ASNT

The predominant emissions highlighted in the aforementioned list are utilized in gamma radiography. These ideally match the gamma-ray attenuation characteristics of smaller or lower density fixtures found in industry that have joints, such as pipelines, f langes, tanks, and their weldments. The useful working thickness range in copper, nickel, or steel alloys is commonly accepted as 3 to 29 mm ( 0.12 to 1.14 in.) (QSA Global 2015). This is variable, depending on the sensitivity requirement and imaging technique. Selenium 75 decays by electron capture with a half-life of 119.79 days to stable arsenic, as shown in Figure 15 (Laboratoire National Henri Becquerel n.d.; Grimm and Kaftal 1996). Natural selenium has six stable isotopes (Element Collection n.d.): selenium 74 (0.86%), selenium 76 (9.23%), selenium 77 (7.6%), selenium 78 (23.69%), selenium 80 (49.8%), and selenium 82 (8.82%). It is selenium 74 that activates to produce selenium 75; however, at only 0.86% natural isotopic abundance, this would make a very poor, low specific activity gamma radiogra- phy source. Natural selenium must be isotopically enriched by gas centrifugation before it can be used to make a practical gamma radiography source. This process typically produces an enrichment >95% of selenium 74. Selenium 74 has a high neutron absorption cross section for both thermal and epithermal

1 mm focal spot size 2 mm focal spot size 3 mm focal spot size

296-316.5 keV peaks

468 keV peaks

589-613 keV peaks

Relative emission intensity

0 100 200 300 400

500 600 700 800

Gamma energy (keV)

Figure 14 Gamma output spectra of iridium 192 source shows compton scatter peaks in detector.

The smaller focal spot size of enriched iridium 192 together with its softer gamma-ray spectrum can increase image contrast and resolution at the lower end of the working range, relative to natural iridium 192 sources, particularly when used in conjunction with digital imaging equipment. Small focal, enriched iridium 192 radiography

sources were first introduced in 2000. Isotopically Enriched Selenium 75

The principal gamma-ray emissions of selenium 75 (Browne and Firestone 1986; Firestone and Shirley 1996; Laboratoire National Henri Becquerel n.d.) are shown in Table 4.

Se-75 5/ 0.00

Table 4 Principal gamma ray emissions of selenium 75

ε decay

119.79 days

100%

Q ε = 0.8636

ε 5

0.4006

Energy

Gamma emission abundance 5.5 photons per 100 decays 1.44 photons per 100 decays 6.35 photons per 100 decays 17.6 photons per 100 decays 59.2 photons per 100 decays 1.48 photons per 100 decays 59.2 photons per 100 decays 25.11 photons per 100 decays 1.38 photons per 100 decays 11.4 photons per 100 decays 0.036 photons per 100 decays

γ 5

24.38 keV 66.05 keV 96.73 keV 121.1 keV 136.0 keV 198.6 keV 264.7 keV 279.6 keV 303.9 keV 400.7 keV 572.2 keV

ε 6 ε 7 ε 8

0.3039 0.2795 0.2646

γ 2

γ 6

γ 7

γ 3

ε 9

0.1986

γ 8 γ 10 γ 11 γ 12

γ 14

0.000

As-75 (stable)

Figure 15 Simplified selenium 75 decay scheme.

CHAPTER 3

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Part 3