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C. Counting technique:
The absolute radioactivity measurements in the Chicago laboratories have been made with
cylindrical thin-wall flow counters. These are shielded with several inches of iron and with
anti-coincidence counters.
The large sample counters have been built after a design by T. Sugihara, R. Wolfgang and
W.F. Libby.*?) These counters have lucite end pieces connected by three thin brass rods at their
periphery. Thin plastic film 2.69 mg/cm’, with a conducting gold-coated surface, is used for
the counter wall.
Miniature flow counters have been constructed with field adjusting rings
at the ends and with external end supports (to avoid internal rods and their field distortion).
These counters have been covered with DuPont Mylar film coated on the inner side with 0.975
mg/cm? aluminum. The Mylar, a poly ester, is very durable, being resistant to moderate
physical stress and common organic solvents. The background for the large counters
(1%x6” to 10’) runs about 6 to 10 counts per minute within the shielding mentioned
above. The miniature counters (1.4x3.0 cm) have a background of 0.4 counts per minute.
(The counters with aluminum-Mylar walls are quite photosensitive, but are ordinarily operated
in the dark.)
Samples are mounted on plastic haif cylinders, two of which completely surround the
sample counter. The geometry is as good as is permitted by the necessity for placing absorbers
between the sample surface and the counter wall. For the large counters the geometry is
38% and for the small ones 33%. The corresponding factors used in conversion of cpm to
dpm are 2.7 and 3.0 respectively. These geometry determinations were made using potassium-40 as a secondary standard and using the figure 28 dps/gm K. Most powered samples
are mounted with the aid of dilute agar in alcohol and water, which provides a nearly weightless binder after drying.
The samples have usually been counted to a standard error of 10 to 15% where activity
is sufficient. If such precision would require an extraordinarily long counting period greater
error is allowed. Backscattering corrections are not included in calculations of rates of disintegration. The plastic sample mounts “reflect” only about 8% of the radiation striking them
and this backscattered radiation has only about half the energy of the original radiation.
Since the samples are of finite thickness, ca. 60 to 80 mg/cm?, most of the backscattered radia-
tion is absorbed. The backscattering contributes about 1% of the observed activity. This is
at least partially corrected for by the geometry determinations where backscattering corrections were not made.
Except for water, activities are routinely reported in “Sunshine units,” based on the ratio
of Sr-90 to elemental calcium in the sample. One “Sunshine unit” is defined for this purpose
as 1x 10~'* curies of Sr-90 per gram of calcium.
Freedom from contamination was demonstrated by analyses of samples of soil and powdered milk obtained by the Department of Agriculture in the years 1937 and 1943, respectively.
“These samples gave the following results:
Soil, sample No. C-2917, leached with NH,Ac _..
ae
.. . 0+0.05 S.U.
sample No, C-2916, leached with HCl after NH,Ac. coe ee) 0.05 8.
Powdered milk, 73.8 gms ash... 6. 0+0.08 8.U.
Results on recent samples of adult leg bones, while not serving as controls, were as low as
0.01+0.006 S.U. These indicate a low upper limit to possible contamination in processing this
type of material.
~~Footnote references on pp. 42-43.
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