roughly spherical in shape, called the "fireball". An "air burst" is defined as one in which the weapon is exploded in the air at an altitude below 100,000 feet, but at such a height that the fireball (at roughly maximum brilliance in its later stages) does not touch the surface of the earth. For example, in the explosion of a 1-megaton weapon the fireball may grow until it is nearly 5,700 feet (1.1 mile) across at maximum brilliance. This means that, in this particular case, the explosion must occur at least 2,850 feet above the earth's surface if it is to be called an air burst.
1.33
The quantitative aspects of an air burst will depend upon its energy yield,
but the general phenomena are much the same in all cases. Nearly all of
the shock energy that leaves the fireball appears as air blast, although
some is generally also transmitted into the ground. The thermal radiation
will travel long distances through the air and may be of sufficient intensity
to cause moderately severe burns of exposed skin as far away as 12 miles
from a 1-megaton explosion, on a fairly clear day. For air burst of higher
energy yields, the corresponding distance will, of course, be greater.
The thermal radiation is largely stopped by ordinary opaque materials:
hence, building and clothing can provide protection.
.
 |
Burns from the thermal radiation at Hiroshima, two kilometer away from ground zero. The man wore a short-sleeves shirt at the time of the blast. Source: L'ULTIMO AIUTO. Le dimensioni mediche della guerra nucleare - International Physicians for the Prevention of Nuclear War - Gabriele Mazzotta Editore - Milano 1983. 1.34 The initial nuclear radiation from an air burst will also penetrate a long way in the air, although the intensity falls off rapidly at increased distances from the explosion. The interactions with matter that result in the absorption of energy from gamma rays and from neutrons are quite different, as will be seen in chapter VIII. Different materials are thus required for the most efficient removal of |
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these
radiations; but concrete, especially if it incorporates a heavy
element, such as iron or barium, represents a practical compromise
for reducing intensities of both gamma rays and neutrons. A thickness
of about 4 feet of ordinary concrete |
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