UPTAKE OF RADIOACTIVE NUCLIDES
ELL
ditions. No
e under experimental con
tors which
logical studies of the fac
ve study
ati
ther species. The compar
m, strontium,
ths, magnesium, calciu
e aspects of
omise of elucidating som
in plants.
ed
err
nsf
tra
absorbed and
polonium
and
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yea
19
life
210 (half
appreciof
s
-wo radioactive substance
radium.
of
ion
nat
222, the gaseous ema
, and has been
), emits alpha radiation
grass grown
of the alpha activity in
this is mainly
has been suggested that
which escape
on
jecay products of rad
igations provide
osphere. Other invest
(2).
{ isotope from the soil
dating” was previously applicable only to samples of considerable antiquity
it is now possible to study the time-course of those carbon cycles which turn
over rapidly in nature, for example the formation of organic matter in soil.
The laborious techniques involved will, however, limit the exploitation of
this opportunity.
OTHER RapIOACTIVE NUCLIDES
Fission products—Apart from strontium 90, caesium 137 and iodine 131,
mixed fission products contain about 180 other radioactive nuclides. How-
ever, if the mixture is deposited in fission yield, strontium 89 (half life
51 days) is the only product which is likely, at any time, to be a more
important source of internal radiation than strontium 90, caesium 137 or
iodine 131. Up till about 6 weeks after fission it may deliver a larger dose
than strontium 90 but it soon becomes of little importance and it contributes
little to the-exposure of the population from world-wide fallout (2). In
fresh fallout, barium 140 (half life 12 days) and short lived isotopes of
ion 14
the atmosphere
n-made, is formed in
cosmic Tays;
e is due to the action of
mainly
formed
neutrons,
of
ae release
carbon 14 and
tive amounts in which
the ratio of total
weapons depends on
14 will become
n yield. Thus, carbon
are €X~larger or “cleaner” weapons
Although
to the extent of fission only.
the evalua>ms through photosynthesis,
since the
lems for the plant physiologist
caratmospheric
the
of
vill reflect that
carbon dioxide
stivity of atmospheric
an important too
1ay, however, provide
(90) has caused
by
rch, The work of Lib
known. Until
ely
wid
ome
dating” to bec
s occurred in
‘oxide from fossil source
bon dioxide 1s
ctivity of atmospheric car
; thus the date at
a considerable period
from its specific
ised could be calculated
years). In the
70
(55
14
life of carbon
carbon dioxide
ctivity of the atmospheric
troposphere
the
in
ted that the mean value
l level, but
ura
nat
the
ve
_few per cent abo
are to re
ses
rea
inc
r
the
Fur
.
) per cent (2)
pounds
com
anic
activity of carbon in org
y convar
l
wil
synthesis in the last decade
whereas
. of formation. Thus,
291
carbon
iodine, for example, iodine 133 (half life 21 hours) may enhance the
exposure of the bone or thyroid, though not being the major sources of
exposure.
Fission products are not, however, always released in fission yield.
Their ratio in the effluent from nuclear establishments may be much altered
by chemical separation. This may cause nuclides which are relatively immobile in biological systems to assume particular interest. Thus, for example, the limits to the release of dilute effluent into the sea at Windscale
Works are largely determined by the extent to which ruthenium 106 (half
life 1 year) is absorbed onto the marine alga Porphyra which is consumed
by limited population groups in Wales (91).
The absorption by plants of nuclides of relatively long half life, other
than strontium 89 and 90 and caesium 137 has been examined, especially
ruthenium 106, zirconium 95 (half life 63 days) and cerium 144 (half life
290 days) (92). All these substances enter plants from the soil considerably
less readily than strontium 89 or 90 though they can be readily adsorbed
onto the surfaces of roots from solutions. Between soil types the absorption
of ruthenium varies widely but it is often of comparable order to that of
caesium 137; except from acid soils, the absorption of cerium is extremely
small. These relationships are to be expected on account both of the valency
of these ions and their other chemical characteristics. Not only is absorption small, but the major fraction of that which enters plants is retained in
roots or stem tissues,little reaching the leaves.
Induced activities——Radioactive isotopes of many elements can be induced by neutron capture. This occurs both in nuclear weapons tests and
in nuclear reactors. The latter is likely to be of particular significance when
large amounts of cooling water are drawn from natural sources and returned to them. Thus at Hanford Works the induced activities of phosphorus 32 (half life 14 days} and zinc 65 (half life 245 days) rank next
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