~ 20 =
The differential rates at which the fallo - has been
occurring probably are best measured by the so-call: 2 "pot
collection" method.
A bucket with vertical wails o: appreci-
able height is placed out in the open and allowed to collect
the total fallout for a given period including the rain, snow,
dust, etc, The bucket is left out whether it has rained or
not and covers the total fallout for a given period.
Figures
6 and 7 give the curves so obtained for New York and Pittsburgh
areas together with the estimated errors of measurement.
It
is interesting to note the changes in slope and to correlate
them with the occurrence of test activities and the relatively
short-lived tropospheric fallout. The minimum slopes which
appear during quiet periods when no one is testing are the
stratospheric fallout of which we heave spoken and these
Slopes when we have enough pots operating all over the world
will, when taken together with the results of the measure»
ments of the amounts of radiostrontium and radiocesium in
the.stratosphere, give an accurate value fer the stratospheric residence time and settle the mixing question.
In addition to the intensity of fallout, the ques-~
tion of the fraction of the radiostrontiwa, and, fer tropospheric falicut, the radioiodine of sight-day half-life, that
is in assimilable form is an important one.
So far most fallout strontiyun appears to be completely water soluble and
therefore most assimilable, though continued tests cn this
point should be made.
Direct leaf pick-up of course promotes
assimilation of the strontium because the plant differentiation
against strontium when it assimilates it from soil thus is
avoided. Another factor is, of course, the concentration of
available calcium in the sozrl. By szvailable calcium we mean
calcium which is available to rlants and not che tetal calcium
in the soil.
I% is known that soils which are high in available calcium produce plants cf lower radioactive strontium
content; that is, the radicactive strontium to calcium ratio
in the plant is lower as a direst consequence of the lower
concentration of radiostrontium in the available soil caiciun.
In addition, as mentioned previously, plants tend to prefer
calcium to strontium with a discrimination facter of about
1.4.
Sheep which grow in certain areas of Wales have shown
concentrations in their bones approaching 150 Sunshine Units,
while sheep and cattle growing in the U. S. have hardly ever
exceeded one-fifth of this. The Welsh soil in certain areas
oD
ee
et
be
FF
«
©
i
£F
.
«©
4
£
1
&€
”
EB
™
ra
(more)