of 4&3, 758-775
(1957
Three types cf fallout are loca:, tropospheric (2-3
weeks), and stratospheric (10 years).
Fallout dissen-
ination is dependent upon firing conditions, i. e.,
Whether there is surface contact, the type of surface,
the height of the fireball, and the size of the weapon.
Fission products from a small bomb fired in Nevada
would fall in latitudes 10-60° N in about 1 month,
be SORIATNTSATEOPIHOO, ABR Re9Bt pitas.
Libby, Willard F.
RADIOACTIVE FALLOUT.
while those from a large bomb would fallout over the
whole earth in about 10
years.
About the same amounts
of radio-Sr and radio-Cs are produced. Radio-Cs data
show the fallout dissemination mechanism and point the
way to Sr-90 behavior. Radio-Cs fallout, except of a
local variety, is carried down largely in the form of
moisture droplets. There ig some pickup by leaves and
grass on surfaces. Some is captured and held tightly
by the top 2 inches of most soils so the water going
into rivers is purified. Because of discrimination
factors against Sr-90 relative to Ca from soil to
leaves (about 1.4), from leaves to milk (about 7)
and from milk to the human body (about from 2 to 8 >
the concentration of radio-Sr derived from milk vs. Ca
in human bone, is not over 1/20 and, possibly, as
little as 1/80 of that in topsoil.
Since a consider-
able portion of fallout may be picked up directly by
leaves, the factor 1.4 does not apply in this case,
and the total protection factor may be reduced to 14.
The high availability of Ca in topsoil reduces Sr
pickup by plants. The maximum permissible concentration of S-=90 for occupational workers and the general
population has been set at 1 and 0.1 pe., respective.
“
The value 1.7 micromicrocuries of Sr-90 per g. of body
Ca is called 00 Sunshine Units (S.U.).
At present,
adults have about 0.1 - 0.2 8.J. and children about
0.5 S.U. The difference in annual cosmic-ray radia-
tion dosage between sea level and 5,000 feet altitude
is equal to 8 S.U. Data show no correlation between
this radiation level and leukemia or bone cancer.
One pe (1,000 S.U.) 1s considered to be safe.
Factors,
other than Sr-90 effect, are general y-radiation effect,
mainly from fission products in local fallout area, or
tropospheric fallout.
The general average intensity of
fallout y-radiation from tests is 1-5 mor.
A brick house
can produce as much as 25-50 mr./year more than a wooden
one. Dosages received are about 1/100,000 of the amount
necessery to give injury symptoms (160-206 r.j}. Genetics
effects are in doubt.
14
i
;