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Disagreement between numbers often is expressed by the ratio of the
measured to manufacturer value. However, caution is advised, as the
uncertainty in each value must be considered. For example, 192Ir seed
activities commonly are expressed with standard deviations as large as
7%. Similarly, a batch of 137Cs pellets in an LDR remote afterloader
has activities matched to within ±5% (Figure 17). Experimental
measurements made of only a sample of sources from a batch must be
evaluated considering the stated spread of source activities in the batch.
Figure 17. Relative activities of 18 137Cs pellets in a Selectron LDR.
Measurements in a well ionization chamber confirmed the
manufacturer’s stated relative activities were within ± 5% of the mean
activity of the 18 pellets. (Courtesy of G. P. Glasgow, Maywood, IL).
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