ASNT

of activity. If the content activity is reported, then its AKR =dose rate/content activity. This is smaller than if output activity is reported because its AKR =dose rate/output activity. As activity is the divisor, the higher AKR comes from the lower activity value that is stated. As an example, an adjusted AKR of 1.8 mSv/h/Ci at 1 m (0.18 R/h/Ci at 1 m) applies to the content activity of selenium 75 sources, whereas an AKR of 2.03 mSv/h/Ci at 1 m (0.203 R/h/Ci at 1 m) applies to the output activity (Shilton 2001). Direct measurement of the content activity of a source is difficult. It can, in principle, be done by measuring only the highest-energy emissions from a source using gamma spectrometry to minimize the self-attenuation effects or by using a sensitive microcalorimeter, which can measure the total heat output from a source; however, such measurements are overly time consuming and impractical to use in routine production. Manufacturers generally report the output activity of sources, not the content activity, because it is easier to measure. In some cases, both may be reported. Except for cobalt 60, the difference between content and output activity of radiography sources is significant. For an example, refer to Table 7.

Activity Limits for Transportation Transport regulations specify maximum limits for radioactivity that are authorized for shipment in Type-A packages. These limits apply to the content activity, except in the unique case of special-form iridium 192 (Table 8). In the International Atomic Energy Agency (IAEA) document Regulations for the Safe Transport of Radioactive Material : Specific Safety Requirements No. SSR-6 ( IAEA TS-R-1/SSR-6), there is an exception for iridium 192. The regulation contains the exception shown in footnote a. of Table 8. Radioisotope Content activity Output activity TBq (Ci) TBq (Ci) Co-60 0.4 (10.81) Ir-192 1.0 (27.03) a Se-75 3.0 (81.08) Yb-169 4 (108.1) a. The activity of iridium 192 in special formmay be determined from a measurement of the rate of decay or a measurement of the radiation level at a prescribed distance from the source. This caveat in the regulation means that uniquely, in the case of special-form iridium 192, source activity for the purposes of transportation can be determined by measuring the output from a source. For instance, internal attenuation need not be considered. Therefore, the output activity value can be used under SSR-6 transport regulations for purposes of transportation. This exception only applies to special form iridium 192, and not to normal form iridium 192 or other radioisotopes. For all others, the content activity must be stated on all regulatory documents, such as shipping manifests. Regulations require the content activity to be specified because it is assumed that in a transport incident involving a breach of containment, dispersal of radioactivity will cause fallout and contamination as a thin, unshielded contamination layer spread over a wide area. In this scenario, the related radiation safety hazard is determined by the radioactivity content and not by the output of a source. It can be argued that the exception applies to iridium because the material is highly inert and dispersal over a wide area is unlikely. Table 8 Special form Type-A1 transport limits

Table 7 Self-shielding of typical radiography sources

Apparent (output) activity

Typical % internal

Real

(content) activity

Radioisotope

attenuation

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

100 100 100

2-5%

102-105 150-180

35-45%

9-12%

110-114

10

15-30%

12-14

An iridium 192 source having a 3700 GBq (100 Ci) output typically contains 5550 to 6660 GBq (150 to 180 Ci) of content. The ratio between content and output may vary from one design to another; it depends on the materials of construction, their thickness, density, and focal dimension. Stack height and diameter of iridium 192 disks have a significant impact on self-attenuation due to the high density of iridium, at 22.56 g/cm 3 (1408.37 lb m /ft 3 ).

CHAPTER 3

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Part 3