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

samples of density less than the standard. It was necessary therefore to
evaluate the magnitude of this error.
The error due to incompatible matching between the densities of the
sample and standard was studied by adding different amounts of stable ZnC12 to
acidified Cs-137 solution. The number of counts in the energy region 0.63 to
0.69 Mev was evaluated at densities of 1.98 g/cc and 2.48 g/cc and compared to
data at 0.99 g/cc. At 1.98 g/cc there were 9.5% fewer counts in the Cs-137
photopeak, at 2.48 g/cc, 13% fewer counts. Since our Cs-137 standard has a
density of 1.6 g/cc, all measurements of Cs-137 in sediment (1.0< p <2/2 g/cc)
have a non-reported error resulting from differences of density of approximately ~ 5%.
The K-40 sediment standard was made from 415.83 g of KC1. This is the
equivalent of 0.185 uCi of K-40 activity. The density of the K-40 standard
was 1.26 g/cc.
-3
The Ra-226 sediment standard contained 8.0 ~ 0.6x10 uCi. It was made
from radium-bearing manganese nodules separated from Lake Michigan sediments.
A U.S. Public Health Service calibrated Ra-226 source was used to compute the
radium activity of the nodules.
Counting Efficiency for Gross Beta Radioactivity
Figure A-6 is a plot of total counting efficiency for Cs-137 beta emissions
2
(Efficiency = cpm/Spm) versus sample weight in a planchet with 20 cm area. The
solids of environmental samples were simulated with NaCl. For each 100 disintegrations of Cs-137 there are 4.7 emissions of 1.180 Mev 6, 95.3 emissions of
0.518 Mev 6, and 9.4 emissions of internal conversion electrons or a total of
1.094 B per disintegration. (33)
In materials of low atomic number the range of beta particles is, to a
first approximation, a function only of the mass interposed in the path of the
radiation and the particles energy. (6)
The maximum range of a 0.5 Mev 6 in 2.2 g/cc concrete is 0.08 cm or 0.176
2 (11) 2
g/cm '. In a planchet of 20 cm area, this infinite thickness is equivalent
to a maximum sample weight of 3.5 g. Figure A-6 shows that there is little
difference in the counting efficiencies for samples with weights greater than
3.5 grams.

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