1.3 THEORY
The gammaradiation emitted from a nuclear detonation may be divided into two portions:
initial radiation and residual radiation.
fallout and neutron-induced activity.
The residual radiation may include radiation both from
In this report, the radiation emitted during the first 30
seconds is termedinitial radiation, and that received after 30 seconds is called residual radia-
tion.
1.3.1 Initial-Gamma Radiation.
For a fission-type device the initial radiations are divided
approximately as shown in Table 1.1 (from Reference 8).
The major contributionsto initial-
gamma radiation are from the fission-product gammas and from the neutron-capture gammas
resulting from the N* (n, y) N® reaction between device neutrons and atmospheric nitrogen.
The prompt gammas are nearly all absorbed in the deviceitself and are of little significance
TABLE 1.1
ENERGY PARTITIONIN FISSION
Reference 8s
Mechanism
Percent of Total
Total Energy
Fission Energy
per Fission
pet
Kinetic Energy of
Fission Fragments
Prompt Neutrons
Prompt Gammas *
Fission-Product Gammas
Mev
81
162
4
4
2.7
Fission-Product Betas
Fission-Product
:
]
8
8
5.4
2.7
5.5
5.4
11
0.1
0.2
100.0
200.0
Neutrinos
Delayed Neutrons
Totals
eal
* Mostly absorbedin the device
outside the device.
The fission-product gammas predominate at close distances (Reference 8).
The N“ (n, y) N° gammas become increasingly important at greater distances and eventually
become the major contributor. This applies only to devices with yields of less than 100 kt, in
which the hydrodynamic effect is small. Figure 1.1 shows the contribution from fission-product
gammas and nit (n, y) N' for a 1-kt surface burst.
Therefore, the fission products become a
more important source of initial-gamma exposure from high-yield fission-fusion devices at
greater distances.
For thermonuclear devices, in addition to gamma radiation from fission-product gammas, it
is necessary to consider the interaction of neutrons fromthe fusion process with N‘*, The radiation caused by the fusion process may vary over wide limits, depending on the design of the
device, For a given yield, the number of neutrons available may be 10 times as great for fusion
as for fission, and therefore a large number of gamma photons are contributed by the ni‘ (n, y)
N® reactions (Reference 9). However, because of the short half life, this gammaradiation
decays before it can be enhanced by the hydrodynamic effect. Gammas from the longer-lived
fission products are greatly enhanced by this effect. Therefore, fission products are the most
important source of initial-gamma exposure resulting from high-yield fission~fusion devices.
The preceding discussion is also in essential agreement with the expanded treatment given in
Refcrence 10.
1.3.2 Residual-Gamma Radiation.
Residual-gammaradiation consists of fission-product
radiation from fallout and radiation from neutron-induced activity.
10
The decay rate of the resid-