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.