T.O. 33B-1-1
6-41
6.5.3.1
Scatter Radiation.
6.5.3.1.1
Description.
When high-energy electromagnetic radiation bombards matter, some of the radiation photons are scattered by electrons,
a process called the Compton effect. If the photon has a greater quantity of energy than necessary to eject an electron
from its orbital path, it continues to travel with a loss of energy at some angle to its original path. The photon energy
must be reduced to a very low value before complete annihilation is possible by photoelectric absorption. In low atomic
number materials the photon direction is changed with little loss of energy and its energy must be reduced to a very low
energy to be absorbed completely. Thus, a single photon may be scattered many times, losing all semblance of its
original path. If this scattered photon strikes the film, it reduces the image definition since it exposes the film at a spot
other than directly under the point where it first entered the test material. High atomic number materials rob the
photon of greater amounts of its original energy and also have much higher photoelectric absorption values. These
more quickly reduce the photon energy to the point where the photon is completely absorbed. For these reasons, low
atomic number materials transmit larger quantities of scattered radiation than high atomic number materials. In actual
radiographic practices, low atomic number materials should be removed from the beam to the extent possible, to
prevent scattering of the primary beam. Wood, concrete or other low atomic number materials in the radiation beam
should be covered with lead or a high atomic number material to reduce the scatter. In actual practice this means that
tables, floors or walls that are behind/beside and close to the test part should be covered with lead.
6.5.3.1.2
Scatter Build Up.
The scattering is due to photon collision with electrons in their path. As material thicknesses increase up to a critical
thickness, the amount of scattered radiation emanating from the material increases. If additional thicknesses of
material are added, the scattered radiation generated in these added layers have insufficient energy to penetrate the
material between them and the film. The amount of scattered radiation emanating from the back of a part being
inspected increases with part thickness up to a total which varies with radiation energy. Since absorption due to the
Compton effect decreases with increasing radiation energy, less scattering occurs at higher radiation energy levels.
Build-up scatter radiation can introduce contrast problems in the radiography of low atomic number materials such as
graphite, plastics, and magnesium. A simple test to reveal the scatter build-up in a test specimen can be made. Choose
a radiation source-to-film distance of three or four feet, make two identical exposures — one with the test specimen at
the X-ray source and one with the specimen at the film. Differences in the densities after processing can be credited to
scatter radiation.
6.5.4
Diffraction Patterns.
In the radiography of very coarse grain structure materials, such as Inconel and cast irons, diffraction patterns are often
revealed in the radiographic image. These patterns are due to the selective diffraction and absorption by the atoms of a
definite pattern in the crystal structure. The definitive pattern of the atoms of a crystal can be aligned with the X-ray
beam at a particular angle so that the radiation is altered in its direction of travel and concentrated upon the film as a
linear indication. These crystalline diffraction patterns are superimposed upon the radiographic image and make
interpretation very difficult. Often these dense, sharp lines caused by the crystal diffraction are interpreted as internal
cracks. If uncertainty exists as to interpretation of a particular indication, a second radiograph can be made at a
slightly different angle (less than 10 degrees difference). It is unlikely the crystal causing the diffraction pattern would
be located to precisely the same relative position as to cause the diffracted line to strike the film in the same relative
position. Changes in radiation energy will also affect diffraction patterns. Often by changing the operating kilovoltage
the problem of diffraction patterns can be reduced.
6.5.5
Material Contrast.
6.5.5.1
Material Contrast Factor.
In consideration of the above discussion on radiation absorption, the most important variable that can be controlled by
the radiographer in industrial X-ray inspection is the kilovoltage. The amount of radiation absorbed by the part being
inspected depends on the atomic number, density, and thickness of the material. The radiographer cannot change these
factors, but can change the energy of radiation. In the attenuation equation, ln (-mx) = IT/I0, it can be visualized that the
