CHAPTER 17
other materials in which radioactivity has been induced by neutron
capture. In the literature, early radiation has been called
"initial," and has been rather arbitrarily defined as all radiation
emitted within the first mimite.
Such a definition may be true for
water-surface bursts, but cannot hold for underwater bursts and conform
with the above definition of residual radiation, since the base surge
may be clearly distinguishable and the fission products in the surge
may be emitting radiations by 30 sec after burst.
Therefore, this
report defines "fireball-plume-cloud radiation” as above, with no
fixed time limit.
For brevity,
the initials, F.P.C. radiation,
will be used in following discussions.
F.P.C. radiations of significance to the total nuclear-radiation
exposure dose for surface or very shallow underwater bursts include
(1) prompt gamma rays and prompt neutrons emitted at the time of
fission or fusion; (2) gamma rays resulting from inelastic scattering
of neutrons; (3) nitrogen-capture gamma rays; (4) early time fissionproduct gamma rays.
The prompt ganmmas and neutrons are liberated in
the process of fission or fusion in a time of less than a microsecond,
and are thus emitted at ea time when the bomb is still almost completely
compacted.
Most of the prompt gamma rays are absorbed by the bomb
fy
materials and casing and thus do not contribute significantly to the
total F.P.C. radiation.
Although many of the prompt neutrons are
slowed down and captured by the bomb residues, a significant number of
neutrons escape to the atmosphere.
As these neutrons traverse the atmosphere, they may undergo either
capture or scatter reactions with atomic nuclei along their paths. If
neutron capture occurs, the energy of the captured neutron raises
that of the capturing nucleus, and the excess energy of the nucleus
may be emitted as ganma radiation.
In the two types of scattering
collisions, the incident neutron loses part of its energy to the
struck nucleus, and a neutron degraded in energy results from the
reaction.
Inelastic scattering occurs when part of the kinetic
energy of the incident neutron is converted into excitation energy of
the struck nucleus. This energy is then emitted as gamma radiation.
Elastic scattering occurs when a portion of the neutron kinetic energy
is transferred to the struck nucleus. In this case the total kinetic
energy of both particles after collision is the same as before, al-
though the energy distribution may be different.
The gamna rays resulting from inelastic scattering of those
neutrons that escape to the atmosphere can contribute significantly
to F.P.C. radiation, particularly for bursts of fusion weapons, where
large numbers of high-energy neutrons are emitted. The high-energy
nitrogen-capture gammas result from the nuclear capture reactions
between atmospheric nitrogen and prompt neutrons at or near thermal
energies.
The early-time fission-product gammas are emitted by
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