162
RADIATION STANDARDS, INCLUDING FALLOUT
itself), and a 2.5 percent per year radiological decay rate. The same factors also
enter into the estimation of external gamma exposure from fallout of shortlived fission products. Assignment of numerical values to the above modifying
factors can be done only on the basis of very broad and highly uncertain assumptions. Since the last hearings, however, there is considerably more agreement as to the assumed values. Some authorities now accept the following factors: Terrain shielding, 0.8; structural shielding, 0.5; body absorption, 0.7; and
loss by weathering, 0.7; making an overall mitigating factor of ~0.2 when converting plane dose to population exposure dose. Gustafson (6) has estimated
that the effective external air dose from deposited Cs'” in 0.08 milliroentgens
per year per millicurie per square mile. This is the infinite plane dose, corrected for weathering and terrain but not for body absorption and shielding
by buildings. His values both for Csand short-lived fission products, multiplied by an additional factor of 0.25 (0.70.5) to give population exposure dose,
are used in the present report.
Eeternat short-lived fission products
Surface deposition of relatively short-lived gamma-emitting fission products
constitutes another component of population exposure from fallout. A list of
the principal isotopes with their respective radiological half-lives is given in
table 1. Because these isotopes emit penetrating gamma rays, they produce
wholebody exposure and constitute both a potential genetic and a somatic
hazard. Present knowledge of this particular component of fallout exposure is
considerably greater than during the 1959 hearings, largely through the efforts
of Gustafson, Martell, Machta, and others. Estimation of population exposure
from short-lived fission products is essentially the same as for external Cs137,
All of the assumed mitigating factors (leaching, weathering, shielding, etc.)
apply. One additional factor is highly important. Because of the relatively
short half-lives of the principal isotopes, the population exposure dose is critically dependent on the time it takes the nuclear debris to travel from the site
of detonation to the point of deposition. For this reason, the exposure dose
varies widely depending on whether injection is into the low polar stratosphere,
the low equatorial stratosphere, or at high altitude. Quantitative treatment of
the differences produced by these different methods of injection is given in
Gustafson’s most recent report (6), which is being inserted into the record of
the present hearings.
TaBLe i.—Significant short-lived gamma-emitting isotopes in fallout
Isotope
Haif-life
(days)
Zirconium 95..02-2--~--------____ 65
Niobium 95. -_-..__.-_-_-- 35
Ruthenium 103__-.-- ~~~ 40
Ruthenium 106__.-~.. 2-2 369
Teotepe
Hatf-life
(days)
Cerium 141-.---_-__ ao
Cerium 144/praseodymium 144____ 285
Barium 140/lanthanum 140_______ 14
Carbon 14
Capture of escaping bomb neutrons in atmospheric nitrogen produces C%,
which emits weak beta rays and has a radiological half-life of 5,700 years.
Carbon, however, is the basic element of all living matter, and the C™ is taken
into the body through the biological cycle and equally deposited throughout,
resulting in whole-body radiation and both a potential genetic and a somatic
hazard. About 2 X 10" atoms of C” are produced per megaton of total weapon
yield; however, about 95 percent of that produced becomes unavailable to the
biosphere through diffusion into the ocean reservoir with a half-time rather
uncertainly estimated as ~20 years. Most of the population dose is delivered
prior to establishment of equilibrium with the ocean reservoir and consequently
to the first generation after a weapon test. The 5 pereent remaining in biospheric equilibrium can, however, continue to contribute to the population
exposure for a mean time of about 8,000 years. This has caused some concert
over genetic consequences. The integrated genetic dose should, however, be
compared with the average natural background dose integrated over the same
period of time.
In these considerations, only the integral dose prior to establish-
ment of equilibrium is considered.
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