White wall showing traces of black rain
Black Rain was oily and sticky and left dark stains on the clothing
of those it fell upon. This white wall, at a house 3.7 km from the hypocenter,
shows clear remnants of black rain.
Radiation resulting from the atomic bomb explosion is roughly classified into two categories: initial radiation which was released within the first minute following the explosion, and residual radiation which was released afterwards for a certain period. This large amount of radiation had an extreme effect on the human body.
Four kinds of radiation are included in initial radiation: alpha rays, beta rays, gamma rays and neutron rays. Those which had an effect on the ground were gamma rays and neutron rays. Alpha and beta rays have a low penetration power and are believed to have been absorbed in the air before reaching the ground.
Radiation dose is measured in units called grays. Seven gray (700 rad) is considered to be a lethal dose. Anyone who receives a whole-body dose more than this level has essentially no chance to live. Four gray is considered to be a semi-lethal dose. Half of those exposed to this level die.
Effect of Radiation on Human Body & Initial Radiation Levels.
High levels of residual radiation were present on the ground for a certain period of time starting one minute after the explosion. Residual radiation came in the form of induced radioactivity produced secondarily as a result of the nuclear reaction of neutrons when initial radiation collided with the soil and building materials, and it also came from nuclear fission products and unfissioned uranium scattered by the bomb.
It is quite likely that anyone entering the area within 1 km of the hypocenter within 100 hours after the explosion to search for people or help with relief efforts was affected by radiation coming from the soil and other such places due to induced radioactivity. In addition, soot and dust saturated with induced radiation from nuclear fission products and unfissioned uranium scattered at the time of the bombing were carried high into the atmosphere and later fell to the ground as radioactive fallout, giving rise to further possibilities for contamination.
As the downtown area erupted in huge fires after the explosion, intense firestorms and whirlwinds developed. After 20-30 minutes, black rain began to fall in a wide area stretching from downtown to the northwest.
Large amounts of fallout, referred to as "Ashes of Death" were contained in the rain in the form of radioactive soot and dust and caused contamination even in areas remote from the hypocenter.
According to a study after the bombing, heavy rain fell in a 19km x 11 km area and light rain fell in a 29km x 15km area, but there have been testimonies that black rain fell in places outside these areas as well.
White wall showing traces of black rain
Black Rain was oily and sticky and left dark stains on the clothing of those it fell upon. This white wall, at a house 3.7 km from the hypocenter, shows clear remnants of black rain.
