• Gain Control - A control which varies the amplification of the ultrasonic system. Also considered the sensitivity control.
• Galvanic Corrosion - Corrosion associated with the current of a galvanic cell consisting of two dissimilar conductors in an electrolyte or two similar conductors in dissimilar electrolytes. Where the two dissimilar metals are in contact, galvanic corrosion may occur.
• Galvanic Series - A series of metals and alloys arranged according to their relative corrosive tendency in a given environment. The most common environment is sea water or other concentrations of salt in water.
• Galvanometer - An instrument with a bar of iron wrapped with an electrical wire and a magnet that detects electrical current.
• Gamma Radiation - high-energy, short wavelength electromagnetic radiation emitted by a nucleus. Energies of gamma rays are usually between 0.010 and 10 mev. X-rays also occur in this energy range, but are non-nuclear origin. Gamma radiation usually accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best attenuated by dense materials like lead and depleted uranium.
• Gas Holes - Holes created by a gas escaping from molten metal.
• Gas Holes (on radiograph) - Appear as round or elongated, smooth-edged dark spots, occurring individually, in clusters or distributed throughout the casting.
• Gas Porosity - A cavity caused by entrapped gas. Essentially a smooth-sided bubble within the metal, where the metal solidified before the gas could escape to the atmosphere. Also called gas pocket.
• Gate - An electronic device for monitoring signals in a selected segment of the trace on an A-scan display. - The interval along the baseline that is monitored.
• Gauss (B) - Unit of flux density or induction. The strength of field induced in a ferromagnetic body is described as being so many gausses. 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 which discharges electrically when ionizing radiation passes through it. Discharges are counted to measure the radiation's intensity.
• Geometry/X-ray Resolution - Radiograph resolution is effected by source to film distance, object to film distance and source size.
• Ghost - An indication arising from certain combinations of pulse repetition frequency and time base frequency. See wrap around.
• Grain - The more common term for crystal, a three-dimensional array of atoms having a certain regularity in its internal arrangement. The grain is composed of many cells, or lattices, in which the atoms are arranged on the metal involved.
• Grain Boundary - The boundary between two grains.
• Grain Noise Modeling - Methods used to predict the electrical voltage signals seen during ultrasonic inspections of metal components.
• Grain Size - Size of the crystals in metal when compared with a standard. Usually referred to as being fine, medium, or coarse.
• Graininess - A film characteristic resulting from the grouping or clumping together of the countless small silver grains into relatively large masses visible to the naked eye or with slight magnification.
• Grains - Those individual crystals that make up the crystalline structure of metal.
• Grass - See background noise.
• Grinding - Pressing down or together with a crushing turning motion, rubbing harshly or gratingly.
• Grinding Cracks - Shallow cracks formed in the surface of relatively hard materials because of excessive grinding heat or the high sensitivity of the material.
• Gross Porosity - In weld metal or in casting, pores, gas holes or globular voids that are larger and in greater number than obtained in good practice.
• Group Velocity - The rate at which the envelope of an ultrasonic pulse (many frequencies) propagates through the medium.