where
Rare = amount of instantaneous radiation required to give the
same effect on lethal scale.
(t) = Time of evaluation of R..,-
r(t,) = Rate at time (t,) of initiation of exposure.
B
= recovery constant, here assumed to be 10% per day or
0.0042 per hour.
Figure 13 is a graphic representation of the above equation for various
times of entry into a fall-out field (t,) up to 19 hours.
This scheme for calculating lethality expectation must be used
with caution, and must be restricted to the job it is intended to perform.
It will be noted that beyond a certain time the effective
roentgen value decreases.
This is not intended to signify that the
individual is able to continue receiving radiation without further danage.
It does signify that for a given radiation experience there is a
maximum
Rare at which point additional radiation will have its maximal
significance with regard to acute lethal effect.
It also signifies that
beyond the point of maximum Rofr the amount of additional radiation
needed to produce a given acute lethal effect does increase.
The as-
pect of damage to the individual which is a function of the total dose
continues to increase during the period of radiation beyond the point
of maximum Rape’
In a real sense, therefore, the biological damage
factor is a function of the total irreparable dose and the R,te is an
index relating lethality expectation of further radiation to the past
radiation experience of the individual.
However, it should not be over-
looked that R.fr is bounded by the lethal dose, and that once this
dose is reached, biological recovery factors no longer play a role, nor
does additional radiation have significance.
Figure 14 is a graphic representation of a fall-out situation
occurring at H+3 hours with an initial dose rate of 40 r/br leading to
a total integrated dose of 600 r which would be LD,69 if received instantaneously.
The upper broken line indicates the cumulative dose
67