chemical procedure, the counting technique, and the calculations involved in making a Sr-90
assay by the milking technique.
2.005 g Sr(NOz)2 were dissolved in 50 mi H.O, and to it was added approximately 22,000
dpm of an acid solution of Sr-90, Y-90 tracer in equilibrium.
A few ml of 85% H;PO, were next
added, followed by 0.304 gm of Nd™carrier in HCl solution. The solution was stirred and heated.
To the hot solution 2N NH,OH was added gradually with vigorous stirring until the precipitation of NdPO,.2H.O was believed to be complete.
(NdPO,.2H.O and YPO,.2H.O precipitate out
completely well below pH 1, while the phosphates of calcium and strontium do not begin to
precipitate until pH 3 or 4.) The precipitate was digested with heating for 15 minutes, filtered
onto a 7-cm filter paper, and washed with water. To the filtrate were added a few mi of 2 N
NH,OH. No precipitation occurred, showing that the precipitation of neodymium had been
complete. Additional NH,OH was now added to the same solution until SrHPO, was precipitated
completely. The SrHPO, was also filtered onto 7-cm paper. The precipitates were dried and
mounted on lucite semicylinders of 1%” LD.
The counting procedureis described in Section C below. The mounted samples were counted
on a gold foil covered Q-gas flow counter of 142” diameter and 10”active length. The geometry
factor for the position that was maintained between the counter and the sample was previously
determined with use of a similarly mounted sample of KC! and was found to be 2.50.
The activities in the two samples were followed for over 10 days. The activity in the
neodymium sample decayed from an initial rate of 4500 cpm with a constant half life of 61+1
hours into a very long lived tail of 102 cpm. An aluminum absorption curve for the same sample
determined within a few hours after milking was found to be absolutely straight to 360 mg/cm?
and gave a half thickness value of 133 mg/em?.
When the Y-—90 activity was subtracted out, the aluminum absorption curve for the strontium
sample, which was also determined soon after milking, also was a straight line as far as 40
mg/cm?.
The half thickness value was 12.0 mg/cm’.
The intensities and identity of the activities in the strontium and neodymium samples corrected to milking time were as follows:
Strontium sample
....
Y-90 activity, com
.
Neodymium sample. .........
Applying the equation,
Sr-90 activity, cpm
263
.
IG s/s’
4540
1068
102
esi"
DTes
where
D-The disintegration rate of the sample in dpm
I=The counting rate of the bare sample
G=The geometry factor=2.50
S=Thickness of air between sample and counter plus
thickness of counter wall (sum—2.70mg/cm?’)
S== Thickness of the sample in mg/cm?
The following results were obtained:
Strontium sample
Neodymium sample
Co,
.
8% , where s% is the half thickness value of the
“0.693 activity in mg/cm?.
8
60.9
15.1
s
17.3
192
D(¥-90)
658
10,970
D(Sr-90)
11,320
447
11,628
11,767
A very satisfactory agreement between the disintegration rates of Y-90 and Sr-90 was
obtained. The recovery of the Y~90 activity from the solution is seen to be 94% and the percentage of Sr-—90 carried over in the necdymium precipitate as contamination is seen to be 3.8%.
24
Gaiam
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