initial nuclear radiation, defined as that produced within a minute or so of the explosion (§ 2.42). The final 10 percent of the total fission energy represents that of the residual (or delayed) nuclear radiation which is emitted over a period of time. This is largely due to the radioactivity of the fission products present in the weapon residues (or debris) after the explosion. In a thermonuclear device, in which only about half of the total energy arises from fission (§ 1.18), the residual nuclear radiation carries only 5 percent of the energy released in the explosion. It should be noted that there are no nuclear radiations from a conventional explosion since the nuclei are unaffected in the chemical reactions which take place.

1.28 The initial nuclear radiation consists mainly of "gamma rays," which are electromagnetic radiations of high energy (see § 1.73) originating in the atomic nuclei, and neutrons. These radiation, especially gamma rays, can travel great distances through air and can penetrate considerable thickness of material. Although they can neither be seen or felt by human beings, except at very high intensities which cause a tingling sensation, gamma rays and neutrons can produce harmful effects even at a distance from their source. Consequently, the initial nuclear radiation is an important aspect of nuclear explosions.

1.29 The delayed nuclear radiation arises mainly from the fission products which, in the course of their radioactive decay, emit gamma rays and another type of nuclear radiation called "beta particles." The latter are electrons. i.e., particles carrying a negative electric charge, moving at high speed; they are followed by a change (neutron > proton + electron) within the nuclei of the radioactive atoms. Beta particles, which are also invisible, are much less penetrating that gamma rays, but like the latter they represent a potential hazard.

1.30 The spontaneous emission of beta particles and gamma rays from radioactive substances, i.e., a radioactive nuclide (or radionuclide), such as the fission products, is a gradual process. It takes place over a period of time, at a rate depending upon the nature of the material and upon the amount present. Because of the continuous decay, the quantity of the radionuclide and the rate of emission of radiation decrease steadily. This means that the residual nuclear radiation, due mainly to the fission products, is most intense soon after the explosion but diminishes in the course of time.

TYPES OF NUCLEAR EXPLOSIONS

1.31 The immediate phenomena associated with a nuclear explosion, as well as the effects of shock and blast and of thermal nuclear radiations, vary with the location of the point of burst in relation to the surface of the earth. For descriptive purposes five types of burst are distinguished, although many variations and intermediate situations can arise in practice. The main types, which will be defined below, are (1) air burst, (2) high altitude burst, (3) underwater burst, (4) underground burst, and (5) surface burst.

1.32 Provided the nuclear explosion takes place at an altitude where there is still an appreciable atmosphere, e.g., below about 100,000 feet, the weapon residues almost immediately incorporate material from the surrounding medium and form an intensely hot and luminous mass,