of decreasing fall-out (Model A)
predictions based on the two
sively, the integrated values
xbout 10 and 40 per cent higher
by equation (1). For Model B
on differ to a smaller extent
»e still smaller if an increasing
ied.
ium-137
sium-]37 is usually subject to a
ition’ in the soil on account of
ice structure of clay minerals
roots muchless readily. It has
t fali-out is the only significant
odstuffs, but this was disproved
y countries the decreasing rate
aller reductions in the levels of
would have been expected if
ely by the current fall-out}.
uation seemed to be provided
137 can enter plants relatively
tain large quantities of organic
inderlying permanent pastures
‘acteristic, and experimental.
ied that caesium-137 continues
s of pastures relatively freely
s deposition??. This led to the
n similar to (1) might be applicctor related: only to the deposit
8, namely :
PF, + paFa
(3)
nean concentration of caesium-
*), fF, is the annual deposit of
r), and Fa is the deposit of
over the preceding two years
of the proportionality factors
ly higher than those for stron.37 is transferred from the diet
‘e times as readily as strontiumsium-137 in milk between 1960
quation (3) agreed reasonably
rved (Table 5) and it was found
caesium-137 in man could be
all-out in a similar manner!*:?9,
wrtionality factors were applied
the calculated level in milk
o that which was observed
ner in which caesium-137 is
: herbage and removed from it
that of strontium-90 (ref. 25),
. lag-rate factor sumular to that
6
used for strontium-90 in equation (2) might be applicable.
The following equation, which differs from equation (2)
only in units used, was therefore applied to the survey
data for caesium-137 from 1960 until 1964:
C = pF, + pki + ‘paFa
(4)
where C is the 12-month mean concentration of caesium137 in milk (pe./l.), #, is the annual deposit of caesium137 (me./km?/year), Fi is the deposit of caesium-137
{mc./km?) in the last half of the previous year, and
fg is the cumulative deposit of caesium-137 in the soil
(mc./km*). Values for the rate and lag-rate factors
derived from survey data by least squares analysis are
shown in Table 4; the soil factor appeared to be zero.
Calculated values for the mean level of caesium-137 in
milk for each year based on these rate and lag-rate factors
agreed more closely with those observed than when equation (3) was used (Table 5).
Although the analysis of the survey data provides no
evidence that the cumulative deposit of caesium-137 has
contributed
hitherto
to
the
contamination
of
milk,
experiments with tracer levels of .caesium-137 indicate
that this nuclide continues to enter the roots of plants,
though to a smal] extent, for many years after its entry
into the soil. Thus if equation (4) is to be used for pre-
dicting levels of caesium-137 in milk in future situations
when the cumulative deposit may exceed the annual rate
by much larger factors than has occurred hitherto, it is
necessary to consider the possible magnitude of the soil
factor. Some basis for so doing is provided by the results
of experiments on several British soils; 3-5 years after
they had been contaminated the ratio in which caesium137 and strontium-90 were absorbed by plants was on
average only one-fortieth of that in the soil’*. Using
Table 4.
ESTIMATES OF PROPORTIONALITY FACTORS FOR THE TRANSFER OF
CAESIUM-137 TO MILK IN THE UNITED KINGDOM
Equation 3
Equation 4
Rate factor (p,)
(pe. moa”per mc./km3/year)
357
3:00
Lag-rate factor (pt)
(pe, 1708/1. per me./km?in
—
6-00
second half of previous year)
Soil factor (pa)
0-65
g3*
(pe. iorelt1, per me./km?*)
(relative to deposit (relative to cumulain Previn years
tive deposit)
omy
* Unlike the other factors, which are based on the analysis of survey data,
the soil factor-in equation (4) has been deduced from experimental resulta.
Table 5. COMPARISON OF CALCULATED AND OBSERVED LEVELS oF CaESIUM-197
In MILK IN THE UNITED KINGDOM
Deposition of caesium-137
Caesium-137 in milk
Year
(mc./km? )
Observed Calculated values as
January
July to
Total
(pc./1) percentage of observed
1959
1960
1961
1962
1963
1964
to June
December
10:7
7
12
8-0
14:3
14-3
31
15
24
8-0
14-4
8-0
Equation 3 Equation 4
13-8
a2
3-6
16-0
28-7
22-3
7
_
26
21
62
135
163
—_—
98
116
100
86
71
_
106
96
101
100
100