CYCLOTRON: A particle accelerator in which the atomic particles are whirled around in a spiral between the ends of
a huge magnet gaining speed with each rotation. The cyclotron is normally used for nuclear research but the particles
can be made to collide with a target to produce X-rays.
D: Symbol for diameter.
d: Symbol for distance.
d/t RATIO: The working distance for the X-ray tube in relation to the film distance. The working distance, d, and the
specimen thickness, t, are both measured with reference to the source side of the specimen.
DAC: Abbreviation for distance amplitude correction; also used to denote electronic distance amplitude correction on
DAMPING: Hindering or decreasing the time of vibrations or oscillations in the motion of a body or in an electrical
system subjected to influences which are capable of causing vibration or oscillation. Compare with attenuation.
DAMPING (UT): Limiting the duration of and/or decreasing the amplitude of vibrations, as in damping of a transducer
element; also designates a bond inspection method in which good bonds are verified by damping ultrasonic energy
transmitted to the back surface.
DAMPING CAPACITY: The ability of a metal to absorb vibration (cyclical stresses) by internal friction converting the
mechanical energy into heat.
DAMPING MATERIAL (UT): Material contained within a search unit in back of the transducer element and used for
DARK ADAPTION: The ability of the eye to adjust so that objects, lights, or colors can be seen in darkened areas.
This is important when performing a fluorescent penetrant, fluorescent magnetic particle inspections or when
interpreting radiographic film.
dB: Abbreviation for decibel.
DC (DIRECT CURRENT): An electrical current that flows continually in one direction through a conductor.
DEAD ZONE: Zone in the test part directly underneath the sound entry surface where discontinuities cannot be
detected; caused by the finite length of the initial pulse, ringing time of the transducer element, and/or electronic
characteristics of the instrument.
DECALESCENCE: A phenomenon, associated with the transformation of alpha iron to gamma iron on the heating
(superheating) of iron or steel, revealed by the darkening of the metal surface owing to the sudden decrease in
temperature caused by the fast absorption of the latent heat or transformation.
DECARBURIZATION: The loss of carbon from the surface of a ferrous alloy as a result of heating in a medium that
reacts with the carbon at the surface.
DECAY (MT): The falling off to zero of the current in an electrical circuit. Magnetic fields can also decay in a similar
manner. This is important in demagnetization.