ASNT

All the ranges overlap, enabling good coverage for the thicknesses, densities, and alloys that commonly occur in industry. This was not always the case. Until selenium 75 was introduced in the mid-1990s, there was a partial gap in performance and range between ytterbium 169 and iridium 192. Air Kerma Rate The air kerma rate (AKR, or alternatively referred to as the specific gamma-ray constant) of a radionu- clide is a measure of the amount of radiation dose generated by a source per unit activity. A high AKR value means that the radioisotope generates a high radiation dose rate per unit of radioac- tivity. Radiation dose can be reported in several different units; for example, as rads/hour (R/h) or millisieverts per hour (mSv/h) at 1 m or 1 ft from a source; or per curie (Ci), per megabecquerel (MBq), per gigabecquerel (GBq), or per terabecquerel (TBq). (1 Ci = 37 000 MBq = 37 GBq = 0.037 TBq). ANSI/HPS N43.9-2015, Gamma Radiogra- phy: Specifications for the Design, Testing, and Performance Requirements for Industrial Gamma Radiography System Equipment Using Radiation Emitted by a Sealed Radioactive Source is the American national standard for gamma radiogra- phy. It lists the industry-accepted AKR values applicable to radiography sources and states that these values apply to the output activity of sources, not to the content activity (Table 6). Different values of AKR constants may be found in published literature for any given radioiso- tope (Eckerman et al. 2012; Jelfs 2009; Ninkovic and Adrovic 2012; Plexus Scientific n.d.; Smith and Stabin 2012; University of Florida 2011). This is because the published values may or may not consider dose rate from low-energy emissions such as X-rays, internal conversion electrons, or auger electrons, which can generate significant radiation doses at a distance if the radioisotope is unshielded or if the dose rate is measured in vacuum or air. If a radioisotope is in the form of a sealed source, its materials of construction partially attenuate its emissions, reducing its AKR. Some published AKR constants significantly overestimate the emitted dose rate from an encapsulated sealed source.

Table 6 Industry-accepted air kerma rate values for the output activity of sources (ANSI/HPS N43 9-2015)

Radioisotope

Air kerma rate (AKR)

3.5 × 10 -4 mSv/h/MBq at 1 m 3.78 × 10 -3 mSv/h/MBq at 1 ft

Co-60

13 mSv/h/Ci at 1 m 140 mSv/h/Ci at 1 ft 1.3 R/h/Ci at 1 m 14.0 R/h/Ci at 1 ft

1.3 × 10 -4 mSv/h/MBq at 1 m 1.4 × 10 -3 mSv/h/MBq at 1 ft

Ir-192

4.8 mSv/h/Ci at 1 m 52 mSv/h/Ci at 1 ft 0.48 R/h/Ci at 1 m 5.2 R/h/Ci at 1 ft.

5.4 × 10 -5 mSv/h/MBq at 1 m 5.9 × 10 -4 mSv/h/MBq at 1 ft

Se-75

2.0 mSv/h/Ci at 1 m 21.8 mSv/h/Ci at 1 ft 0.20 R/h/Ci at 1 m 2.18 R/h/Ci at 1 ft

3.3 × 10 -6 mSv/h/MBq at 1 m 3.6 × 10 -5 mSv/h/MBq at 1 ft 1.25 mSv/h/Ci at 1 m

Yb-169

13 mSv/h/Ci at 1 ft 0.125 R/h/Ci at 1 m 1.3 R/h/Ci at 1 ft

Radiographers use the values listed in ANSI/HPS N43.9-2015 , which have been widely accepted and used by the industry to represent the dose rate per unit output activity. The values are used to calculate “shot time” at a known distance from imaging media using the output activity value that has been certified by a manufacturer. The AKR is also used to assess the radiation hazard when setting up a safety exclusion zone around a work site. Content Activity and Output Activity All emissions from sources are partially internally attenuated. Due to this, the content activity of a source will always be higher than its output activity. It is possible to determine the content activity of a source if the internal attenuation coefficients of all its emissions are known. These can be calculated by computational modeling. Using such coefficients, a manufacturer can derive an adjusted value for an AKR constant that would apply to its content activity, rather than its output activity. The dose rate from a source is constant. It doesn’t change if activity is reported as content activity or output activity, and AKR is the dose rate per unit

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Part 3