ASNT

neutrons (Kopecky et al. 1997); however, this is not as high as iridium 191. Because of this, and its longer half-life, it is necessary to irradiate selenium 74 in a much higher neutron f lux than is necessary to make iridium 192. Most of the isotope production reactors throughout the world that are used to make iridium 192 do not have a high enough f lux to make high-specific activity selenium 75; therefore, the choice of reactors is more limited. The cost of the enrichment process and the high-f lux irradiations do impact the price and availability of selenium 75; however, its longer half-life and softer spectrum provide many other technical and safety benefits when applied to the radiography of smaller or lower density fixtures. Elemental selenium is soft and malleable and has a low melting point of 217 ° C (422.6 ° F). It boils at 685 ° C (1265 ° F). It is highly toxic, volatile, reactive, and corrosive at high temperatures, but it is stable and insoluble in water at low temperatures. There are three physical forms: amorphous, monoclinic, and metallic, with densities of 4.3, 4.5, and 4.8 g/cm 3 (268.44 lb m /ft 3 , 280.93 lb m /ft 3 , and 299.65 lb m /ft 3 ), respectively. It has a large coefficient of expansion close to its melting point, when it expands by 12%. Selenium 75 sources are tested to verify that they meet the same performance, safety, and regulatory standards that other gamma radiography sources meet, notwithstanding these properties. Selenium can be combined with one or more nonactivating metals to form more stable metal alloys or composites. Radiography sources contain- ing either elemental selenium 75 or metal alloy composites have undergone special fire tests to verify that they can withstand the conditions of a hypothetical 1200 ° C (2192 ° F) petrochemical fire without leakage. In 2019, elemental selenium 75 sources were not approved for transportation within North America. However, they are approved elsewhere. Radiography sources containing highly enriched elemental selenium 75 were first introduced in the mid-1990s (Unger and Trubey 1982); sources containing selenium 75 as a metal alloy composite were first introduced in 2000 (Shilton 2001).

The enriched selenium 74 irradiation-target material must be pre-encapsulated to isolate it from the canister in which it is irradiated. This is done in order to prevent chemical or physical interaction or volatilization under the extreme conditions of irradiation. This is not needed when iridium or cobalt are irradiated, because those materials are inert. It is impossible to irradiate unencapsulated disks or pellets of selenium 75 and then stack them like an iridium 192 source because of its physico- chemical properties. This means that the activity of selenium 75 sources cannot be adjusted after irradiation in the way iridium 192 sources can, by stacking disks. Selenium 75 sources are made available in batches several times a year, whereas iridium 192 sources are made daily. Isotopically Enriched Ytterbium 169 The principal gamma-ray emissions of ytterbium 169 are shown in Table 1 (Browne and Firestone 1986; Firestone and Shirley 1996; Laboratoire National Henri Becquerel n.d.). The predominant emissions that are highlighted in the table are utilized in gamma radiography. These ideally match the gamma-ray attenuation characteristics of small- diameter pipes and weldments or other small low-density alloys with a useful working thickness range in copper, nickel, or steel alloys of approxi- mately 2 to 20 cm (0.79 to 7.87 in.) (QSA Global 2015). This is variable, depending on the sensitivity require- ment and imaging technique. Ytterbium 169 sources are used in just a few specialist applications. For example, they are used in radiography of small-bore piping and weldments in naval nuclear vessels in difficult-to-access spaces where microfocal X-ray equipment cannot go. Ytterbium 169 decays by electron capture to stable thulium 169 with a half-life of 32.018 days (Figure 16). Natural ytterbium has seven stable isotopes: ytterbium 168 (0.126%), ytterbium 170 (3.023%), ytterbium 171 (14.216%), ytterbium 172 (21.754%), ytterbium 173 (16.098%), ytterbium 174 (31.896%), and ytterbium 176 (12.887%) (Element Collection n.d.). It is ytterbium 168 that activates to produce ytterbium 169; however, at only 0.126% isotopic abundance, this would make a very poor, low-specific activity gamma radiography source.

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Part 3