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Since your pancake probe has a 20 cm2 detection
area, your measured efficiency corrected activity is in dpm/20cm2. Since
surface contamination limits (e.g. Reg Guide 1.86, DOE Order 5400.5, 10CFR835 or
10CFR834(draft)) are given in terms of dpm/100cm2, it makes sense to multiply
your result by 5 to convert to dpm/100cm2. Yes it is a geometry
factor.
Note that this assumes that the area of
contamination exceeds 100 cm2. Indeed the regs specify that total
average surface contamination measurements be averaged over not more than 1
m2 if compared with the average surface contamination limits. Maximum
surface contamination measurements should be averaged over not more than
100 cm2 if compared with the maximum surface contamination limits.
If the object being measured and/or the area of
contamination is between 20 cm2 and 100 cm2, I would say you would be justified
in multiplying by an appropriate factor between 1 and 5 depending on the actual
area. If the object being measured and/or the area of
contamination is less than 20 cm2, one could argue that you should
proportionally reduce your measured valued by the ratio of the object or
area of contamination to 100 cm2. This would be somewhat analogous to
the guidance for removable contamination.
Although the last paragraph seems logical as I
write it, don't be surprised if your DOE masters disagree. My opinions
alone.
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