ASNT

PART 3 Radioisotope Sources and Exposure Devices

The half-life of the radioisotope is important. A shorter half-life results in higher specific activity (meaning high activity in a small volume, resulting in a small focal spot size). In contrast, a long half-life results in low specific activity (meaning low activity in a large volume, resulting in a large focal spot size). The half-life must be short enough to achieve the high specific activity that is necessary, but not so short that the working life of a source is impractical. Half-lives for useful gamma radiography radioisotopes range from 32 days (ytterbium 169) to 5.27 years (cobalt 60). Finally, manufacturing must be feasible and economic on a global scale capable of producing millions of curies of radioactivity and tens of thousands of sources per year. Sources must be designed for maximum safety and durability, requiring source materials to be inert and resilient. Taking all these factors into account, the practical choices are limited to just a few suitable radioisotopes (Table 1).

GAMMA RAY SOURCES There are hundreds of natural and artificial radioisotopes; however, very few have suitable properties to be of any use in gamma radiography. Those that are suitable need to have abundant gamma-ray emissions with energies that match the attenuation characteristics of an object being radiographed. Thick or dense objects need high-energy radioisotopes; thin or low-density objects need low-energy radioisotopes to optimize image contrast and resolution. Radiation sources must contain very high specific activity material so that the focal spot size can be small. This is necessary to achieve high-resolution images without having to place a source a long way from an object. This rules out most radioisotopes, either because they activate inefficiently, because their gamma-ray emission abundance is too low, or their half-lives are so long that they activate too slowly.

Table 1 Radioisotopes for gamma radiography

Specific activity Ci/mm 3

Specific activity

Mean

Focal

Isotopic enrichment of target material

Nuclide Co-60 Ir-192 Ir-192 Se-75 Yb-169

t 1/2

energy (keV)

Ci/g 225 400 700

size (mm)

5.27y

~1250

2 8

5-10

Made from natural cobalt Made from natural iridium Made from ~80% Ir-191 Made from ~99% Se-74 Made from ~80% Yb-168

74d 74d

~375-385 ~370-380 ~215-230

1-5 1-3

15

120d

5

1000 3000

1.5-5

32d

~120

25

0.5-2

CHAPTER 3

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Part 3