ASNT

lower-energy emissions of ytterbium 169. Source activities are typically available up to 20 Ci.

Titanium Vanadium

3.5 3.1 2.7 2 1.4 Diagonal (mm) 80-120Ci 50-70Ci ~40Ci ~20Ci ~8Ci

HALF- AND TENTH-VALUE LAYERS Tables of X-ray mass attenuation coefficients and mass energy-absorption coefficients from 1 keV to 30 MeV for elements Z = 1 to 92, and 48 additional substances of dosimetric interest, are published by the US Department of Commerce, National Technical Information Service order NTIS PB95-220539 ( Report No. NISTIR 5632 ). Selected tables of mass attenuation coefficients can be found in Chapter 17 of this handbook. Data from these tables can be used to determine the amount of attenuation of gamma rays of any energy through many different materials. The half-value-layer (HVL) in a material is the thickness that attenuates the dose rate from a source by 50%. The tenth-value-layer (TVL) in a material is the thickness that attenuates the dose rate by 90%. Approximate HVL and TVL values in several materials of relevance to gamma radiogra- phers are summarized in Table 11 (QSA Global 2015; Canadian Nuclear Safety Commission 2017; University of Florida 2011). Mathematically, the TVL would be 3.3 × thicker than the HVL (= ln 10/ln2) if the source emission was a single-energy gamma ray. However, in practice, radiography isotopes emit a spectrum

Se-75 metal alloy/composite

Figure 22 Metal alloy selenium 75 sources.

The extremely high neutron activation cross section of ytterbium 169, coupled with its shorter half-life, enables very high specific activity sources to be made with very small focal spot size (Table 1). This enables radiography to be carried out in close proximity to fixtures that are under investigation. An example of an ytterbium 169 source assembly is shown in Figure 24. The active component is an inert oxide ceramic bead or pellet, which can be quasi-spherical or cylindrical in shape. The capsule is titanium. This has lower density than stainless steel, reducing internal attenuation and maximizing emission of the

Figure 23 Metal alloy selenium 75 source assemblies.

CHAPTER 3

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Part 3