icted by the United Kingdom
y*®-3, by multiplying the mean
m all sites for which quarterly
» by the meanrainfall throughout
16 average ratio of caesium-137 :
is relatively constant, and the
7 has therefore been calculated
taking the ratio of caesium-137 :
| 1961 and 1-5 since then!®, The
s of radioactivity in milk were
of the Agricultural Research
Laboratory !4-2°,
ontium-90
‘or many years that strontium-90
t of both the ‘direct’ contamina-
her forage crops with airborne
2 of strontium-90 through plant
e magnitude of these two com-
ectively, to the recent rate of
ve deposit in the soil. Equations
e, therefore, been widely used to
tium-90 to calcium in milk to
nulative deposit of strontium-90:
°F, + paFa
Kingdom between 1958 and 1964 are compared with
those which would have been expected from the measured
deposit of fall-out on the basis of the proportionality
factors for equation (1), shown in Table 1, which were
derived by a least squares analysis of the results of surveys
between 1958 and 1961. The average agreement between
calculated and observed values was good (Table 2) but
for individual years there were appreciable divergences.
The calculated values were too low for years when the lag
effect was expected to be greatest because of high fall-out
in the previous year (that is, 1960, 1963 and 1964);
conversely, the calculated values were too high when the
rate of deposition had previously been relatively low.
An approximately linear relationship was found between
the magnitude of these discrepancies and the extent of
fall-out in the second half of the previous year. Only a
sraall improvement was obtained when the rate factor in
equation (1) was related to the deposition of fall-out in
the twelve months ending in June or September instead
of to that in the current year.
have been shown to provide a
g both the averagelevels of con-
1umberof countries and also the
Nonetheless, the relationship
fall-out and the contamination
nore complex than the equation
n that the soil factor is constant
: penetration of strontium-90
th will reduce its availability to
her defect is that the equation
‘act that an appreciable, though
diet of cattle consists of stored
as year; thus the rate of fall-out
» the extent to which the current
directly contaminated by aireas this ‘lag’ effect of the rate of
due to the absorption by plants
3 lodged in the basal tissues and
months without being incorportion (1) from this point of view
mM annual ratios of strontium-90
The following equation
was, however, found to give a significantly better fit to
the data (P < 0-01), the residual standard error being
reduced from 16 to 6 per cent:
(1)
1 mean ratio of strontium-90 to
7, is the annual deposit of stron4 is the cumulative deposit of
ldie of the year (mec./km?*), and
© ‘rate’ and ‘soil’ proportionality
2
to calcium observed in milk throughout the United
C= pF, + oli + paka
(2)
where F; is the deposit of strontium-90 in the second half
of the previous year (mec./km*) and p is the ‘lag-rate’
factor, the other symbols being defined as in equation (1).
Whenthe lag-rate factor was related to the deposit in the
whole of the previous year, or in the summer monthsonly,
a poorer fit was obtained. Proportionality factors for
equation (2), derived by least squares analysis, are
shown in Table 1. The levels of strontium-90 in milk
calculated on this basis agree closely with those observed
in each year (Table 2) and it is evident that equation (2)
describes the situation in past years more satisfactorily
than equation {1}.
The introduction of the lag-rate factor leads to a lower
value for the soil factor, and a amaller fraction of strontium90 in milk is thus attributed to absorption from cumulative deposit; it appears from equation (2) that strontium90 from this source was responsible for from 20 to 50
per cent of the contamination of milk in past years (Table
Table 1.
ESTIMATRS OF PROPORTIONALITY FACTORS FOR THE TRANSFER OF
STRONTIUM-90 To MILE IN THE UNITED KINGDOM
Equation 1 Equation 2
Rate factor (p,-)
(pe. Sr/g Ca per me./km?/year}
0-76*
0-70
Lag-rate factor (pr)
(pe. *Sr/g Ca per me./km*in
—_
118
second half of previous year)
Soil factor (4g)
(pe. "Sr/g Ca per me,/km?)
Q-19*
Oal
* Calculated from survey data for the period 1958 to 1961 (ref. 28}.
4