tate the oxalate of yttrium. About 15 minutes of digestion of this precipitate with heating results
in a well coagulated sample, easily filterable for the mounting technique to be described.
Milking of the phosphate samples was generally analogous to that described for carbonates,
except that the phosphate of yttrium was utilized as the first precipitate rather than the hydroxide. This step was somewhat more difficult since pH control was necessary to achieve
separation of the yttrium phosphate from the phosphate of calcium and strontium. By slow
addition of 2 N NH,OH to the acid solution of calcium phosphate to which yttrium carrier had
been previously added, with heating and vigorous stirring, complete precipitation of yttrium
phosphate occurred below pH of 1.5, while negligible amounts of calcium or strontium phosphate
appeared until about pH of 2.5. Thus care in thefirst precipitation was necessary to obtain good
separation. The precipitate was filtered and the filtrate stored for later milking. The precipitate was then treated like the yttrium hydroxide, dissolved off the filter paper, recrystallized,
barely redissolved, and by addition of oxalic acid, yttrium oxalate precipitated.
C. Sample mounting:
The technique used of mounting samples for counting was essentially that employed by the
Health and Safety Laboratory, p. 32. The oxalate precipitate was filtered by using a Fisher
Filtrator with the Tracerlab stainless steel sampie funnel and Whatman #42 14” diameterfilter
paper. The precipitate was washed with distilled water and dried by the vacuum. It was found
that about 5 minutes of standing in the funnel, with the vacuum pump on,sufficiently dried the
samples. The mounted samples were then dropped onto brass disks (Tracerlab) covered with
Pliofilm and sealed with brass rings. The resulting mount was found to be compact and sturdy
and greatly simplified the counting technique.
D. Counting method:
Counting is performed with a 1 inch Anton flat halogen-filled Geiger counter.
Shielding
consists of a complete ring of cosmic ray counters in anticoincidence with the sample counter
and an outer shield of lead brick. A special housing was constructed for this work, made entirely
of lucite to cut background toa minimum. The sample slides under the counter window and in
operation is about 2 millimeters from the window.
To illustrate the efficiency of the shielding,
the G.M. tube had a backgroundat the centerof its plateau of about 10 cpm when exposed on the
bench. Placed in the lead shield this was cut toabout6cpm. Activation of the anticoincidence
ring has cut the background to 2 cpm at which it has remained constant over a period of about
2 months.
The counter has been standardized with mounts of yttrium oxalate prepared from calibrated
standards of Sr-90 obtained from the National Bureau of Standards. Samples were prepared
by both the phosphate and carbonate method and yielded an efficiency of 31.6%+2.0%.
Hight
such standard samples have been counted over periods of several half-lives and have yielded an
average half-life of 64+1 hours.
Later runs on these samples have shown that less than 1% of the total activity was due to
Sr-90 carried through the sample preparation. For example, one sample which initially counted
about 1100 cpm decayed with a half-life of 63+1 hours. Again checked about 3 weeks after
preparation, this sample counted 9 cpm.
E. Outline for analytical procedures:
a. Procedure for the determination of Sr—90 in water samples.
1. Add carrier strontium.
until completely in solution.
(SrCl.6H.O about 20 gms. per gal. water).
COnmees
Stir thoroughly
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