Report of the British Mission to Japan
Chapter III
THE ACTION OF THE ATOMIC BOMB
17. It was officially announced by President Truman at the time of the Hiroshima attack that the explosive energy of the bomb was equivalent to that of 20,000 tons of T.N.T. Estimates of the equivalent amount of T.N.T. were made by the Mission, based on the damage observed and on extrapolation from experience with much smaller charges. The blast damage was judged to be somewhat less than would have been expected from 20,000 tons, particularly at Hiroshima. This effect may have resulted from the present uncertainties regarding the extent to which damage per ton of explosive falls off as, the weight exploded increases.
18. The source and the manifestation of energy in the atomic bomb differ from those in conventional explosions. The energy stored in an explosive such as T.N.T. is chemical, and when liberated is used in the rapid conversion of the explosive to a gas. This gas, seeking to occupy a volume many times larger than did the solid explosive, exercises intense pressure on the surrounding air, which in turn passes it on to the next layer of air, and so on. The belt of high pressure thus rapidly moving outward from the explosion is the blast wave, and is the major cause of damage from high explosive. All explosions also release heat, but its contribution to the damaging effect is usually negligible.
19. In the process of atomic fission energy is actually created by the destruction of an infinitesimal fraction of the material which is undergoing fission. Some of this energy is communicated to the products of fission, and to the bomb case, but the greater portion is communicated to the surrounding air, which is thereby raised to an extremely high temperature. As a result, the air expanding under the influence of this temperature rise exerts enormous pressure. This pressure moves outward as a blast wave in the conventional manner.
20. At the same time a further large part of the energy created in atomic fission is radiated. This radiation is of various wave lengths including that of ordinary light, and also heat radiation, that is, radiation of longer wave length, and X-rays and possibly gamma rays, that is, radiation of shorter wave length than that of light. Heat radiated by the atomic bomb (which should not be confused either with flame or with heated air) and penetrating short wave radiations are intense enough to kill thousands of people. By contrast, conventional explosions release no penetrating radiation, and the heat released is dangerous only at small distances—for example, to gun crews exposed to the flash at the mouth of a gun.
THE ACTION OF THE ATOMIC BOMB
17. It was officially announced by President Truman at the time of the Hiroshima attack that the explosive energy of the bomb was equivalent to that of 20,000 tons of T.N.T. Estimates of the equivalent amount of T.N.T. were made by the Mission, based on the damage observed and on extrapolation from experience with much smaller charges. The blast damage was judged to be somewhat less than would have been expected from 20,000 tons, particularly at Hiroshima. This effect may have resulted from the present uncertainties regarding the extent to which damage per ton of explosive falls off as, the weight exploded increases.
18. The source and the manifestation of energy in the atomic bomb differ from those in conventional explosions. The energy stored in an explosive such as T.N.T. is chemical, and when liberated is used in the rapid conversion of the explosive to a gas. This gas, seeking to occupy a volume many times larger than did the solid explosive, exercises intense pressure on the surrounding air, which in turn passes it on to the next layer of air, and so on. The belt of high pressure thus rapidly moving outward from the explosion is the blast wave, and is the major cause of damage from high explosive. All explosions also release heat, but its contribution to the damaging effect is usually negligible.
19. In the process of atomic fission energy is actually created by the destruction of an infinitesimal fraction of the material which is undergoing fission. Some of this energy is communicated to the products of fission, and to the bomb case, but the greater portion is communicated to the surrounding air, which is thereby raised to an extremely high temperature. As a result, the air expanding under the influence of this temperature rise exerts enormous pressure. This pressure moves outward as a blast wave in the conventional manner.
20. At the same time a further large part of the energy created in atomic fission is radiated. This radiation is of various wave lengths including that of ordinary light, and also heat radiation, that is, radiation of longer wave length, and X-rays and possibly gamma rays, that is, radiation of shorter wave length than that of light. Heat radiated by the atomic bomb (which should not be confused either with flame or with heated air) and penetrating short wave radiations are intense enough to kill thousands of people. By contrast, conventional explosions release no penetrating radiation, and the heat released is dangerous only at small distances—for example, to gun crews exposed to the flash at the mouth of a gun.