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  • Narrow Banded - A relative term denoting a restricted range of frequency response.
  • Natural Frequency - The frequency at which an object will vibrate freely when set in motion.
  • Naturally Occurring Radiation - cosmogenic (extraterrestrial) and terrestrial radiation with an activity range that is usually around 5-10 micro roentgens per hour, The term "naturally occurring" needs to be differentiated from "background radiation," which now includes the impact of the cumulative deposition from stratospheric fallout and nuclear accidents such as Chernobyl. In some contaminated areas, such accumulations of long-lived artificially produced radionuclides exceed natural background radiation levels.
  • NDE - Acronym for Nondestructive Evaluation
  • NDT- Acronym for Nondestructive Testing.
  • NDT Methods - A process used to test an object for flaws and other defects that does not harm the object.
  • Near Field - The distance immediately in front of the transducer in which the ultrasonic beam exhibits complex and changing wavefronts. Also called the Fresnel Field or Fresnel Zone.
  • Neper - The natural logarithm of a ratio of two amplitudes (equal to 8.686 dB) used as a measure of attenuation. Power ratios are expressed as one-half the natural logarithm.
  • Neutron - One of the basic particles which make up an atom and is about the same size as a proton but has no electrical charge.
  • Neutron Activation - The process of bombarding a material with neutrons.
  • Neutron source - A source that emits neutrons, e.g. a poloniumberyllium source, sometimes used in radiography where background fields are high, e.g. in a reactor room.
  • Newton's Inverse Square Law - The intensity of the influence at any given radius (r) is the source strength divided by the area of the sphere.
  • Nitriding - An elevated-temperature process (but lower than carburizing or carbonitiding) by which a ferrous metal absorbs nitrogen atoms into the surface when exposed to a high-nitrogen environment. Nitrogen atoms actually diffuse, or flow, into the metal to form a high-nitrogen surface layer.
  • Nodal Points - In angle beam testing, the location of reflections at opposite surfaces as a wave progresses along a test object.
  • Node - A point in a standing wave where a given characteristic of the wave field has zero amplitude.
  • Noise - Any undesired signal that obscures the signal of interest.
    - It might be electrical noise or a signal from specimen dimensional or property variations.
  • Nonaqueous - Made with liquids other than water.
  • Nondestructive Evaluation (NDE) - The use of non-invasive techniques to determine the integrity of a material, component or structure, or to quantitatively measure some characteristic of an object. (Making quantitative measurements is what distinguishes NDE from NDT.)
  • Nondestructive Testing (NDT) - Testing to detect defects in materials using techniques that do not damage or destroy the items being tested.
  • Nonferrous - Describing a metal that is less than 50% iron, such as aluminum, copper, magnesium, and zinc and their alloys.
  • Nonrelevant Indication - An indication that has no relation to a significant discontinuity.
  • Normal Beam Inspection - Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure
  • Normal Incidence - A condition in which the axis of the ultrasonic beam is perpendicular to the entry surface of the test object.
    - A condition where the angle of incidence is zero.
  • Normal Incidence Shear Wave Transducers - A transducer that allows shear waves directly into a test piece without the use of an angle beam wedge.
  • Nuclear Fission A process which involves the splitting of nuclei with the accompanying release of energy.
  • Nuclear Fusion A process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones.
  • Nuclear Radiation Radiation emitted from an unstable atomic nucleus.
  • Nuclear Reaction - A reaction involving an atom's nucleus, such as fission, neutron capture, radioactive decay, or fusion, as distinct from a chemical reaction, which is limited to changes in the electron structure surrounding the nucleus.
  • Nuclear Reactors - Devices that control fission reactions producing new substances from the fission product and energy.
  • Nucleon - A nuclear particle-either a proton or neutron or both collectively.
  • Nucleus The center of an atom containing protons and neutrons.
  • Nuclide - Any species of atom that exists for a measurable length of time. A nuclide can be distinguished by its atomic weight, atomic number, and energy state. The term is used synonymously with isotope. A radionuclide is a radioactive nuclide.

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