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).
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.
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.
EFFECTIVE RADIOGRAPHIC INSPECTIONS.
EFFECTIVE RADIOGRAPHIC INSPECTIONS.
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.
Factors Affecting Image Quality.
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