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

radionuclide in a composite spectrum, because each isotope may interfere with
all the others. Further, the background, which is subtracted from the gross
spectrum, changes from time to time. We have therefore devised the method,
explained in the Appendix, page A-36, to determine the minimum detectable activity for Cs-137 and Zn-65.
As a result of the analyses summarized in Table A-9, it appears doubtful
Table A-9
Minimum Detectable Activities and Minimum Detectable
Levels in Biological and Water Samples
MDL
MDA (uCi) Biological*(uCi/g) Water**(uCi/ml)
Cesium-137 7xlO6 l.OxlO-6 3.5x10-9
Zinc-65 16x10-6 2.2x10-6 8.0x10-9
*Weight of heaviest sample 7.2 grams of ash.
**For a sample volume of 2000 ml.
that Cs-137 or Zn-65 could be observed frequently in water samples of 2-liter
size from Lake Michigan. In Section II.B. the maximum Cs-137 activity in water
-9
from fallout is predicted to be 1.2x10-9 uCi/ml, only 1/3 the minimum detect-9
able level of 3.5x109 uCi/ml. Therefore, water samples would have to be at
least three times their present size in order to measure Cs-137 in Lake Michigan.
Since the expected Zn-65 activity in water is much less than the MDL is greater
than that for Cs-137, Zn-65 should also be observed infrequently.
Gamma Spectrometer for Sediment Containers
-5
The minimum detectable activity for Cs-137 in sediment is 6x10 uCi and
-7
the minimum detectable levle is lxlO uCi/g. All Ra-226 and K-40 activities
in sediment are very significant so it is neither possible, nor necessary to
determine for these radionuclides a minimum detectable activity or level. The
Ra-226, K-40 and Cs-137 sediment standards are described in the Appendix, page A-36.
Beckman Low Beta II - Planchet Sources
The chief advantage of gross beta analysis is its relative sensitivity
and simplicity in obtaining environmental trends of radioactivity concentrations.
In addition, it is useful as a method of choosing samples for specific analysis.

A-21