ASNT

sources, although it may be combined with other elements to form a glassy ceramic. This material is highly inert, insoluble in water and acids, and has a very high melting point of 2355 ° C (4271 ° F) (Element Collection n.d.). Activation Burn-up Not only do the stable target isotopes (cobalt 59, iridium 191, selenium 74, and ytterbium 168) have high neutron absorption cross sections, but their activated products (cobalt 60, iridium 192, selenium 75, and ytterbium 169) may also have high- activation cross sections. In these cases, the product radioisotope “burns up” as it is made, significantly reducing its yield. This may be in addition to decay loss that also occurs during irradiations. Skilled nuclear physicists at reactor sites optimize the irradiation conditions for each radioisotope to

Yb-169 7/2 + :0 32.018 days

472.8841

12.6

433.524 430.124 379.268 367.66 345.031 332.119 316.1463

0.0044 0.121

82.2

5.1 0.0142 0.0138

138.9331

118.1894

8.4102 Stable 1/2 + :0

maximize yields and minimize losses. Useful Working Thickness Range

Tm-169

Figure 16 Simplified ytterbium 169 decay scheme.

The useful working thickness range for gamma radiography radioisotopes are compared in both Figure 17 and Table 5 (QSA Global 2015). Ranges are somewhat variable, depending on the sensitivity requirement and imaging techniques.

Natural ytterbium must be isotopically enriched before it can be used for gamma radiography. Gas centrifugation cannot be used for enrichment because there are no stable gaseous ytterbium molecules that can be enriched by this technique. Other costlier enrichment techniques are therefore used, including electromagnetic or laser enrichment techniques, such as atomic vapor laser isotope separation (AVLIS). An enrichment of approximately 80% ytterbium 168 can typically be produced. Ytterbium 169 has an extremely high neutron absorption cross section for both thermal and epithermal neutrons, even higher than iridium. High specific activity with very small focal spot size can be achieved, such as 370 GBq to 555 GBq (10 to 15 Ci) with 1.0 to 1.4 mm (0.04 to 0.06 in.) focal spot size. Elemental ytterbium is a soft, malleable, and reactive metal. It easily oxidizes and reacts with water; therefore, the elemental form is unsuitable for use in gamma radiography sources. Ytterbium forms a hard, dense sesquioxide: Yb 2 0 3 . This is the form most commonly used in gamma radiography

t 1/2 Source

Range

1.17 – 1.33 MeV 206 – 612 KeV

5.27y 74d

60 Co

192 Ir

66 – 401 KeV

60 Se

120d

63 – 308 KeV

32d

60 Yb

0 20 40 60

80 100 120 140

Thickness (mm)

Figure 17 Useful working thickness range in copper, nickel, and steel alloys.

Table 5 Useful working ranges in steel, copper, and nickel-based alloys

Radioisotope

Useful working range

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

50 – 150 mm 12 – 63 mm 3 – 29 mm 2 – 20 mm

CHAPTER 3

72

Part 3