1.3 THEORY
The gamma radiation emitted from a nuclear detonation may be divided into two portions:
initial radiation and residual radiation. The residual radiation may include radiation both from
fallout and neutron-induced activity. In this report, the radiation emitted during the first 30
seconds is termed initial radiation, and that received after 30 seconds is called residual radiation.
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 contributions to initialgamma radiation are from the fission-product gammas and from the neutron-capture gammas
resulting from the N* (n, y) N'5 reaction between device neutrons and atmospheric nitrogen.
The prompt gammas are nearly all absorbed in the device itself and are oflittle significance
TABLE 1.1
ENERGY PARTITION IN FISSION
Reference 8
.
Mechanism
Percent of Total
«es
Fission Energy
Kinetic Energy of
Total Energy
ta:
per Fission
pet
Mev
81
162
Fission Fragments
Prompt Neutrons
4
8
Prompt Gammas *
4
8
Fission-Product Gammas
2.7
5.4
Fission-Product Betas
2.7
Fission-Product
5.5
Neutrinos
Delayed Neutrons
0.1
0.2
100.0
200.0
Totals
5.4
11
*Mostly absorbed in the device
outside the device. The fission~product gamimas predominate at close distances (Reference 8).
The N'4 (n, y) N“ gammas becomeincreasingly 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 N“{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 from the fusion process with N“*, The radlation 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 availaole may be 10 times as great for fusion
as for fission, and therefore a large number of gamma photons are contributed by the N* (n, y)
N' reactions (Reference 9). However, because of the short half life, this gamma radiation
decays before it can be enhanced by the hydrodynamic effect. Gammas from the longer-lived
fission products are grcatly 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 treatmentgiven in
Reference 10.
1.3.2 Residual-Gamma Radiation. Residual-gamma radiation consists of fissfon-product
radiation from fallout and radiation from neutron-induced activity. The decay rate of the resid10