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

Preparation
The biological samples were preserved by freezing. The sediment and
water samples were stored in polyethelene bags and bottles, respectively,
until analyzed bythe Radiological Health Group.
Sediment samples were oven-dried for 48 hours at 110~C. The dried cake
was then broken up in a motor-driven mill and the granules were placed in
one-pint plastic freezer containers. Each container was filled with 330 cubic
centimeters of sediment. The mean density of dried sediments was approximately 1.6 g/cc (range 1.0-2.2 g/cc).
The two-liter water samples were acidified with HC1 and evaporated to
dryness on 2" diameter planchets. The residue, consisting of dissolved and
suspended solids, was then counted for activity.
Samples of phytoplankton, zooplankton and benthos were defrosted and
placed on a Whatman No. 1 filter to remove excess water. The organisms were
then scraped from the filter, placed in a crucible, weighed, oven dried for
16 hours at 110~C, and then reduced to ash at 550~C. The ash was transferred
to planchets, weighed, and counted for radioactivity.
Fish samples were defrosted in a crucible. They were then dried, ashed,
and transferred to planchets in the same manner as the other biological samples.
All samples were counted for 8,000 seconds on a 5" x 5" NaI (Tl) scintillation crystal connected to a 256-channel Nuclear Data Series 2200 spectrometer.
The gamma-ray spectra were transferred to punched paper tape and then to computer magnetic tape. The radionuclides which could be identified in sediment
spectra were Ra-226 and its daughters, natural K-40, and Cs-137. Only Zinc-65
and Cs-137 were observable in water and biological samples.
Gross beta measurements of water and biological specimens were made with
a Beckman Low Beta II thin-window (0.5 mg/cm2) proportional counter. The
length of the beta count was 3,000 seconds.
Calibration
Gamma Spectrometer for Planchet Sources
A description of the calibration procedures for planchet sources and a
summary of the least squares computer program used to analyze gamma-ray spectra
are presented in the Appendix, page A-29.
It is difficult to specify the minimum detectable activity (MDA) of each

A-20