THE NATURE OF AN ATOMIC EXPLOSION - 1

THE NATURE OF AN ATOMIC EXPLOSION
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 socalled
heat radiations of very long wave length to the socalled
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.