ASNT

activate efficiently. Further beneath the surface, the neutron f lux becomes depleted, making activation inefficient. It is, for this reason, that iridium sources are made by irradiating thin disks and then stacking them to form a compact cylinder inside a source. In this way, a disk stack has uniform high activity throughout its volume and not just on its surface. Manufacturers often report the “stack height” of an iridium 192 source. This refers to the height of the stack of disks inside. The focal spot size is the diagonal of a stack when used in the radial (sideways) direction, which is most common (Figure 13). In a 3700 GBq (100 Ci) source containing natural isotopic abundance iridium, this is typically in the range of 3.5 to 4.5 mm (0.134 to 0.177 in.), although the precise focal spot size depends on the neutron f lux of the reactor that irradiated the disks and the age (or amount of decay) of the iridium 192 since it was activated. Exactly 62.7% of natural iridium is iridium 193, which also has an extremely high neutron absorption cross section for both thermal and epithermal neutrons. Its presence during activations, combined with iridium 191, results in the production of a large quantity of unwanted radioactive iridium 194. This consumes neutrons, and its presence with iridium 191 reduces the activation yield of iridium 192.

Iridium 194 emits high-energy gamma rays with a short half-life of 19.3 h. It decays to stable platinum 194. Reactor operators must hold freshly activated iridium in a cooling pond for about four days to allow most of the iridium 194 to decay away (this is referred to as “cooling off”) before supplying the activated iridium 192 disks to source manufac- turers. By the time manufacturers assemble and ship sources, the amount of remaining iridium 194 is trivially small and can be ignored. Iridium is a hard, brittle, silver-gray noble metal. It has the highest resistance to chemical attack of all the elements that exist in nature and it has an extremely high melting point of 2466 ° C (4471 ° F). Its density of 22.56 g/cm 3 (1408.38 lb m /ft 3 ) is the second-highest of all known substances (Element Collection n.d.). Due to its high density, a typical 3700 GBq (100 Ci) source may self-attenuate approximately 35 to 45% of its own gamma-ray emissions. The precise percentage depends on its focal spot size. A typical 3700 GBq (100 Ci) source with an output of 3700 GBq (100 Ci) has an activity content that is likely to be in the range of 5550 to 6660 GBq (150 to 180 Ci). Isotopically Enriched Iridium 192 Iridium of natural isotopic composition can be isotopically enriched by gas centrifugation to produce iridium containing approximately 80% iridium 191. This more than doubles the concentra- tion of iridium 191 and reduces to approximately one-third the amount of iridium 193 contained in it. Gamma radiography sources made using enriched iridium generally have a higher specific activity and a smaller focal spot size than natural iridium 192 sources. Gamma-ray spectra from iridium 192 sources with focal spot sizes of 1, 2, and 3 mm (0.04, 0.08, and 0.12 in.) show differences in their spectral outputs (Figure 14). The relative abundances of high-energy emissions at 589 to 613 keV are similar in all sources; however, the relative abundances of lower energy emissions at 296 to 316.5 keV are increased in the case of the smaller focal spot size sources. This is due to their reduced self-attenuation.

Focal dimension

Stack height

Diameter

Figure 13 Iridium 192 disk stack; height varies with activity.

CHAPTER 3

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Part 3