All nuclear weapons results in fallout, but the amount of fallout depends on the height of the explosion. Thermal radiation
causes a fireball to be created. If the fireball touches the ground, soil, dirt, and debris are taken into the fireball. These
particles mix with radioactive materials in the bomb. Then these pieces fall to the ground and constitute the local radioactive
fallout. But if the bomb explodes in the air, then the fallout decreases. The fallout is a radiation hazard long after the
explosion occurs. The dosage rates from the fallout at various times after the explosion are shown in Table 3. As
time goes on, the dose rate decreases.
Radiation from Neutrons: The initial number of neutrons produced
per unit of explosive power as well as the energy distribution is different in fusion and fission bombs. In fusion reactions,
the number of neutrons produced is about ten times greater than fission reactions. After the bomb breaks up, the neutrons
collide with the atomic nuclei in the materials of the bomb. The energy lost in these collisions is captured by nuclei that
emit gamma-rays. A dose of radiation is given when these neutrons reach the human body and are partially absorbed. The dosage
of radiation varies with the energy contained in the neutrons. The graph shown (Figure 4) shows neutron dose at the surface
tissue of the human body as a function of the slant distance. The slant distance takes into account many factors including
the height of the burst. The graph shows that a greater slant distance delivers less rads.
Radiation from Gamma-rays:
The initial gamma-ray dose from a fission bomb comes mainly from radioactive capture. Neutrons that collide with nitrogen
atoms ultimately release energy in the form of gamma rays. Fission product decay also releases gamma rays over time. Gamma
rays from other sources are emitted before the shock wave, unlike the fission bomb which is emitted at a later stage.
of these components contribute to the total dose of radiation. At a short distance from the Earth, neutrons contribute more
the dosage and at greater distances the gamma rays contribute more.
Barnaby, Frank; Joseph Rotblat.
“The Effects of Nuclear Weapons” in Nuclear War: The
Aftermath Vol. 11 No. 2/3, 1982,
Hachiya, Michihiko. Hiroshima Diary. Chapel Hill, NC: The University of North
Carolina Press, 1945: pp. 14-15
Hiroshima-Nagasaki: A Pictorial Record of the Atomic Destruction. Tokyo, Japan:
Committee, 1978: pp. 26-27