Lake Michigan environmental survey : final report / [edited] by John C. Ayers.

of less than 50%, i.e., a 2 sigma counting error of less than 100%.
The frequency of occurrence of X picocuries is given above the noted
activity. The frequency of occurrence for activities which are
not considered significant (50% < o < 99%), i.e., a 2-sigma counting
error greater than 100%, are given below the noted activity.
NOTE: In a frequency distribution such as this, the frequencies of
occurrence of all activities considered non-significant should be
given. However, it is not necessary to present the frequencies of
occurrence for those activities considered significant but of greater
activity than the maximum activity in the non-significant group.
3. Choose a minimum detectable activity (MDA) which separates reasonably the significant activity from the non-significant activity. The
MDA for Cs-137 by such an analysis is approximately 7x10 uCi, that
for Zn-65, 16x10 uCi per sample. Notice that statistical fluctuations in the sample activities permit the occurrence of a non-significant activity greater than the MDA as well as a significant activity
less than the MDA.
4. Find the minimum detectable level (MDL) in water by dividing the MDA
by 2000 milliliters.
MDLw = M uci/ml
5. Find the minimum detectable level (MDL) in a biological sample by
dividing the minimum detectable activity by the weight of the heaviest
sample in the group of samples. The MDA in the heaviest sample represents the lowest concentration of activity that can be measured.
MDL = MDAuCi/
B weight of heaviest sample uCi/g
Ra-226, K-40, Cs-137 Sediment Standards
-2
The Cs-137 sediment standard contained 5.1 ~ 0.2x10 uCi (one standard
deviation) of added activity. The density of this standard was 1.6 g/cc (330
ml of 0.5 N HC1 plus 198 g stable ZnCl2). Sediment samples which have a density
greater than 1.6 g/cc would have their Cs-137 concentrations reported incorrectly because of increased scatter of photons. The opposite situation occurs for

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