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Gamma RADIATION Exposures FRoM FaLtoutr
et renee oo ene sO ry,
589
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ship for timed doses versus biological
effects; yet there are sufficient convincing
data to pernit an attempt at estimating the
effect of this phenomenon.
Blair (6, 7,) Smith (8), Davidson (9),
rt.
and others have made extensive analyses
of existing data on the effects of timespaced doses for several species oi animals.
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accumulated radiation dose for persons
living normally in a contaminated area
(Graph 3). Since Graph 3 is based on an
essumed dose rate of 1 r per hour at the
time of fafiout, the accumulated doses may
be linearly extrapolated to any other dose
rate at fallout. For example, if fallout befins at three hours after detonation and the
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Graph 4.
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OURATION OF EXPOSURE
TIMED DOSES AND BIOLOGICAL EFFECTS
It has been recognized that, in general,
the longer the period over which a given
radiation dose is delivered, the less is the
resultant biological effect, except for such
aspects as the genetic. Since past experiments usually have been designed for
other purposes, the data from these do not
readily elucidate the rate of repair or the
proportions of reparable and irreparable
damage resulting from differently timed
doses. Varying relationships have been
demonstrated, depending upon the species
or even the strain of animal, as well as the
criteria selected for study, such as skin
damage, life shortening, and LD 50 values.
Our present knowledge does not permit
establishment of a precise overall relationoat
po
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1000
fs
Ratio of total accumulated equally fractionated daily gamma whole-body doses to a one-day exposure
* to produce the same whole-body effects.
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dose rate at that time is 10 r per hour, then
about 90 r might be accumulated by personnel continuing to live normally in the
contaminated area.
~~
st pt
100
Generally, the recovery rate for larger
mammals, such as dogs,is significantly less
than for mice. One estimate places the
half-time recovery for man at four weeks
(9). The most conservative estimate of
the effect of time-spacing of doses, for
application to the problems under discus-
sion, is that of Davidson.
On the basis of
his analysis, a plot has been constructed
(Graph 4) of accumulated, equally fractionated daily doses versus an acute ex-
posure which would result in the same
whole-body effect (death or sickness).
This analysis indicates, for example, that
if a radiation exposureis divided into equal
daily doses, the total amount accumulated
over eighty days would be twice the
amount required by a one-day exposure to
produce death or sickness.
The calculations necessary to incorpo-
rate the factor of timed doscs into those for
radiological decay, weathering, and shield-
ing are rather tedious.
An approximation