(column 4) and in terms of the amount deposited per unit area (column 5). The amounts of Sr®®,
cs and Ce4 are expressed relative to the sr*° activity.
The cumulative curves of deposition of Sr®*® and Sr have been computed from the data in
Table 2 and are plotted in Fig. 2. It will be seen that the deposition rate of Sr’ has been main-
tained! and that the total deposition in 1956 was about 8% greater than that in 1955 although the
annual rainfall was 4% less.
7
— 4
—
6
—y4
5
12
—4 10
x
4
=
x
a
=
x
Oo 4
s4eai6
=
=
o
a
—__
wo
3
—1 6
2
— 4
{
a
a
ni
— 2
ba |
__]
Litt ti ee PP PPP EEE EE
MJJASONDIVFMAMJSASONDIVFMAMJ JASON DIVEMAMJ GA
1954
1955
1956
1957
=
Fig. 2— Cumulative deposition of Sr® and Sr?" at Milford Haven.
The specific Sr** content of the Milford Haven rain water has been plotted against time in
Fig. 3 in which the figures have been grouped in approximately two monthly intervals to smooth
out shorter period variations. The graph reveals a marked Seasonal variation in the specific
activity of the rain water, with peaks in the late spring and troughs in the late autumn of both
1955 and 1956 and in indication of an approaching peak in the late spring of 1957. There are
three possible explanations for such a result:
(1) The curve is primarily a function of the amount and of the type of rain falling during
the sampling periods, the controlling factors being dry deposition and the higher washout
efficiency of small showers (para. 3).
(2) The form of the curve is determined by the dates of weapon testing programmes, each
peak being associated with an immediately preceding series of explosions. This interpretation implies that the fallout is primarily tropospheric* and has been airborne for a relatively
short time.!
* Tropospheric fallout refers to radioactive dust particles which have never risen above the tropopause.
Stratospheric fallout refers to particles which have spent part of their life in the stratosphere; these return
ultimately to the troposphere to be deposited by the same processes as the tropospheric failout.
236