T.O. 33B-1-16-166.2.5 PracticalConsiderationsWhenChoosingEquipment.6.2.5.1 ChoiceofRadiationEnergy.The relation of X-ray voltage to the penetration for steel or other common materials depends upon the density of thematerial and the absorption characteristics of the material in the X-ray beam. In general, Table 6-2 can be used as aguide for applying X-rays to inspection problems, assuming average radiographic results are expected. It is necessaryto establish lower limits as well as upper limits on material thickness because using voltages higher than what isrequired to penetrate a given thickness will reduce the radiographic contrast.Table 6-2. Appropriate Radiation Energies for Radiography ofSteel.Kilovoltage Range Material Thickness5-50 kV*50-150 kV100-200 kV200-400 kV1000 kV2000-6000 kV15-24 MeVExtremely thin, such as foil up to 1/8 in.1/8 to 3/4 in. steel1/4 to 2 in. steel3/4 to 3 in. steel1 to 5 in. steel2 to 8 in. steel3 to 18 in. steel* This energy range is also useful for composite structures. Note that forX-ray energies of 15 kV or less, scatter in the air path may be a problem.6.2.5.2 ChoiceofEquipment.The equipment choice should depend upon the circumstance under which radiographic inspection is to be conductedand the technique requirements.6.2.5.2.1 ChoiceofTubeType.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 notedthat scattered radiation is greater with the rod anode and additional personnel protection is often necessary. Thedirectional X-ray tubes restrict the radiation to a smaller area and have a comparatively smaller focal spot resulting inbetter quality radiographic images.6.2.5.2.2 ChoiceofWindow.When the X-ray absorption of a test object is low, lower energy radiation is required. To take advantage of the highercontrast provided at lower energies, an X-ray tube with a beryllium window should be used since beryllium transmitsthe low energy radiation. Up to 150 kVp the beryllium window offers advantages. Above 150 kVp the typical glasswindow should prove satisfactory. Therefore, radiographic inspections using 150 kVp or less SHALL use a berylliumwindow X-ray tube. The beryllium window and the resultant soft (low energy) spectrum SHALL also be used for theinspection of composite laminates. For example, a graphite-epoxy composite laminate 0.100-inch thick might requirethe use of an X-ray energy in the order of 10-20 kV for optimum sensitivity. (Reasonable exposures with standardportable X-ray equipment are often difficult below 25-30 kV.) It should be noted that, at X-ray energies of 15 kV orless, the air between the source and object would scatter the X-rays. If the X-ray equipment will operate that low, oneway to displace the air is to stuff a helium-filled plastic bag between source and object.
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