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