MARKERS: A series of square waves, or other beam deflections displayed on the cathode-ray tube screen of the
ultrasonic equipment used to determine the distance from the test surface of the article being inspected to a subsequent
discontinuity or boundary.
MAS: Milliampere seconds, utilized to standardize radiographic exposures. Example: 5 MA X 60 seconds = 300
MASKING (RT): Surrounding specimens or covering thin sections with absorptive material to eliminate scatter and
halation on the film image.
MASS ABSORPTION COEFFICIENT (RT): A numerical expression of the absorption characteristics of a given
material. The mass absorption coefficient is different for different materials, and is dependent on kilovoltage. It is
equal to the Linear Absorption Coefficient divided by the mass density.
MASS ATTENUATION COEFFICIENT (RT): The fraction of uncharged ionizing particles that experience
interactions in traversing a unit distance in a material of density.
MASS NUMBER: The sum of the neutrons and protons in a nucleus. The mass number of uranium-235 is 235. It is
the nearest whole number to the atoms actual atomic weight.
MATERIAL NOISE: Extraneous signals caused by the structure of the material being tested.
MAXIMUM PERMISSIBLE DOSE (MPD): That dose of ionizing radiation which competent authorities have
established as the maximum that can be absorbed without undue risk to human health.
MEAN FREE PATH: Average distance a particle travels between collisions.
MEAN LIFE: The average time during which an atom or other system exists in a particular form.
In fatigue testing, the algebraic mean of the maximum and minimum stress in one cycle. Also called the
steady stress component.
In any multiaxial stress system, the algebraic mean of three Principal Stresses; more correctly called mean
MECHANICAL PROPERTIES: The properties of a material that reveal its elastic and in-elastic behavior where force
is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile
strength, elongation, hardness, and fatigue limit.
MEDIAN LETHAL DOSE (RT): The whole-body dose, resulting from a single short exposure (minutes or hours), that
will cause the death, within a specified period of time, of 50 percent of the individuals irradiated. The dose sufficient to
cause death to 50 percent of the individuals within 30 days is indicated as LD and is on the order of 300 rads.
MEGACYCLE: One million cycles; often used to express one million cycles per second. Abbreviation is mc.
MEGAHERTZ: One million hertz. Abbreviation is MHz.
METAL SCREEN (RT): A screen consisting of a foil of dense metal (usually lead) that emits secondary electrons when
exposed to X- or gamma radiation. It also reduces the undesirable effects of scattered radiation.
METALLOGRAPH: An optical instrument designed for both visual observation and photomicrography of prepared
surfaces of opaque materials at magnifications ranging from about 25 to about 1500 diameters.