true fall-out pattern is likely to deviate from the idealized contours.
The total dose accumulated by an individual in a fall-out field
varies with the time of entry.
For example, a reading of 50 r per
hour in an area taken 50 minutes post detonation will decrease to
22 r/br at one hour.
If an individual remains in this area during the
period between H+30 minutes and .Ht+l hour, he will accumulate a dose of
16 roentgens.
If a reading is 50 r/hr at one hour and he enters at
B+l hour and stays 30 minutes, he will accumulate 20 roentgens.
This
difference at an early time post detonation is characteristic of the
rapid decay of a fission product field at early times.
However, in
another area with a reading of 50 r/hr at H+8 hours and a stay of 30
minutes, an individual will accumulate a dose of 24 roentgens.
Thus,
at later times with the same reading in roentgens per hour, a larger
amount of radiation is accumulated for the same time of stay.
The variation of the energy spectrum with time has been the subject of field evaluations at several test operations.
It was found at
castiett/ that a considerable fraction of the gamma energy in fall-out
is in the vicinity of 0.1 Mev.
After 10 days there is an accenturtion
in the regions of 0.5 Mev and 1.6 Mev.
- tatto radiations.
This is attributed to the Barto
As this decreases a concentration of the spectrim
in the 0.7 Mev region is noted.
The military situation requires a more detailed discussion of the
immediate fall-out area for a surface burst and the area around ground
zero for air bursts.
The radiological problems encountered in these
two areas will exert a strong influence on decisions for weapon employment, as well as on deployment of troops and exploiting the advantages
of the use of nuclear weapons.
Following an air burst, no radiological fall-out of consequence
will be encountered.. The dust swept up into the cloud from the blast
effect on the ground will fall out within a short period of time.
How-
14/ Cook, C.S., Fall-out Symposium (Confidential), Armed Forces Special
Weapons Project, AFSWP-895, SECRET Restricted Data.
45