44
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
THEORETICAL CALCULATIONS OF THE GAMMA RADIATION SPECTRUM » ET,
at 1,000 yards, as represented by a histogram
Wilz,E) = Frac tion
e nergy
with
chart plotting fraction of total dose within the
energy range 4 HE against photon energy.
Note that the spectrum is a “hard” one with
prominent high energy contributions. Despite
some degradation through 1,000 yards of air,
many of the discrete source components of the
nitrogen capture radiations are still prominent.
of dose from source
» delivered hy photons
less
than Tl.
DANE}
=
i
The spectrum of Figure L1 has been calculated
for 1,500 yards, a range of some biological
interest for weapons in this yield range. Two
salient features are apparent init:
1. This is a very “hard" or energetic
spectrum indeed. Although these data are
calculated for spherical or 44 geometry, the
tendency of very energetic photons to scatter
Fraction
from
source energy E4, delivered
by scattered photons
womele
pibsteury
tne
tow
Bal Lye t0d
SOURCE ENERGY €j {Mer}
Frours 4,—Poini isotropic source, interpolation curves, at 1,000 yards.
manner previously developed.
In Figure 8 is arepresentative fission product
source spectrum used for these calculations.
At the times of gamma ray emission which are
of interest from the point of view of initial bomb
radiations, the fission product gamma source
spectrum can be characterized by an exponential expression as is seen here. (See also reference 4and Dr. Zobel’s paperat this conference).
It appears that the source spectrum corresponding too ~+ "cin the figure is the best one to use:
that is, the middle curve. For application to
the transport equation solutions the continuous
fission product spectrum presented here can be
approximated by a discrete distribution, if
desired. Note that on the logarithmic chart
of the figure, the vast majority of fission product photons leave the source with energies of
only afew Mevorless.
SOURCE ENERGY Ey (eed
Fraure 5.—Potnt isotropic source, interpolation curves, at 1,500 yards.
The decay scheme of excited N" is shown by
Figure 9.
[5].
The column listing relative
numbers of photons defines the source strength
swurce
lites
of the nitrogen capture gammas. Incontrast to
the continuousfission product spectrum, the nitrogen capture gammasource is seen to consist of
relatively few discrete types of photons, many
of which are exceedingly energetic, at around
10 Mev or more. It may be anticipated that
the so-called “‘hard’’ or energetic nature of this
nitrogen capture source will be reflected in the
gamma doso spectrum at various distances
Ot)
gamma spectra are normalized to known
weapons parameters, and then treated in the
from a nuclear device, and this will be further
indicated later.
Using the appropriate normalization factors,
initial gamma spectra can be calculated at
various distances from actual nuclear weapons.
As a representative example, a fairly typical
small yield weapon might generate gamma dose
spectra in air of the following nature:
Figure 10 showsthe differential dose spectrum
45
SOURCE ENERGY E,IMow)
Figure 6.-—Point isotropic source, interpolation curves, al 3,000 yards,