ASNT

the emission spectrum of the radioisotope and the density of the material the gamma rays are passing through. The denser the material and the wider the spectral range of the emissions, the more the spectrum is hardened. As an example, a selenium 75 source has a TVL/HVL ratio of approximately 3.3 in concrete, approximately 3.4 in steel, and approxi- mately 4.3 in lead. SOURCE EXCHANGE AND RETURN Radiographers generally utilize sources for more than a half-life, but rarely for more than two half-lives. A 3700 GBq (100 Ci) iridium 192 source is typically exchanged for a new source when its activity decays to between 740 to 1480 GBq (20 to 40 Ci). Replacement sources may be shipped in a two-hole source changer (Figures 25 and 26). This is a Type-B shipping container that holds a new source in one hole and enables the used source to be retracted from an exposure device into the empty hole in the changer. This operation can be carried out in the field or at a local service center. Bulk shipments of sources can also be supplied to larger customers, distributors, and service centers in Type-B approved multihole source changers (Figures 27 and 28).

Figure 24 Example of a ytterbium 169 source.

of energies. Lower energies are preferentially attenuated when thickness increases. This “hardens” the spectrum (that is, the average energy of the spectrum shifts to higher energies as the gamma rays penetrate deeper into a material). Because of this, in practice, the ratio of TVL/HVL is larger than 3.3. How much larger depends on

Table 11 Half- and tenth-value layers in materials relevant to gamma radiographers

Approximate half-thickness layer (in /mm)

Density (g/cm 3 )

Yb-169

Se-75

Ir-192

Co-60

Concrete

2.35 2.65

1.14

29.00

1.17 1.10

30.0

1.70

43.2

2.40

61.0

Aluminum

27.00

Steel Lead

7.8

0.17

4.30 1.20

0.315 0.039

8.0

0.512 0.20

13.0

0.827 21.0

11.34

0.047

1.0

5.10 3.30

0.50 0.31 0.27

12.70 7.90 6.80

Tungsten

17.8 18.7

0.032

0.80

0.13

Depleted uranium

0.05

1.3

Approximate tenth-thickness layer (in /mm)

Density (g/cm 3 )

Yb-169

Se-75

Ir-192

Co-60

Concrete

2.35 2.65

3.9

99.0

5.6

142.0

8.0

203.0

Aluminum

Steel Lead

7.8

0.56

14.0

1.06

27.0

1.70

43.0

2.7

69.0 43.0 25.4 22.9

11.34

0.156

4.0

0.17

4.25

0.664 17.0

1.7

Tungsten

17.8 18.7

0.4 0.2

102.0

1.0

Depleted uranium

5.1

0.9

CHAPTER 3

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Part 3