288
RUSSELL
The extent of fixation can be reduced by the addition of stable caesium
(56, 80) and sometimes by potassium (79). The latter observation suggests
that the process of fixation may be similar to that for potassium, though
some results suggest the contrary view (81). There is evidence that the
fixation process is not instantaneous and that the availability of caesium
137 may decrease progressively over two or more years (82). Thus, whereas
the results of short-term experiments suggest that caesium 137 usually
enters plants from typical temperate soil to about one-tenth the extent of
strontium 90, the corresponding value after three or more years may be
about one-twentyfifth (2). Until relatively recently it was assumed that
caesium 13/7 was fixed to a considerable degree in ail soils. This, however,
has now been disproved. Fredriksson conducted an extensive series of
experiments on soils from tropical areas in South America; many of these
soils contained very small quantities of clay minerals and no evidence of
fixation was obtained (64).
Entry of caesium 137 into plants from world-wide fallout—The relative
extent to which caesium 137 enters plants as a result of the direct con-
tamination of vegetation as opposed to absorption from the soil, has been
studied in considerably less detail than that of strontium 90. Because the
latter ion is considerably more freely absorbed from the soil, and the extent
of direct contamination of both is relatively similar, it would be expected
that the content of caesium 137 in plants would be much moreclosely related
to the rate of fallout in the immediate past. While this is, in general, borne
out by surveys of dietary contamination the levels of caesium 137 in milk
from some regions did not decrease in the manner expected, relative to
those of strontium 90, when the rate of fallout declined in 1960 (2). A
possible explanation is suggested by the two “delaying” mechanisms to
which reference has already been made, namely the retention of caesium
137 in the plant-base zone for a longer period than strontium 90 and the
slow rate at which fixation may proceed in the soil. Considerably more
detailed investigations must be undertaken before relationships between
the levels of caesium 137 in plants and both the rate and the cumulative
total of fallout can be elucidated, even to the extent which has been
achieved for strontium 90.
Tue URANIUM AND THORIUM SERIES
The most important naturally occurring alpha-emitting nuclides belon;
to the uranium and thorium series; the actinium series is much less abun
dant. Because of the complex decay schemes of these series, measurement
have frequently been reported as total alpha activity. Uncertainties as t
the extent of isotopic equilibrium can cause difficulties in the interpretatio
of such observations, but it is apparent that radium is the element prit
cipally absorbed by plants (2, 83, 84). Thorium 232, the parent member
the thorium series, has not been detected in living tissues; thorium 228 h:
been observed but its presence can be attributed to the decay of the pare’
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