Radiography is based on the ability of X-rays and gamma rays to pass
through metal and other materials opaque to ordinary light, and produce
photographic records of the transmitted radiant energy. All materials
will absorb known amounts of this radiant energy and, therefore, X-rays
and gamma rays can be used to show discontinuities and inclusions within
the opaque material. The permanent film record of the internal
conditions will show the basic information by which weld soundness can
be determined.
X-rays are produced by high-voltage generators. As the high voltage
applied to an X-ray tube is increased, the wavelength of the emitted
X-ray becomes shorter, providing more penetrating power. Gamma rays are
produced by the atomic disintegration of radioisotopes. The radioactive
isotopes most widely used in industrial radiography are Cobalt 60 and
Iridium 192. Gamma rays emitted from these isotopes are similar to
X-rays, except their wavelengths are usually shorter. This allows them
to penetrate to greater depths than X-rays of the same power, however,
exposure times are considerably longer due to the lower intensity.
When X-rays or gamma rays are directed at a section of weldment, not
all of the radiation passes through the metal. Different materials,
depending on their density, thickness and atomic number, will absorb
different wavelengths of radiant energy.
The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. The opaque material absorbs a certain amount of radiation, but where there is a thin section or a void (slag inclusion or porosity), less absorption takes place. These areas will appear darker on the radiograph. Thicket areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter.