ASTM: Abbreviation for American Society for Testing and Materials.
ASTM BLOCK: Specific type of reference standard, cylindrically shaped and having a specified size FBH at a specified
metal travel distance from the top of the block. See ASTM.
ASTM HARDNESS NUMBER: The depth (in thousandths of an inch) of penetration of an indentor into a rubber
specimen under loads and conditions specified in ASTMD-314. While suited for most common grades of rubber,
ASTM hardness number is not applicable to extremely hard or soft rubbers.
ATOM: The smallest particle of an element that can enter into a chemical combination. All chemical compounds are
formed of atoms, the difference between compounds being attributable to the nature, number and arrangement of their
ATOMIC MASS UNIT (AMU): 1.66 x 10-24 grams. Arbitrarily defined as 1/12th of a carbon-12 atom. An AMU is
approximately the mass of a proton (1.0073 AMU) or a neutron (1.0087 AMU).
ATOMIC NUMBER: An integer that expresses the positive charge of the nucleus in multiples of the fundamental
electronic charge. In present theory, it is the number of protons in the nucleus.
ATOMIC WEIGHT: The relative weight of the atom of an element, referred to some element taken as a standard. An
atomic weight of 16 for oxygen is the one usually adopted as a basis for reference.
ATTENUATION (RT): Reduction in the intensity of a beam of ionizing radiation due to passage through matter.
ATTENUATION (UT): Loss of energy caused by scattering of the sound beam within a material or at an interface or an
electronic device in or attached to the instrument.
ATTENUATION COEFFICIENT (RT): Average rate that a beam of radiation changes as it passes through a body.
ATTENUATOR: A device that causes a known loss in energy of a beam that is passed through it. It may be calibrated
AUGER ELECTRON (RT): An orbital electron emitted by an atom, instead of a photon of characteristic radiation,
when a vacancy in an inner electron shell is filled.
AUSTENITIC STEELS: Steels whose constituents remain in solution with each other at room temperature and are,
therefore, non-magnetic and corrosion resistant.
AUTORADIOGRAPH (RT): The image of an object obtained on a photographic emulsion by means of radiation
emitted by the object itself.
AUTORADIOGRAPHY (RT): A test in which the object being inspected is radioactive, or made radioactive, and the
inherent radiation so produced is used to produce the image on a film.
AUTOTRANSFORMER (RT): A special type of transformer in which the output voltage can be easily varied. The
autotransformer is thus employed to adjust the primary voltage applied to the step-up transformer that produces the
high voltage applied to the X-ray tube.
AVERAGE GRADIENT (RT): (of a film) The steepness of the characteristic curve of a film. Usually measured as
average gradient between two levels of density; e.g., the average gradient between a density of 0.25 and a density of 2.0
is the slope of a straight line connecting these points. Most x-ray films have a gradient of 2.5 to 4.0, and any film with
a gradient over 1.0 amplifies the subject contrast.