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MARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION
289
square mile. (Apparently, these estimates are based on the assumption that
only two thirds of the Sr-90 and Cs-137 produced are available for worldwide distribution and that the United States, because of its latitudinal
position, will receive about twice the world average.)
Rafter & Fergusson (1957) have reported that the C-14 content of wood
formed in 1957 was 6.7% higher (in New Zealand) than that of wood formed
in 1954. The atmosphere normally contains 2 x 10°° atoms of C-14 produced
by nuclear reactions between cosmic rays and nitrogen molecules (Broecker
and Walton 1959). Using the data cited by Dunning (1962) and Leipunsky
(1959), I have estimated that all nuclear detonations in the atmosphere
prior to January 1962 have produced approximately 27 x 1078 atoms of C-14.
If 10% of this total were absorbed by sea water and the remainder were uniformly distributed in the atmosphere, this would amount to an increase of
about 12% over background. The current (Jan. 1962) concentration of
bomb-produced C-14 is probably greater in the northern hemisphere than in
the southern, and most of the total in both hemispheres is probably confined
to the stratosphere. The time required for thorough mixing is unknown,but
it is probably short in comparison to the 5500-year half-life of C-14.
The estimated amount of H-8 produced by nuclear detonations is quite
impressive but may be somewhat misleading. While H-3 may make a major
contribution to the over-all increase of environmental radiation, its half-life
is relatively short; and it is so diluted by rain, by surface and ground
waters, and bythe water in plant and animaltissues that its concentrations
in organisms (Table 4) is generally quite small in comparison to the concentrations of other radionuclides.
While many of the preceding estimates are based on over-simplified assumptions, they should provide an indication of the contrast between natural and man-madesourcesof ionizing radiation in the biosphere and of the
relative concentrations of the former in air, soil, water, plants, and animals.
The sections which follow deal primarily with the initial distribution of
radionuclides produced by nuclear detonations in the atmosphere and with
the subsequent redistribution of these materials by environmental processes
and by organisms.
Formation and dispersal of fallout. FALLOUT FoRMATIoN. When a nuclear weapon is detonated, about 50% of the energy released is dissipated
as blast and shock, 35% as thermal radiation and light, and 15% as nuclear
radiation (Glasstone 1957). About one third (5%) of the nuclear radiation
is dissipated during the first 60 seconds after the detonation. The remainder
(10% ) maybe incorporated in or on the fallout particles which are formed
during the development of the familiar ‘‘mushroom’”’ cloud.
Immediately after the detonation (Fig. 1) an extremely hot, intensely
luminous ‘‘fireball’’ is formed. Because of its high internal pressure and