ASNT

Figure 18 Example of cobalt 60 chain-link source.

Figure 19 Example of an iridium 192 source. For size comparison, disk diameters range from 0.5 mm (0.02 in.) to 3.6 mm (0.14 in.).

depends on the disk activity, which is determined by the isotopic enrichment; the reactor f lux, time, and activation yield of iridium 192; and how many days of decay there have been before source assembly. One hundred Ci-enriched iridium 192 sources can be made using 2 mm (0.08 in.) diameter disks with a stack height of approximately 2 mm (0.08 in.) and a focal dimension (diagonal) of 2.7 mm (0.11 in.). Lower activity, smaller focal spot size iridium 192 sources can be made by irradiating solid 0.5 × 0.5 mm (0.02 × 0.02 in.) or 1 × 1 mm (0.04 × 0.04 in.) pellets of iridium 192. This typically produces source activities ranging from 370 to 740 GBq (10 to 20 Ci). Source assemblies can be attached to a f lexible wire with drive cable connector (sometimes referred to as a pigtail). Examples are shown in Figures 20 and 21. Male and female connections between the drive cable and the source wire are critical to the safety of devices to avoid unplanned disconnects. It is recommended that users ensure both male and female parts are supplied by the same manufacturer/designer to assure compatibil- ity of parts and tolerances, which reduces the risk of unintended disconnects. Iridium 192 sources may also be connected to tungsten-shielded, chain-link assemblies, as shown in Figure 19.

Irradiations to produce amounts of HSA cobalt 60 may take over a year using a very small selection of high-f lux reactors. Consequently, the availability of HSA cobalt 60 can sometimes be sporadic, due to periodic maintenance or unscheduled shutdowns during irradiation cycles. Due to its long half-life of 5.27 years, customers can typically use sources for 10 to 15 years before they need to be replaced. Manufacturers generally offer new-for-old source exchanges and will take back spent sources at the end of their working lives for dispositioning. IRIDIUM 192 SOURCES Iridium 192 sources range in activity up to approxi- mately 8880 GBq (240 Ci). The most common activity used in North America is 3700 GBq (100 Ci). Such sources typically have a focal spot size in the range of approximately 3 to 4 mm (0.12 to 0.16 in.) when they contain natural iridium 192, or approximately 2 to 3 mm (0.08 to 0.12 in.) when they contain enriched iridium 192. Sources generally contain cylindrical disk stacks (Figure 19). Disk diameters may vary, with the most common being 2.7 or 3 mm (0.11 or 0.12 in.) in 3700 GBq (100 Ci) sources with stack heights which typically vary from approximately 1 to 3 mm (0.04 to 0.12 in.) The stack height

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Part 3