ASNT

The technique of gamma radiography was first conceived and developed at the beginning of the last century after radium (radium 226) was first discovered. For the next 50 years, this was the only choice of radioisotope. Radium 226 was extracted from pitchblende, a natural mineral that is rich in uranium. This was far from ideal; its overly long half-life of 1600 years meant that the specific activity was low and, therefore, the focal spot size was large. To obtain clear images, source-to-film distance needed to be large and exposure time had to be long. During this era, the fundamental principles of gamma radiography were established and bourgeoning new applications were developed, primarily in the field of medical imaging and brachytherapy. Synthetic radioisotopes started to become available after World War II during the “Atoms for Peace” era, when the first high-flux isotope production reactors were commissioned. Gamma radiography rapidly transitioned from radium 226 to the shorter half-life, higher-specific activity radioisotopes of cesium 137, cobalt 60, and iridium 192. By the end of the 1960s, thulium 170 and ytterbium 169 had also been introduced. In the 1990s, selenium 75 first became available. In 2019, radium 226, cesium 137, and thulium 170 were no longer used, while ytterbium 169 was only very rarely used in some niche applications. The predomi- nant radionuclides are cobalt 60, iridium 192, and selenium 75. COMMERCIAL RADIOISOTOPES AND THEIR PROPERTIES Cobalt 60 The principal gamma-ray emission energies of cobalt 60 (Browne and Firestone 1986; Firestone and Shirley 1996; Laboratoire National Henri Becquerel n.d.) are shown in Table 2. Cobalt 60 decays by beta emission to stable nickel with a half-life of 5.272 years (Figure 11). The 1173.2 and 1332.5 keV emissions are utilized in gamma radiography. The very low-abundance, higher-energy gamma emissions can be ignored by the radiographer; however, they do need to be considered by designers and

Table 2 Principal gamma ray emission energies of cobalt 60

Energy

Gamma emission abundance 99.9 photons per 100 decays 99.82 photons per 100 decays 1.14 × 10 -3 photons per 100 decays 2 × 10 -6 photons per 100 decays

1173.2 keV 1332.5 keV 2158.9 keV 2505.7 keV

Co-60

0.31 MeV β 99.88%

5.272 a

1.48 MeV β 0.12%

1.1732 MeV γ

1.3325 MeV γ

Ni-60

Figure 11 Simplified cobalt 60 decay scheme.

manufacturers of devices and transport containers from the perspective of surface dose rate and radiological safety. Cobalt is a hard, brittle, silver-gray transition metal. It has a high melting point of 1495 °C (2723 °F) and is hard wearing and stable at high tempera- tures. Natural cobalt contains 100% of the stable isotope cobalt 59 (Element Collection n.d.). Its absorption cross section for thermal and epithermal neutrons (such as neutrons with energies below about 100 eV) is very high. Nevertheless, to achieve high specific activity and small focal spot size, a high neutron f lux and a long irradiation time are both needed. Irradiations may take over a year because the half-life of cobalt 60 is long (5.272 years), so it activates slowly. The form of cobalt 60 that is used in gamma radiography sources and devices is referred to as HSA cobalt (high specific activity cobalt 60), as opposed to LSA cobalt (low specific activity cobalt 60). The latter is produced in lower-f lux Canada Deuterium Uranium (CANDU) power reactors after long-term (multiyear) activation of its cobalt control rods (Gamma Industry Processing Alliance n.d.). Megacuries of LSA cobalt are used in the process

CHAPTER 3

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Part 3