ASNT

industry for industrial, medical, and agricultural sterilization. LSA cobalt is unsuitable for gamma radiography because its specific activity is too low. HSA cobalt has sufficiently high specific activity and small enough focal spot size in the range of 5 to 10 mm (0.20 to 0.39 in.) to enable useful gamma radiography sources to be made with activities in the range of 3700 GBq to 11 100 GBq (100 to 300 Ci). Natural Iridium 192 Natural iridium (iridium 192) undergoes two simultaneous modes of decay with a combined half-life of 73.83 days. Iridium 192 decays 95.13% of the time by beta emission to stable platinum 192. The remainder of the time, iridium 192 decays by electron capture to stable osmium 192, as shown in Figure 12 (Browne and Firestone 1986; Firestone and Shirley 1996). The principal gamma-ray emission energies of iridium 192 are shown in Table 3. High-abundance emissions that are highlighted in the list are utilized in gamma radiography. They range in energy from 296 to 612 keV. This range ideally matches the gamma-ray attenuation characteristics of many common fixtures found in industry that have joints, such as pipelines, f langes, tanks, and their weldments. This, coupled with its high specific activity and small focal spot size, has led to iridium 192 becoming the radioisotope of choice for most

Table 3 Principal gamma ray emission energies of iridium 192

Energy (decay to)

Gamma emission abundance 0.2 photons per 100 decays 0.47 photons per 100 decays 3.34 photons per 100 decays 0.27 photons per 100 decays 28.7 photons per 100 decays 29.7 photons per 100 decays 82.8 photons per 100 decays 0.73 photons per 100 decays 0.67 photons per 100 decays 47.8 photons per 100 decays 3.2 photons per 100 decays 0.4 photons per 100 decays 0.5 photons per 100 decays 8.2 photons per 100 decays 5.3 photons per 100 decays 0.3 photons per 100 decays 1.2 × 10 -3 photons per 100 decays 1.2 × 10 -3 photons per 100 decays

136.0 keV (Pt) 201.0 keV (Os) 205.8 keV (Os) 283.3 keV (Os) 296.0 keV (Pt) 308.5 keV (Pt) 316.5 keV (Pt) 374.5 keV (Os) 416.5 keV (Pt) 468.1 keV (Pt) 484.6 keV (Os) 489.1 keV (Os) 588.6 keV (Pt) 604.4 keV (Pt) 612.5 keV (Pt) 884.5 keV (Pt) 1090 keV (Pt) 1378 keV (Pt)

gamma radiography applications. The practical working thickness range in copper, nickel, or steel alloys for iridium 192 is commonly accepted as 12 to 63 mm (0.47 to 2.48 in.) (QSA Global 2015). This may be variable, depending on the sensitivity requirement and imaging technique. The low-abundance, high-energy gamma emissions of iridium 192, between 884.5 and 1378 keV, do not impact gamma radiographs; however, they do have an impact on the surface dose rate and radiological safety of devices. Manufacturers take these into consideration when designing the shielding of exposure devices, source exchangers, and other transport containers. Natural iridium contains two stable isotopes: 37.3% iridium 191 and 62.7% iridium 193 (Element Collection n.d.). It is the iridium 191 that activates to produce iridium 192. Iridium 191 has an exceptionally high neutron absorption cross section for both thermal and epithermal neutron energies (Kopecky et al. 1997). Its affinity for neutrons is so high that during irradiation, only the first few fractions of a millimeter below the surface of a substrate can

4.87%

95.13%

Ir-192

1.453

1.453

β 1

EC 1

β 2

1.201

EC 2

1.100 1.000 0.900

β 3

0.921 0.785

0.613

0.613

Ground state

0.410

Os-192

0.317

Ground state

0

0

Pt-192

Figure 12 Simplified iridium 192 decay scheme. EC = electron capture; β = beta emissions.

CHAPTER 3

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Part 3