Neutron Radiography

Figure 6-22. Radiographs of Honeycomb Showing Effect of Kilovoltage on Contrast

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 6-50 6.6.5 Neutron Radiography. 6.6.5.1 Description. Neutrons are useful for radiography because the attenuation of thermal neutrons is very different from that of X-rays. In general terms, the attenuation pattern is reversed in that many light materials (e.g., hydrogen, lithium, boron) have high attenuation of thermal neutrons while many heavy materials (e.g., bismuth, lead) are relatively transparent. Therefore, in this sense, neutron radiography can serve as a complementary inspection technique to X radiography. The advantages of thermal neutron radiography include excellent sensitivity to materials containing low atomic number elements (particularly hydrogen, lithium, and boron), some additional high attenuation materials (examples include silver, cadmium, indium, and gold), and rare earth elements (particularly samarium, gadolinium, and dysprosium). 6.6.5.2 Applications. Sensitivity to low atomic number materials opens up neutron inspection to a variety of applications involving water, explosives, fluids, rubber, plastics, and corrosion products (usually a hydroxide). An example of this type of inspection is neutron radiography of small explosive devices in metal cases to assure the presence of the explosive. Lead-covered explosive lines represent such an example. Inspection applications involving materials like cadmium have been demonstrated in the nuclear industry for cadmium reactor control materials. Cadmium plating inspection can also be considered. A major application involving rare earth materials is the inspection of investment-cast turbine blades to detect residual ceramic core left in cooling passages after leaching. 6.6.5.3 Disadvantages. Disadvantages of neutron radiography include the relatively high cost and additional radiation safety problems. Where high volume applications exist, for example turbine blade inspection, cost need not be a prohibitive factor. The additional radiation safety issues arise mainly from the generation of radioactivity in the inspection sample. These problems are rare and where they exist are usually easily handled by shielding and/or short waiting-time periods for the activity in the sample to decay. SECTION VII EFFECTIVE RADIOGRAPHIC INSPECTIONS. 6.7 EFFECTIVE RADIOGRAPHIC INSPECTIONS. 6.7.1 Introduction. This section describes the factors that determine whether or not a particular radiographic inspection is sufficiently sensitive to detect small defects. Sensitive radiography requires maximum subject contrast resulting from correct kilovoltage and alignment of the beam with the plane of the likely flaw; a sharp image due to good geometry and control of secondary radiation; and optimum density to give good film contrast. Each of these factors is described in turn and, finally, a description is given of quantitative transformations to allow exposure and density changes with a minimum of experimentation. 6.7.2 Factors Affecting Image Quality. 6.7.2.1 Radiation Energy. The radiation energy chosen must be compatible with absorption of the subject. For low-absorbing subjects, low energy radiation produces final radiographic images with good contrast. Conversely, for inspection of thick, highly absorbing subjects, the radiation must have sufficient penetrative capability to produce an image within a reasonable period of time. For high contrast, 96 to 99 percent of the incident radiation should be absorbed by the subject. Increasing kilovoltage reduces contrast because the quantity of radiation at any given energy increases and, perhaps more importantly, the proportion of radiation with a short wavelength (high energy) increases disproportionately. Figure 6- 21 shows these two relationships. High energy radiation can penetrate the subject more readily and thus reduces subject contrast. Figure 6-22 shows the effect on the final image of low or high contrast. The right diagram in Figure

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