Table 1-5 shows various radioactive sources for industrial NDT. The most commonly used ones are Cobalt, Iridium and increasingly Selenium. Seleneum is very attractive while it permits lighter containers than Iridium. Due to its average energy level it often is a good alternative for an X-ray tube, also attractive while no electricity is needed.
Average energy level (nominal value): The spectrum of a source has one or more energy lines, as shown in figure 2-2. For sources with multiple energy lines an average energy level is assumed, the so-called nominal value.
On the basis of these spectra data it is clear that Co60, CS137 and Ir192 sources produce high-energy radiation and are therefore well suited to irradiate thick materials.
Yb169, on the other hand, is a source that produces relatively soft radiation and is of a very small size (0.5 mm), which makes it particularly suitable for radiographic examination of circumferential welds in pipes of a small diameter and thin wall thickness, with the source centrally positioned so that the weld can be exposed uniformly in one exposure, as shown in figure 8-5.