ASNT

Sealed source

Flexible leader

Stop ball

Control cable

Source assembly connector

Control cable connector

Figure 20 Schematic example of source assembly.

selenium 75 in the form of a metal alloy were introduced in 2000. The use of elemental selenium in radiography sources became a concern due to its high volatility and chemical reactivity. This was first emphasized by a Belgian health physicist, C. Ballaux, in his paper titled “Gamma Radiogra- phy with Selenium 75 Sources: Consequences of a Violent Fire” (Ballaux 2000). However, both metal alloy selenium 75 (Figure 22) and elemental selenium 75 sources have been reported to maintain their structural integrity at or above 1200 ° C (2192 ° F) (Shilton 2009). Selenium 75 sources are generally assembled as wire- or chain-link assemblies (Figure 23). These are compatible with different types of exposure devices. Elemental selenium 75 sources containing pressed or fused selenium powder in titanium or vanadium capsules are also available outside of North America. YTTERBIUM 169 SOURCES Ytterbium 169 sources have a relatively short half-life of 32 days. Their low-energy gamma-ray emission spectrum is ideal for carrying out radiography of thin or low-density alloys that may be located in difficult- to-access places. An example is the radiography of small-bore pipework in nuclear naval vessels. X-ray equipment and other imaging modalities may be too large to fit into the narrow spaces between pipes, and high background radiation levels may limit the application of other technologies.

Figure 21 Radiography source holder.

SELENIUM 75 SOURCES Selenium 75 has a softer gamma-ray spectrum than iridium 192, and it has a significantly longer half-life (Table 1). Gamma radiography using selenium 75 provides performance benefits relative to iridium 192 at the lower end of its working range (Figure 17 and Table 5). This relative performance benefit can extend up to 60 mm (2.36 in.) of steel when using digital or computed radiography techniques (Jelfs 2009). Selenium 75 is designated for use over its working range under ISO 5579:2013 Radiographic Testing of Metallic Materials Using Film and X- or Gamma-Rays—basic rules ; it is preferred over iridium 192. The first selenium 75 source designs that came onto the market in the mid-1990s contained elemental selenium 75. Sources containing

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Part 3