[Illustration: _USAEC—Argonne National Laboratory_ 13. Cutaway section of a nuclear reactor. The heart of the reactor is a small region at the center where the fission energy is generated. Most of the weight and volume are needed for cooling apparatus and shielding material to keep in nuclear radiation.] For the radioactivity to affect areas at a large distance from the point of the explosion, considerable time must elapse while the atomic cloud rises and drifts in the horizontal winds. During this time more disintegrations occur, due mainly to the short-lived nuclei. The rate at which they occur keeps diminishing as the short-lived nuclei disappear. Roughly speaking, the rate diminishes simply in proportion to the time. More precisely, the rate drops somewhat faster, decreasing by a factor of ten when the time increases by a factor of seven. A minute after the explosion the activity is less than one per cent of what it is at a second. After an hour it is less than one per cent of its value at a minute. This law for the decrease in activity of fission products is, of course, quite different from the simple law of radioactive decay. The latter law applies to a single radioactive species. The fission products consist at any instant of many different radioactive species. Each one obeys the simple law of radioactive decay, but the totality follows a different law. It should be kept in mind that the product nucleus of a radioactive disintegration may itself be radioactive with a different half-life. For example, there is strontium⁹⁰. Only a small amount of this isotope is made directly in the fission process. The fission process yields large quantities of krypton⁹⁰, which decays with a half-life of one-half minute into rubidium⁹⁰. The latter has a half-life of three minutes and decays into strontium⁹⁰. This is how practically all of the strontium⁹⁰ is made in the explosion. Thus both the intensity and the nature of the radioactivity keep changing with time. These facts are important because they determine the magnitude and the character of the danger when the radioactivity finally falls out of the cloud and is deposited on the surface of the earth. Those radioactive particles which disintegrate while still in the cloud need not worry us since this radiation can have no effect on living organisms that may be underneath. Provided that the cloud is more than a few hundred feet above the ground, the beta and gamma rays released in these disintegrations merely dissipate their energy in ionizing the air.