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NATIONAL INSTITUTES OF HEALTH CLINICAL STAFF
4C is a trivial fission product, but because
of the intense neutron source there is 2
nitrogen-neutron reaction giving *C. It was
estimatedseveral years ago that at that time
the amount of “C in the atmosphere was
roughly 70% greater than before the first
atomic explosion.
significant only in relatively short-term fallout because of its 8-day half-life, and for
many years it was not considered to be a
serious hazard. **] has about 4.5% yield;
it has a 2.5-hr half-life, so it is only im-
portant in very short-term fallout. #41 has
a substantial yield, 6%, and a 21-hr half-
life, so it is of some importance over the
course of perhaps a week. 485] is another
radioisotope of iodine; this has a 6.7-hr
half-life and is important only for a few
days.
*oSr, for example, has a 5.3% fission yield;
There are some other isotopes from fission explosions that must be considered.
One is #*7Cs, which still can be detected
in most exposed individuals in the Marshall
Islands. It is also a potential problem because of relatively long-term storage in
muscle. Cesium is further worrisome in
meat eaters such as Eskimos who eat caribou. There is cerium, which has almost a
8-yr half-life and a substantial fission yield,
and finally barium.
The amounts of these isotopes formed are
perfectly enormous. Just one example: Explosion of a megaton bomb of the fission
type produces enough radioactivity so that
if it is evenly distributed over 1,000 square
miles it will give in 1 hr a dose rate of the
that is, of 100 atoms of uranium, 5.3 atoms
order of 1,000 rads/hr. Hence, a 30-min ex-
24Na is another isotope that is largely an
induced isotope, and it is of particular importance in underwater blasts or blasts that
are at all close to seawater because of the
sodium in the ocean. Under these conditions, a substantial amount of *4Na, which
has a 1]4-hr half-life, can be formed, and
in short-term fallout this can be of some
importance.
There are three or four other isotopes
that are of importance for several reasons.
(if normalized for mass) end up as °°Sr, so
this is a significant fission product. It also
has a 28-yr half-life so that it will persist
for a long time. Finally, it localizes in bone
so that it can be of importancebiologically.
The iodine isotopes are clearly of importance. Let me give a few numbersto illustrate variations in fission yield with the type
of device and the magnitude of release of
radioactive
iodine.
For
#38U
and
high-
energy neutrons as the explosive device,
there is a 4.7% yield of #81]. With #*5U fission the yield is 3%, and for *85U with
thermal neutronsit is 2.9%, so that depending on the device there can be a fairly substantial difference in the amount of production of any given isotope; but in each case
81] ig an important product. ''I is clearly
posure to this amount of radiation would,
in general, be lethal. This is for a 1-megaton bomb, but you mayrecall that the
Russians exploded a 100-megaton bomb a
few years ago. The explosion to which the
Marshallese were exposed was of the order
of 15 megatons.
The lifetime of these fission products is
very complicated because there are literally
hundredsof isotopes formed, all of different
half-lives. There is, however, an empirical
“rule of 7” that states that at any given
time for the first 8 or 10 days the amount
of radioactivity remaining after a fission
explosion decreases by a factor of 10 after
7 hr, by a factor of 10? at 7? hr, and by a
factor of 10° at 78 hr. Therefore, if at 1 hr
there are 100 units of radioactivity, at 7 hr
Te
they decay, frequently in a very compli-
cated way. The average numberofisotopes
in a chain before any given fission fragment
reaches a stable isotope is through six different daughter generations.
There are a numberofisotopes of major
biological importance. First, there are two
induced isotopes that should be considered.
4
Annals of
Internal Medicine
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around it, producing a variety of isotopes.
Most of these isotopes are radioactive and
.
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