Practical Considerations When Choosing Equipment.
Choice of Radiation Energy.
The relation of X-ray voltage to the penetration for steel or other common materials depends upon the density of the
material and the absorption characteristics of the material in the X-ray beam. In general, Table 6-2 can be used as a
guide for applying X-rays to inspection problems, assuming average radiographic results are expected. It is necessary
to establish lower limits as well as upper limits on material thickness because using voltages higher than what is
required to penetrate a given thickness will reduce the radiographic contrast.
Table 6-2. Appropriate Radiation Energies for Radiography of
Extremely thin, such as foil up to 1/8 in.
1/8 to 3/4 in. steel
1/4 to 2 in. steel
3/4 to 3 in. steel
1 to 5 in. steel
2 to 8 in. steel
3 to 18 in. steel
* This energy range is also useful for composite structures. Note that for
X-ray energies of 15 kV or less, scatter in the air path may be a problem.
Choice of Equipment.
The equipment choice should depend upon the circumstance under which radiographic inspection is to be conducted
and the technique requirements.
Choice of Tube Type.
The choice of a directional or a rod anode tube type should depend upon the type of radiographic inspection conducted.
Circumferential specimens, such as pipe weldments, are compatible with the rod anode radiation. It should be noted
that scattered radiation is greater with the rod anode and additional personnel protection is often necessary. The
directional X-ray tubes restrict the radiation to a smaller area and have a comparatively smaller focal spot resulting in
better quality radiographic images.
Choice of Window.
When the X-ray absorption of a test object is low, lower energy radiation is required. To take advantage of the higher
contrast provided at lower energies, an X-ray tube with a beryllium window should be used since beryllium transmits
the low energy radiation. Up to 150 kVp the beryllium window offers advantages. Above 150 kVp the typical glass
window should prove satisfactory. Therefore, radiographic inspections using 150 kVp or less SHALL use a beryllium
window X-ray tube. The beryllium window and the resultant soft (low energy) spectrum SHALL also be used for the
inspection of composite laminates. For example, a graphite-epoxy composite laminate 0.100-inch thick might require
the use of an X-ray energy in the order of 10-20 kV for optimum sensitivity. (Reasonable exposures with standard
portable X-ray equipment are often difficult below 25-30 kV.) It should be noted that, at X-ray energies of 15 kV or
less, the air between the source and object would scatter the X-rays. If the X-ray equipment will operate that low, one
way to displace the air is to stuff a helium-filled plastic bag between source and object.