II.
PHYSICAL ASPECTS OF THE FALL-OUT PROBLEM
A.
Radioactive Materials Formed In A Nuclear Detonation.
The quantity of radioactive material created as fissicn products in the detcnation of a fission weapon is known to depend upon the
total energy release of the weapon.
Some methods of calculation of
total fission yields utilize the ntmber of fissions per kiloton, which
gives approximately 1.38 x 10°) fissions occurring per KT of fission
yiela/, resulting in 2.76 x 10°? radioactive fission fragments per KT.
This value includes all of the immediate fission products, some of
which decay with very short half-lives, while others my give rise to
decay chains consisting of several isotopes, some of which may have
longer half-lives, resulting in their decay over a period of months or
years.
All emit one or more beta particles and most emit one or more
gamma rays.
There sare approximately 170 isotopes ultimately formed
from a total of 35 elements known to result from the fission process.
Some of these isotopes are formed directly from fission; others form
in part from direct fission and partly from decay chains.
If the same
isotope is formed by the two processes, the total isotope concentration
is the sum of the two.
Two exemples of decay chains which form the same
elements but different isotopes as indicated by their different halflives are:
Germeniun-77 pont Arsenic-77 win Selenium-77 (Stable)
Germaniun-78 ="———» Arsenic-78 Gs
Selenium-78 (Stable)
It is convenient to discuss the quantity of radioactive materials
formed in a nuclear detonation in terms of the value at one hour after
detonation, expressed in curies per KT.
At this H+l hour reference
time, there are approximately 3.0x 108 curies of gamma-active fission
1/ Spence, R. W., Bowman, M.C., Radiochemical Efficiency Results of
SANDSTONE Tests. Scientific Director's Report of Atomic Weapons
Test Annex 1. SECRET Restricted Data.
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