The most striking difference between the explosion of an atomic bomb
and that of an ordinary T.N.T. bomb is of course in magnitude; as the
President announced after the Hiroshima attack, the explosive energy of
each of the atomic bombs was equivalent to about 20,000 tons of T.N.T.
But in addition to its vastly greater power, an atomic explosion has
several other very special characteristics. Ordinary explosion is a
chemical reaction in which energy is released by the rearrangement of
the atoms of the explosive material. In an atomic explosion the
identity of the atoms, not simply their arrangement, is changed. A
considerable fraction of the mass of the explosive charge, which may be
uranium 235 or plutonium, is transformed into energy. Einstein's
equation, E = mc^2, shows that matter that is transformed into energy
may yield a total energy equivalent to the mass multiplied by the
square of the velocity of light. The significance of the equation is
easily seen when one recalls that the velocity of light is 186,000
miles per second. The energy released when a pound of T.N.T. explodes
would, if converted entirely into heat, raise the temperature of 36
lbs. of water from freezing temperature (32 deg F) to boiling
temperature (212 deg F). The nuclear fission of a pound of uranium
would produce an equal temperature rise in over 200 million pounds of
water.
The explosive effect of an ordinary material such as T.N.T. is derived
from the rapid conversion of solid T.N.T. to gas, which occupies
initially the same volume as the solid; it exerts intense pressures on
the surrounding air and expands rapidly to a volume many times larger
than the initial volume. A wave of high pressure thus rapidly moves
outward from the center of the explosion and is the major cause of
damage from ordinary high explosives. An atomic bomb also generates a
wave of high pressure which is in fact of, much higher pressure than
that from ordinary explosions; and this wave is again the major cause
of damage to buildings and other structures. It differs from the
pressure wave of a block buster in the size of the area over which high
pressures are generated. It also differs in the duration of the
pressure pulse at any given point: the pressure from a blockbuster
lasts for a few milliseconds (a millisecond is one thousandth of a
second) only, that from the atomic bomb for nearly a second, and was
felt by observers both in Japan and in New Mexico as a very strong wind
going by.
The next greatest difference between the atomic bomb and the T.N.T.
explosion is the fact that the atomic bomb gives off greater amounts of
radiation. Most of this radiation is "light" of some wave-length
ranging from the so-called heat radiations of very long wave length to
the so-called gamma rays which have wave-lengths even shorter than the
X-rays used in medicine. All of these radiations travel at the same
speed; this, the speed of light, is 186,000 miles per second. The
radiations are intense enough to kill people within an appreciable
distance from the explosion, and are in fact the major cause of deaths
and injuries apart from mechanical injuries. The greatest number of
radiation injuries was probably due to the ultra-violet rays which have
a wave length slightly shorter than visible light and which caused
flash burn comparable to severe sunburn. After these, the gamma rays
of ultra short wave length are most important; these cause injuries
similar to those from over-doses of X-rays.
The origin of the gamma rays is different from that of the bulk of the
radiation: the latter is caused by the extremely high temperatures in
the bomb, in the same way as light is emitted from the hot surface of
the sun or from the wires in an incandescent lamp. The gamma rays on
the other hand are emitted by the atomic nuclei themselves when they
are transformed in the fission process. The gamma rays are therefore
specific to the atomic bomb and are completely absent in T.N.T.
explosions. The light of longer wave length (visible and ultra-violet)
is also emitted by a T.N.T. explosion, but with much smaller intensity
than by an atomic bomb, which makes it insignificant as far as damage
is concerned.
A large fraction of the gamma rays is emitted in the first few
microseconds (millionths of a second) of the atomic explosion, together
with neutrons which are also produced in the nuclear fission. The
neutrons have much less damage effect than the gamma rays because they
have a smaller intensity and also because they are strongly absorbed in
air and therefore can penetrate only to relatively small distances from
the explosion: at a thousand yards the neutron intensity is negligible.
After the nuclear emission, strong gamma radiation continues to come
from the exploded bomb. This generates from the fission products and
continues for about one minute until all of the explosion products have
risen to such a height that the intensity received on the ground is
negligible. A large number of beta rays are also emitted during this
time, but they are unimportant because their range is not very great,
only a few feet. The range of alpha particles from the unused active
material and fissionable material of the bomb is even smaller.
Apart from the gamma radiation ordinary light is emitted, some of which
is visible and some of which is the ultra violet rays mainly
responsible for flash burns. The emission of light starts a few
milliseconds after the nuclear explosion when the energy from the
explosion reaches the air surrounding the bomb. The observer sees then
a ball of fire which rapidly grows in size. During most of the early
time, the ball of fire extends as far as the wave of high pressure. As
the ball of fire grows its temperature and brightness decrease.
Several milliseconds after the initiation of the explosion, the
brightness of the ball of fire goes through a minimum, then it gets
somewhat brighter and remains at the order of a few times the
brightness of the sun for a period of 10 to 15 seconds for an observer
at six miles distance. Most of the radiation is given off after this
point of maximum brightness. Also after this maximum, the pressure
waves run ahead of the ball of fire.
The ball of fire rapidly expands from the size of the bomb to a radius
of several hundred feet at one second after the explosion. After this
the most striking feature is the rise of the ball of fire at the rate
of about 30 yards per second. Meanwhile it also continues to expand by
mixing with the cooler air surrounding it. At the end of the first
minute the ball has expanded to a radius of several hundred yards and
risen to a height of about one mile. The shock wave has by now reached
a radius of 15 miles and its pressure dropped to less than 1/10 of a
pound per square inch. The ball now loses its brilliance and appears
as a great cloud of smoke: the pulverized material of the bomb. This
cloud continues to rise vertically and finally mushrooms out at an
altitude of about 25,000 feet depending upon meteorological conditions.
The cloud reaches a maximum height of between 50,000 and 70,000 feet in
a time of over 30 minutes.
It is of interest to note that Dr. Hans Bethe, then a member of the
Manhattan Engineer District on loan from Cornell University, predicted
the existence and characteristics of this ball of fire months before
the first test was carried out.
To summarize, radiation comes in two bursts--an extremely intense one
lasting only about 3 milliseconds and a less intense one of much longer
duration lasting several seconds. The second burst contains by far the
larger fraction of the total light energy, more than 90%. But the
first flash is especially large in ultra-violet radiation which is
biologically more effective. Moreover, because the heat in this flash
comes in such a short time, there is no time for any cooling to take
place, and the temperature of a person's skin can be raised 50 degrees
centigrade by the flash of visible and ultra-violet rays in the first
millisecond at a distance of 4,000 yards. People may be injured by
flash burns at even larger distances. Gamma radiation danger does not
extend nearly so far and neutron radiation danger is still more limited.
The high skin temperatures result from the first flash of high
intensity radiation and are probably as significant for injuries as the
total dosages which come mainly from the second more sustained burst of
radiation. The combination of skin temperature increase plus large
ultra-violet flux inside 4,000 yards is injurious in all cases to
exposed personnel. Beyond this point there may be cases of injury,
depending upon the individual sensitivity. The infra-red dosage is
probably less important because of its smaller intensity.